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Title:
A METHOD FOR PRODUCING DIAMOND MATERIAL
Document Type and Number:
WIPO Patent Application WO/2008/007336
Kind Code:
A3
Abstract:
The present invention relates to synthesised diamond material having at least two of the following characteristics: (i) a nitrogen content of at least 100 ppm; (ii) a crystal morphology index (CMI) of either less than four (4) or greater than six (6); (iii) a uniform distribution of nitrogen in the majority volume of the crystal where the majority volume is at least 50% of the total volume of the crystal; (iv) a low transition metal impurity content, where 'low' is less than 100 ppm by mass; and (v) a crystal shape with absence of minor facets, where 'minor facets' are all facets other than {100} and {111} facets. The invention further relates to a method of producing diamond material including the step of providing a reaction mass including sufficient nitrogen to result in diamond material having a nitrogen content of at least 100 ppm in combination with a transition metal vector and subjecting the reaction mass to a temperature of greater than 12000C and a pressure greater than 5 GPa.

Inventors:
DODGE CARLTON NIGEL (GB)
TSHISIKHAWE FRANCIS ALIZULI (ZA)
SUMMERTON GRANT CHARLES (ZA)
HANSEN JOHN OLAF (ZA)
GUY KEITH BARRY (GB)
NAUDE LOUIS LOUWRENS (ZA)
BURNS ROBERT CHARLES (ZA)
LAWSON SIMON CRAIG (GB)
Application Number:
PCT/IB2007/052751
Publication Date:
May 15, 2008
Filing Date:
July 10, 2007
Export Citation:
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Assignee:
ELEMENT SIX TECHNOLOGIES PTY L (ZA)
DODGE CARLTON NIGEL (GB)
TSHISIKHAWE FRANCIS ALIZULI (ZA)
SUMMERTON GRANT CHARLES (ZA)
HANSEN JOHN OLAF (ZA)
GUY KEITH BARRY (GB)
NAUDE LOUIS LOUWRENS (ZA)
BURNS ROBERT CHARLES (ZA)
LAWSON SIMON CRAIG (GB)
International Classes:
B01J3/06; C30B29/04
Domestic Patent References:
WO2005084334A22005-09-15
Foreign References:
US3442616A1969-05-06
US4174380A1979-11-13
EP0346794A11989-12-20
US20010043903A12001-11-22
EP0239239A11987-09-30
Other References:
KIFLAWI I ET AL: "The effect of the growth rate on the concentration of nitrogen and transition metal impurities in HPHT synthetic diamonds", DIAMOND AND RELATED MATERIALS, ELSEVIER SCIENCE PUBLISHERS, AMSTERDAM, NL, vol. 11, no. 2, February 2002 (2002-02-01), pages 204 - 211, XP004336318, ISSN: 0925-9635
Attorney, Agent or Firm:
SPOOR & FISHER et al. (0001 Pretoria, ZA)
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Claims:

CLAIMS

1. Synthesised diamond material having at least two of the following characteristics (i) to (v):

(i) a nitrogen content of at least 100 ppm;

(ii) a crystal morphology index (CMI) of either less than four (4) or greater than six

(6);

(iii) a uniform distribution of nitrogen in the majority volume of the crystal where the majority volume is at least 50% of the total volume of the crystal;

(iv) a low transition metal impurity content, where 'low' is less than 100 ppm by mass; and

(v) a crystal shape with absence of minor facets, where 'minor facets' are all facets other than {100} and {111} facets.

2. Synthesised diamond material according to claim 1 having at least three of characteristics (i) to (v).

3. Synthesised diamond material according to any one of claims 1 and 2 having at least four of characteristics (i) to (v).

4. Synthesised diamond material according to any previous claim wherein at least two of characteristics (i) to (v) are exhibited by the material prior to post synthesis treatments including but not limited to cutting, polishing or grinding.

5. Synthesised diamond material according to any previous claim suitable for use as a cutting or machining tool, an optically activated switch, a blade for surgery, a designer anvil, a gemstone, a substrate for the synthesis of single crystal diamond by chemical vapour deposition (CVD) and other optical material.

6. Synthesised diamond material according to any previous claim wherein δN over an area of 1 mm x 1 mm is less than 100%.

7. Synthesised diamond material according to any previous claim having two dimensions of at least 3 mm.

8. A method of producing diamond material including the step of providing a reaction mass including sufficient nitrogen to result in diamond material having a nitrogen content of at least 100 ppm in combination with a transition metal vector and subjecting the reaction mass to a temperature of greater than 1200 0 C and a pressure greater than 5 GPa.

9. A method according to claim 8 further including the step of including a solvent catalyst alloy treated such that a component or components of the solvent catalyst alloy that are usually regarded as being unable to form nitrides, at the conditions usually used to form nitrides in steel to increase the nitrogen content of the component or components of the solvent catalyst alloy to be between 300 and 900ppm.

10. A method according to claim 9 wherein the solvent catalyst contains as a major component either nickel or cobalt or nickel and cobalt.

11. A method according to claim 10 wherein the solvent catalyst contains as an additive at least one of the elements manganese, chromium and copper.

12. A method according to any previous claim wherein the carbon source material from which the diamond is grown is diamond.

13. A method according to any previous claim wherein the diamond material is according to claim 1.

14. A method according to any previous claim wherein greater than 10% of diamond material stones from a single run have a CMI of less than 5.

15. A method according to any previous claim wherein the variation in the maximum edge length (MEL) from any one synthesis run is such that greater than

25% of diamond material stones show a maximum variation expressed as a percentage of the average of less than 85%.

16. A method as claimed in claim 8 wherein the diamond material has a CMI of less than 4 and the temperature is from 1220 deg C to 1250 deg C.

17. A method as claimed in claim 8 wherein the diamond material has a CMI of greater than 6 and the temperature is higher than 1350 deg C.

18. Use of nitrogen to control the morphology of diamond material synthesised at a pressure greater than 5 GPa.

19. Use as claimed in claim 18 wherein the nitrogen content is at least 100 ppm.

20. Use as claimed in claim 18 or 19 wherein the diamond material is according to claim 1.

21. Use as claimed in any one of claims 18 to 20 wherein the nitrogen is used together with temperature to control the morphology of synthesised diamond material.

Description:

A METHOD FOR PRODUCING DIAMOND MATERIAL

INTRODUCTION

This invention relates to a method for producing diamond material, in particular this invention relates to a method for producing diamond material of defined and useful form and to diamond material produced to have such predetermined qualities. The present invention therefore relates to the growth or synthesis of large single crystals of diamond by either the temperature gradient method under high pressure high temperature (HPHT) conditions, or otherwise by the phase-balance or graphite conversion process where excess pressure is the motive force for growth.

By the teachings of this invention it is possible to produce diamonds having nitrogen content of more than 500 parts per million (ppm) and a shape that is useful for the production of products such as plates, rods and prisms with a high volumetric conversion efficiency of the as-synthesised diamond material into final product.

BACKGROUND TO THE INVENTION

It is well known to those skilled in the art of diamond synthesis that nitrogen is often present in the reaction mixture by way of an impurity. The presence of this nitrogen in the reaction mixture allows the HPHT growth process to occur at a higher growth rate, expressed in milligrams per hour of diamond deposited, than in the absence of nitrogen. It is also known that when nitrogen impurity is present in the reaction mixture, growth temperature affects the morphology of crystals grown, whereas in the absence of this nitrogen impurity, the crystal morphology changes much less with growth temperature.

Maximising the fraction of the total volume of each crystal that can be used in a finished product is an important challenge in diamond synthesis. For a range of products ranging from synthetic gem stones to substrates for homoepitaxial growth (discussed in more detail later), this challenge provides an important motivation for tuning growth processes to yield crystals that are either as cubic or as octahedral as possible because such shapes give a high yield for the finished product. In addition, for many of these products there is a size requirement that makes it advantageous for such shapes to be achieved for crystals above particular size thresholds.

In some applications it is important for the diamond material in the finished product to have uniform optical properties. It is well known that the optical properties (for example optical absorption) of diamond may be strongly affected by its nitrogen content and that this is in turn sensitive to the crystallographic orientation (e.g. {100} or {111}) of the growth surface of the diamond. This has suggested that synthesis of material with low nitrogen content is more suitable for applications of this type.

As discussed above, the morphology of HPHT synthesised diamond crystals can be tuned to a certain extent by varying the growth temperature. There is, however, a limit to the extent to which extremely cubic or octahedral crystals can be produced using temperature alone to tune the morphology. There is therefore a need for a method to achieve crystals with these extremes of crystal morphology, preferably

with sizes above particular thresholds and for some applications with uniform optical, electrical and mechanical properties as well.

For ease of subsequent discussion we propose to use a scale for the measurement of crystal morphology which has been is use for several years within the industry and is known as the 'crystal morphology index' (CMI) as applied to crystals of the cubic system. In essence, the CMI is the fractional cut back at each cube corner seen in plan made by the matching {111} plane on a scale of 0 to 8. A cube, bounded by six {100} types faces, has a CMI value of 0 and an octahedron, bounded by eight {111} type faces, has a CMI value of 8. Crystals which are bounded by a mixture of {100} and {111} type faces have CMI values that are greater than 0 and less than 8. A graphical representation of the CMI index is given in Figure 1.

In the growth of large diamond crystals by the reconstitution or temperature gradient method it is important to maintain the position of the seed crystals at the lower end of the temperature gradient. For this reason, the seeds from which diamond crystals are grown are generally retained in a ceramic carrier. The presence of the carrier means that crystals can generally only grow into half of the full solid angle. After growth the seed crystal remains exposed on the seed face of the grown large crystal. The CMI is defined using the view of the crystal from the side furthest from the seed. (See Figure 2)

SUMMARY OF THE INVENTION

The present invention relates to a process for synthesising large diamond crystals of a defined and useful form, with low metallic impurity content and at a high rate of growth. The crystal growing process is conducted at High Pressure and High Temperature (HPHT) using either the temperature gradient process or alternatively using the phase balance (graphite conversion) process and where materials to be used for the growth of the diamonds are augmented by a controlled quantity of one or more components or vectors that have the effect of introducing nitrogen into the growing diamonds in a controllable and reproducible way. One such vector is a cobalt metal powder that has preferably been treated in ammonia at a temperature of not less than 700 0 C before combining with the other metal components used in

the HPHT synthesis solvent catalyst. By the teachings of this invention it is possible to produce diamonds having nitrogen content of more than 500 ppm and a shape that is useful for the production of products such as plates, rods and prisms with a high volumetric conversion efficiency of as-synthesised diamond material into final product.

According to a first aspect to the present invention there is provided a method of producing diamond material including the step of providing a reaction mass including sufficient nitrogen to result in diamond material having a nitrogen content of at least 100 ppm in combination with a transition metal vector and subjecting the reaction mass to a temperature of greater than 1200 0 C and a pressure greater than 5 GPa.

According to a second aspect of the present invention there is provided synthesised diamond material having at least two, preferably at least three, more preferably at least four of the following characteristics (i) to (v): (i) a nitrogen content of at least 100 ppm;

(ii) a crystal morphology index (CMI) of either less than four (4) or greater than six (6);

(iii) a uniform distribution of nitrogen in the majority volume of the crystal where the majority volume is at least 50% of the total volume of the crystal; (iv) a low transition metal impurity content, where 'low' is less than 100 ppm by mass; and

(v) a crystal shape with absence of minor facets, where 'minor facets' are all facets other than {100} and {111} facets.

What is not known in the art and the understanding of which is the essence of this invention is that increasing the nitrogen concentration in the growth environment can be used to accentuate the effect of temperature on crystal morphology, with higher concentrations of nitrogen in the growth environment giving crystals with even more cubic morphology at low temperature and crystals of even more octahedral morphology at high temperature. In addition to the effect that increasing the nitrogen concentration has on the relative sizes of the {100} and {111} crystal faces,

the inventors have also found that the tendency for formation of crystal faces with other orientations (such as {113} and {115}) can be suppressed at low temperatures by increasing the nitrogen concentration avoiding the growth of stones of undesirable morphology (Figure 4).

According to a third aspect to the present invention there is provided the use of nitrogen to control the morphology of synthesised diamond material.

The nitrogen concentration in the grown diamond is also affected by the growth temperature and the nitrogen concentration in growth environment. The outcome is that diamond with a nitrogen concentration in a given range can be grown in forms that depend on the growth temperature in the way indicated in Table 1. The morphology of the stones varies continuously from fully-cubic to fully octahedral and the boundaries between the temperature bands in Table 1 are consequently somewhat blurred.

Table 1 : Combinations of morphology and nitrogen content achieved in different growth temperature ranges.

A graphical representation of table 1 can be seen in Figure 3 where entries in the grid of Figure 3 match the corresponding entries in Table 1.

This invention relates to the growth or synthesis of large single crystals of diamond by the either the temperature gradient method under high pressure high temperature (HPHT) conditions, or otherwise by the phase-balance or graphite conversion process where excess pressure is the motive force for growth.

In this specification, synthesised diamond material may be interchangeably referred to as diamond, diamond crystal, crystal, stone, diamond material and synthesised diamond material.

DETAILED DESCRIPTION

When growing diamonds of the type Ib, it is well-known in the art to have nitrogen contents generally greater than one part per million (1 ppm), and more generally greater than 50 ppm. The nitrogen that enters the growing diamond is derived principally from the atmospheric air that is entrapped in the starting materials used for synthesis, and to a lesser extent it is derived from the small amount of nitrogen, typically less than 50 parts per million by mass, that is present as an impurity in the metals used as catalyst solvents. Nitrogen is present because the catalyst metals and other materials were prepared in most cases using processes that are at least in part open to the earth's atmosphere, and that atmosphere contains a constant percentage of nitrogen of near to 80 percent by volume. Some of this nitrogen is inevitably dissolved or physically entrained in the metals giving low but finite nitrogen contents of typically a few tens of parts per million by mass.

In growing diamond in High Pressure High Temperature processes using the temperature gradient method, growth temperature is known to affect the morphology, the rate of nitrogen incorporation and growth rate (see for example H. Kanda, in 'Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials', Ed. P. Capper. (2005), p416). A correlation is seen between growth rate and the level of nitrogen incorporation and it has been concluded, rightly or wrongly, that the growth rate is the cause and the nitrogen level is the effect (I. Kiflawi, H. Kanda, S. C. Lawson, Diamond Relat. Mater., 11 (2002), p204). With decreasing growth temperature, the total nitrogen content increases, cubic morphologies are preferred and the relative distribution of nitrogen between growth sectors changes so that the nitrogen concentration in the cubic sectors, N{ioo} > is greater than the nitrogen content in the octahedral sectors, N{in} (S. Satoh, H. Sumiya, K. Tsuji, S. Yazu, 'Science and Technology of New Diamond' Proc. 1st Int. Conf. on New Diamond Science and Technology, Tokyo, 24-26 Oct 1988, p351-355).

In all further discussion we will use the words "nitrogen content" to denote the total content of nitrogen within the synthetic diamond. It is known to those skilled in the art that the nitrogen can be present in one of several forms in the diamond. One such form is the single substitutional nitrogen atom, where a nitrogen atom replaces a carbon atom at a site in the diamond lattice. Diamond in which this is the major form of nitrogen present, and where the nitrogen content is generally greater than one part per million (1 ppm) is classified as being 'Type Ib' (for a description of the diamond classification system, see Field, The Properties of Natural and Synthetic Diamond', Academic Press, London, 1992, p669). Almost all diamond grown by the HPHT method, and in particular in the regimes of temperature and pressure normally applied for this method, is of type Ib. When the crystal has been subjected to subsequent processing at high temperatures and high pressures, the form of the nitrogen present in the crystal may become altered. It should be clear that the present invention relates to diamond crystals containing high total concentrations of nitrogen, irrespective of the form of the nitrogen. When the nitrogen is present in the form found in type Ib diamonds, the concentration of nitrogen can be measured with accuracy using the techniques of FTIR absorption spectroscopy using the 1130 cm "1 feature in the infrared region of the absorption spectrum and an absorption coefficient-concentration relation of 22 ppm/cm "1 (G. S. Woods et al, Phil. Mag., B62 (1990), p589-595). The concentration of nitrogen in aggregated forms (such as A- centres composed of nearest neighbour nitrogen atoms) can also be measured using FTIR absorption spectroscopy. The total nitrogen concentration can be measured using techniques such as Secondary Ion Mass Spectroscopy (SIMS).

In HPHT synthesis it cannot be considered straightforward to control the nitrogen in the growth environment. One obvious problem already referred to is the entrainment of atmospheric nitrogen during the fabrication of the raw materials.

Whilst there are techniques known in the art for reducing the nitrogen content by the use of 'getters' and pre-treatment of raw materials, the same is not true of controllably and reproducibly increasing the nitrogen content.

The possibilities for enriching the gas space within the reaction cell are limited because air is already 80% nitrogen, N 2 . The addition of ammonia to the reaction

cell or capsule is not effective because one molecule of ammonia carries half the number of nitrogen atoms by comparison with the dinitrogen molecule in air. The toxicity of ammonia poses further problems.

The use of a nitrogen-containing organic compound, as has been discussed in the prior art (Burns RC, Davies GJ, ZA patent 89/7820 Diamond synthesis (1990)), is not straightforward because most organic compounds containing nitrogen are also hydrogen-rich and hydrogen (and hydrocarbons) can act as poisons in HPHT synthesis. There are further difficulties associated with the uniform distribution of the organic compound or inorganic salts within the reaction volume of the synthesis capsule, and their stable persistence through the steps of capsule preparation, which often involve stages of vacuum drying and purification, as would be well known to those practiced in the art.

The addition of a stable metallic nitride such an iron nitride, has been suggested (Borzdov Y, Pal'yanov Y 1 Kupriyanov I, Gusev V, Khokhryakov A, Sokol A, Efremov A, 1 HPHT synthesis of diamond with high nitrogen content from an Fβ 3 N-C system 1 , Diamond Relat, Mater., 11 (2002), p1863-1870), but this method is not preferred for the following reasons:

1. High concentration - stable nitrides such as Fβ 3 N contain tens of thousands of ppm of nitrogen and so only a small quantity of the dopant must be added and distributed rather well. This is difficult to achieve on a controllable and reproducible basis and so is unsuited to anything other than research.

2. Stability of the nitride - stable nitrides require high temperatures for the release of the nitrogen component. Further, the published work cited above uses temperatures of higher than 1700 0 C at pressures of 7 GPa. Apart from the cost of achieving these conditions, which are higher by several hundred degrees than the conventional HPHT synthesis, this range of temperatures makes it impossible to achieve the near-cubic morphology that is required for the production of one of the useful morphologies of this invention.

Surprisingly, we have found that a good result can be achieved by treating that component (or those components) of the solvent catalyst alloy that are usually regarded as being unable to form nitrides, at the conditions usually used to form nitrides in steel and other metals. These metals are typically cobalt and nickel, and could include other metals used as additives such as manganese, chromium and copper. Preferably the solvent catalyst contains either nickel or cobalt or nickel and cobalt as a major component. Preferably in addition to either cobalt or nickel or cobalt and nickel, the solvent catalyst contains at least one of the elements manganese, chromium and copper as an additive. A major component in a solvent catalyst is present at a level of more than about 5% by weight. An additive in a solvent catalyst is present at a level of less than about 1% by weight. This has the effect of increasing the nitrogen content in the treated metal powder, as measured by a technique such as the 'Leco Elemental C/H/N TruSpec' analysis method or similar combustion analysis method, to be between 300 and 900 ppm. At this level, the addition of between 10 and 100 grams of the nitrogen-enriched metal powder to the synthesis reaction volume has the following beneficial actions:

1. The nitrogen in the diamond so produced is increased from a level of 100 to 150 ppm in the case where no additive is used, to over 100 ppm, preferably greater than 150 ppm, preferably greater than 200 ppm, preferably greater than 250 ppm, preferably greater than 300 ppm, preferably greater than 350 ppm, preferably greater than 400 ppm, preferably greater than 450 ppm, preferably greater than 500 ppm, preferably greater than 550 ppm, preferably greater than 600 ppm, preferably greater than 650 ppm, preferably greater than 700 ppm, preferably greater than 750 ppm, preferably greater than 800 ppm, preferably greater than 850 ppm, preferably greater than 900 ppm, preferably greater than 950 ppm, preferably greater than 1000 ppm.

2. The shapes of the diamonds grown under these conditions is best described as a 'blocky shape', predominantly comprising of either {100} facets or {111} facets, with little or no minor facets such as {110}, {113} and {115} and little or no curvature of the facets and edges.

3. After synthesis at low temperatures typically below 1250° C, the diamonds are strongly cubic in shape (i.e. exhibit predominantly {100} facets) and this shape favours the efficient production of plates, logs and prisms as might be important in technical applications. In practice this means the materials morphology can be described by having a CMI less than 5, preferably a CMI less than 4, preferably a CMI less than 3, preferably a CMI less than 2.

4. After synthesis at high temperatures typically above 1350 0 C, the crystals are strongly octahedral in shape (i.e. exhibit predominantly {111} facets) as might be useful when there is a requirement for this form. Such a requirement might be in the gem industry, where the octahedral shape is more usually seen in the natural diamond crystals most often used in that industry. In practice this means the materials morphology can be described by having a CMI greater than 5, preferably a CMI greater than 6, preferably a CMI greater than 7.

Contrary to the prior art and the expectations of the inventors, the material of this invention is found to have a low total transition metal concentration dissolved in the lattice. The total transition metal content in the lattice is less than 100 ppm, preferably less than 80 ppm, preferably less than 60 ppm, preferably less than 50 ppm, preferably less than 40 ppm, preferably less than 30 ppm, preferably less than 20 ppm, preferably less than 10 ppm, preferably less than 5 ppm, preferably less than 2 ppm, preferably less than 1 ppm.

In particular the total Ni concentration is less than 100 ppm, preferably less than 80 ppm, preferably less than 60 ppm, preferably less than 50 ppm, preferably less than 40 ppm, preferably less than 30 ppm, preferably less than 20 ppm, preferably less than 10 ppm, preferably less than 5 ppm, preferably less than 2 ppm, preferably less than 1 ppm.

In particular the total Al concentration is less than 100 ppm, preferably less than 80 ppm, preferably less than 60 ppm, preferably less than 50 ppm, preferably less than 40 ppm, preferably less than 30 ppm, preferably less than 20 ppm, preferably less

than 10 ppm, preferably less than 5 ppm, preferably less than 2 ppm, preferably less than 1 ppm.

In particular the total Co concentration is less than 100 ppm, preferably less than 80 ppm, preferably less than 60 ppm, preferably less than 50 ppm, preferably less than 40 ppm, preferably less than 30 ppm, preferably less than 20 ppm, preferably less than 10 ppm, preferably less than 5 ppm, preferably less than 2 ppm, preferably less than 1 ppm.

In particular the total Fe concentration is less than 100 ppm, preferably less than 80 ppm, preferably less than 60 ppm, preferably less than 50 ppm, preferably less than 40 ppm, preferably less than 30 ppm, preferably less than 20 ppm, preferably less than 10 ppm, preferably less than 5 ppm, preferably less than 2 ppm, preferably less than 1 ppm.

These transition metal impurities can be quantified and characterized using techniques such as SIMS and photoluminescence. For example it is reported in the literature that under 325 nm excitation PL features thought to be associated with Co are observed at 523.8, 544.5, 561.7, 580.7 nm. With excitation at 632.8 nm further peaks at 657.7, 669.2, 720.8, 722.8, 793.2, 807.6, 863.9, 868.8, 882.6, 884.7 nm 887.4 and 992.3 nm are also thought to be associated with Co.

Excitation at 632.8 nm in samples grown with a nickel catalyst show nickel related peaks at 657.7, 722.8, 793.2, 807.6, 868.8, 882.6, 884.7 nm. Further nickel related features are produced under 532 nm excitation at 728, 707 and 751 nm. There is a feature believed to be due to Ni + in an interstitial site at 881 nm. The FTIR spectra can have an absorption feature at 1332 cm due to N + (ionised single substitutional nitrogen). This is believed to be formed by charge-transfer between Ni and N yielding N + and Ni " . As there is a correlation between the two, the absorption coefficient at 1332 cnrr " ! can be used to determine the concentration of Ni". In some prior art it has been reported that the Ni concentration is greater than 100 ppm in certain regions of the stone.

As a corollary of this invention, it should be clear that the carbon source material from which the diamond is grown could also be diamond. This includes diamond synthesized through this route. This provides an additional route to further enhance the nitrogen in the grown material by starting with a source material containing a high nitrogen concentration.

As a further corollary of this invention, it should be clear that it is possible to use the diamond grown by the teachings of this invention as the vector for nitrogen (where the term 'vector' has the meaning of 'the means by which the nitrogen is transported from the synthesis environment into the growing diamond crystal') in a subsequent synthesis cycle when using the reconstitution, or temperature-difference method of diamond growth. In this case, the diamond that is used as the vector for nitrogen is also the source of carbon for the growth of the large diamond crystals. This may not appear to be as attractive as the use of a metal or metal-alloy vector, in the application as an economical solution to an industrial problem. It would be clear to one skilled in the art, that the use of diamond as a source material may confer other advantages in the design of an HPHT synthesis process. For that reason, the vector for the addition of nitrogen to the crystal growth process, according to the teachings of this invention, could be diamond of high nitrogen content, whether grown by the teachings of this invention or produced in some other way.

The nitrogen content of diamond when grown by the HPHT method can be controlled to range from less than 1 ppm of nitrogen through the action of nitrogen getters and other techniques, through to levels of greater than 1000 ppm by adding additional nitrogen to the catalyst (Z. Z. Liang, X. Jia, H. A. Ma, C. Y. Zang, P. W. Zhu, Q. F. Guan, H. Kanda, Diamond Relat. Mater., 14 (2005), p1932). We have grown diamond in Co-Fe catalysts where we have increased the nitrogen content of the environment by a factor of between 3 and 10 times the normal atmospherically equilibrated value, yielding samples containing up to 1000 ppm of nitrogen. The external crystal morphologies, as well as the internal sector structure and nitrogen content, have been measured to provide data to develop our understanding of how the growth variables interact. The results have indicated that the incorporation of nitrogen into {100} sectors is more sensitive to changes in the growth conditions than the {111} sector. Measurements that we have made on whole crystals suggest

that increasing nitrogen within the catalyst at these high nitrogen levels decreases the overall growth rate. Our experience is that at low levels of nitrogen, increasing the nitrogen increases the growth rate. There is generally a maximum growth rate in the region of 150 ppm of incorporated nitrogen, but this can be affected by the nature of the catalyst solvent and the growth temperature. A similar observation has been reported for growth of CVD diamond (J. J. Schermer, F. K. de Theije. Diamond Relat. Mater., 8 (1999), p2127). The added nitrogen also promotes octahedral growth when the temperature of the synthesis process is in the high range of temperatures available for growth. The range of temperatures that are available for growth will depend upon the pressure at which the growth occurs. Generally, a higher pressure allows for growth over a larger range of temperatures.

The inventors have studied diamond growth within two growth regimes, one capable of producing high CMI (near 8) crystals the other, low CMI (near 1). We find that when pursuing conditions for high CMI growth, the action of increasing the nitrogen content of the catalyst leads to increased nitrogen incorporation within the grown diamond as well as a higher CMI (i.e. more octahedral crystals). Further we have found that the action of increasing the growth temperature had the effect of increasing the CMI, leading to more octahedral crystals, to a greater extent than could be achieved solely by adding nitrogen. This result is unexpected.

Further, the inventors have found that when pursuing conditions for low CMI growth, a lower growth temperature was required, as has been disclosed in the prior art, but that here, surprisingly, addition of nitrogen increases the tendency towards lower CMI growth, the opposite to what was seen in the high CMI regime. In this case, the action of reducing the growth temperature favours the growth of crystals with a more cubic morphology, but at a reduced growth rate.

Thus it has been established that combining the effects of temperature and nitrogen allows more efficient synthesis of diamond with either highly cubic or highly octahedral morphology.

Examination of sectioned samples grown under both regimes highlights an interesting difference. In the case of samples with a high CMI, i.e. octahedral or

nearly octahedral crystals, the concentration of nitrogen in the octahedral sectors, indicated by [Ntøn}, is greater than the concentration of nitrogen in the cubic sectors, indicated by [N]{ioo>- In samples of low CMI, i.e. cubic or nearly cubic crystals, the opposite holds true and the [N] { i O o} is greater than the [N] {1 n}. Slight increases in growth temperature reduce the nitrogen incorporation in the {100} sector but it remains at a much more constant level in the {111}. Further we have noticed that the nitrogen concentration within the central {100} sector, that sector which grows in the direction of the carbon source material, has a tendency to decrease during the course of the growth cycle, whereas it remains more constant in the {111} sector. The cause of this change is at present not known. It could be due to a loss of the added nitrogen from the HPHT growth cell, or to temperature or pressure changes. The temperature of the growing face will increase as it moves upward and towards the carbon source, because of the temperature gradient used to promote crystal growth, but we would not expect this temperature difference to be sufficient to result in such a distinct change in morphology. This decrease in [N] {1O o } /[N] { iii } ratio is accompanied by an increase in the octahedral form which could be explained by an increase in the relative growth rate of the {100} sector as compared to the {111} sector (Figure 7).

Morphologies of the grown crystals were recorded from captured images using the CMI system as defined above. Selected samples were sectioned along {110} planes and polished on a diamond polisher's scaife. The sector structures were examined both optically and using a photoluminescence microscope such as the DTC Diamond View™ instrument. The distribution of nitrogen between sectors was determined from the polished plates by FTIR spectra using a Nicolet 750 Magna IR spectrometer with a 0.5 mm aperture. More detailed scans of the change in nitrogen within sectors were determined using a Nicolet Nic plan microscope through a 0.1 mm aperture. These measurements enable the change of morphology during growth to be related to the nitrogen incorporation between sectors.

A high degree of uniformity in the nitrogen concentration as measured by FTIR using the method described below. In particular, the frequency distribution of nitrogen measurements taken by FTIR over a representative sample taken from the layer

must be such that 90% of the measurements vary by less than 50%, and preferably by less than 30%, expressed as a percentage of the mean. In all later discussion this is what is defined as δN ('delta N').

A bulk measurement of the uniformity of the concentration of nitrogen in a parallel sided diamond sample can be made using FTIR absorption spectroscopy in the following way. A representative map of the infrared absorption characteristics over the whole sample is built up by collecting FTIR spectra at room temperature with a 0.5 cm "1 resolution and an aperture size of 0.1-0.5 mm, the map containing a minimum of 10 data points. One of the relationships above is then chosen based on the average measurement taken and used to derive the concentration of nitrogen for each position. The uniformity is then judged from the frequency plot of the concentration measurements taken, assessing the percentage of measurements further away from the average than the limit to the deviation set.

SIMS analysis was typically performed using an O 2 + primary beam, with a primary voltage of 10 kV, a beam current of typically 1 μA and a spatial resolution of better than 50 μm. Mapping was typically completed by stepping the analysis point on a 0.5 mm or 1 mm pitch over the face of the layer, obtaining from each face typically a minimum of 20 points and more preferably a minimum of 40 points. Calibration was by comparison with implant standards. Data from the SIMS was analysed by finding the mean of the dataset, and then finding the full range of the data expressed as a percentage of the mean for the different % fractions of the dataset, with the two opposite major faces of a layer given approximately equal weighting in order to characterise a volume. Reproducibility of the SIMS is typically of the order of 3-5%, dependent on conditions.

The material of this invention can be used in wide range of applications all of which have specific requirements on the geometry of the diamond used for their fabrication. It will thus be seen that controlling the morphology of the stone using the invention allows more efficient use of the synthesised material. The applications described below are non-limiting and provided merely to demonstrate the required attributes of the material.

Diamond as a Cutting or Machining Tool

Using the method of the current invention it is possible to synthesise large diamond crystals,containing high concentrations of nitrogen and with near-cubic morphology. Such material is considered beneficial in a range of different applications.

Single crystal cutting tool products are fabricated from a range of shapes and qualities of diamond depending on the application, market and cost requirement. The most common shapes are rectangles for cutting applications and 'logs' for dressing grinding wheels. Other shapes are also used such as circles and triangles. The majority of products have a four-point orientation for the largest planar face. This nomenclature refers to the sides of the products which are predominantly cut with edges parallel to a cubic or <100> direction. Tools with two-point <110> edges can also be made. Dresser logs, which are high aspect ratio rectangular prisms, for example have four-point all around and a four-point tip.

HPHT synthesis processes are developed to meet market requirements of cost, shape, quality and size. Some products are sold near net synthesis shape and some are highly engineered, high tolerance products. The challenge for HPHT synthesis is to maximise the usable volume of each crystal and stones from each synthesis run. Theoretically, the ideal way is to grow crystals without missing corners and truncated edges. Small crystals, of less than 4 mm edge length, should be grown near-net cubic shape requiring little removal from the large planar faces. There are two reasons for loss of volume. In the first case, the corners may be bound by octahedral ({111}) facets, as is seen in the central entry in the upper row of Table 1 and Figure 1. In the second case, the edges are curved and made less sharp by the presence of false facets, of the type shown in Fig 4. In both cases, more material has to be removed from both planar faces to open up the usable area of the crystal and reduce the truncation. Consequently, the usable volume yield from truncated crystals is considerably lower and the cost to process to usable shapes is higher. The same principle applies to crystals with a larger edge length

(typically greater than 4 mm). In this case, we would expect to be able to process the crystal to yield multiples of planar plates with larger areas. The extent to which this is possible depends upon the extent of the truncated corners and edges; therefore it is apparent that being able to control the morphology and reduce the impact is a significant advantage.

The teachings of this invention are central to the growth of a crystal showing an external morphology with a well defined shape, preferably a cubic shape, and well defined sharp edges. Whilst the nitrogen level is not the only factor contributing to the growth of crystals of the desired form, it is an important one. When using the teachings of this invention it becomes possible to control the amount of nitrogen added to the synthesis process in a systematic and reproducible way.

The teachings of this invention not only produce material with controlled and more uniform shape, but also increases the effective yield in terms of the number of stones from each synthesis run which fits certain size and morphology criteria. This is an obviously important commercial factor.

In detail this invention enables greater than 10%, preferably greater than 20%, preferably greater than 30%, preferably greater than 40%, preferably greater than 50%, preferably greater than 60%, preferably greater than 70%, preferably greater than 80%, preferably greater than 90%, of the stones from a single run to have a CMI of less than 5.

Preferably this invention enables greater than 10%, preferably greater than 20%, preferably greater than 30%, preferably greater than 40%, preferably greater than 50%, preferably greater than 60%, preferably greater than 70%, preferably greater than 80%, preferably greater than 90%, of the stones from a single run to have a CMI of less than 4.

Preferably this invention enables greater than 10%, preferably greater than 20%, preferably greater than 30%, preferably greater than 40%, preferably greater than 50%, preferably greater than 60%, preferably greater than 70%, preferably greater

than 80%, preferably greater than 90%, of the stones from a single run to have a CMI of less than 3.

Preferably this invention enables greater than 10%, preferably greater than 20%, preferably greater than 30%, preferably greater than 40%, preferably greater than 50%, preferably greater than 60%, preferably greater than 70%, preferably greater than 80%, preferably greater than 90%, of the stones from a single run to have a CMI of less than 2.

In an alternative aspect of this invention where high CMI is achieved by increasing the growth temperature in addition to increasing the nitrogen in the growth process through the method of this invention.

In detail this invention enables greater than 10%, preferably greater than 20%, preferably greater than 30%, preferably greater than 40%, preferably greater than 50%, preferably greater than 60%, preferably greater than 70%, preferably greater than 80%, preferably greater than 90%, of the stones from a single run to have a CMI of greater than 5.

Preferably this invention enables greater than 10%, preferably greater than 20%, preferably greater than 30%, preferably greater than 40%, preferably greater than 50%, preferably greater than 60%, preferably greater than 70%, preferably greater than 80%, preferably greater than 90%, of the stones from a single run to have a CMI of greater than 6.

Preferably this invention enables greater than 10%, preferably greater than 20%, preferably greater than 30%, preferably greater than 40%, preferably greater than 50%, preferably greater than 60%, preferably greater than 70%, preferably greater than 80%, preferably greater than 90%, of the stones from a single run to have a CMI of greater than 7.

In addition to factors that depend on the shape and morphology of the stone this invention enables the size distribution from any one synthesis run to be decreased

compared with growing stones under the same conditions except for the nitrogen level. This size distribution is expressed as a percentage of the average dimensions of the stones which have a maximum edge length (MEL) (as measured in the plane of the growth, i.e. perpendicular to the growth direction) as determined by measurements using digital callipers (or something similar) of the maximum dimensions.

In particular the variation in the maximum edge length (MEL) is such that greater than 25% of the as grown stones show a maximum variation expressed as a percentage of the average of less than 85%, preferably less than 70%, preferably less than 50%, preferably less than 40%, preferably less than 25%, preferably less than 15%, preferably less than 10%, preferably less than 5%.

In particular the variation in the maximum edge length (MEL) is such that greater than 40% of the as grown stones show a maximum variation expresses as a percentage of the average of less than 85%, preferably less than 70%, preferably less than 50%, preferably less than 40%, preferably less than 25%, preferably less than 15%, preferably less than 10%, preferably less than 5%.

In particular the variation in the maximum edge length (MEL) is such that greater than 60% of the as grown stones show a maximum variation expresses as a percentage of the average of less than 85%, preferably less than 70%, preferably less than 50%, preferably less than 40%, preferably less than 25%, preferably less than 15%, preferably less than 10%, preferably less than 5%.

In particular the variation in the maximum edge length (MEL) is such that greater than 75% of the as grown stones show a maximum variation expresses as a percentage of the average of less than 85%, preferably less than 70%, preferably less than 50%, preferably less than 40%, preferably less than 25%, preferably less than 15%, preferably less than 10%, preferably less than 5%.

In particular the variation in the maximum edge length (MEL) is such that greater than 85% of the as grown stones show a maximum variation expresses as a

percentage of the average of less than 85%, preferably less than 70%, preferably less than 50%, preferably less than 40%, preferably less than 25%, preferably less than 15%, preferably less than 10%, preferably less than 5%.

In terms of numbers of stones this means that in terms of target MEL and CMI (defined above) that greater than 5 stones, preferably greater than 10 stones, preferably greater than 20 stones, preferably greater than 30 stones, preferably greater than 40 stones, preferably greater than 50 stones, preferably greater than 60 stones, preferably greater than 70 stones, preferably greater than 80 stones, preferably greater than 100 stones, preferably greater than 150 stones, preferably greater than 175 stones, preferably greater than 200 stones, preferably greater than 300 stones, preferably greater than 400 stones, preferably greater than 500 stones, within the synthesis run meet the chosen specifications.

Optically Activated Switches

Nitrogen containing diamond material may be used in optically activated switches in which photons of energy less than the diamond band gap excite carriers from nitrogen-related defects to the valence or conduction bands. For this application it is beneficial for the diamond to be processed into plates contain a high and relatively uniform concentration of nitrogen. Since nitrogen concentration varies significantly between growth sectors, it is beneficial for such plates to be produced in such a way that they do not cross the boundaries between growth sectors. In crystals with near- cubic morphology, the central <100> sector, formed above the seed crystal, has an area (measured perpendicular to the local growth direction) that increases as growth proceeds. When measured perpendicular to the local growth direction, the nitrogen concentration in the <100> sectors is found to be uniform. This means that, by sawing perpendicular to the local growth direction, it is possible to process diamond plates of a range of different sizes from the central <100> sector that have a nitrogen content that is both high and uniform.

The areas of such plates may range from approximately 1 mm x 1 mm to more than 5 mm x 5 mm. The nitrogen concentration may be greater than 100 ppm, preferably greater than 150 ppm, preferably greater than 200 ppm, preferably greater than 250

ppm, preferably greater than 300 ppm, preferably greater than 350 ppm, preferably greater than 400 ppm, preferably greater than 450 ppm, preferably greater than 500 ppm, preferably greater than 550 ppm, preferably greater than 600 ppm, preferably greater than 650 ppm, preferably greater than 700 ppm, preferably greater than 750 ppm, preferably greater than 800 ppm, preferably greater than 900 ppm, preferably greater than 1000 ppm.

Blades for Surgery

Diamond material is used as a material for sharp blades for applications such as eye surgery. For such sharp blades it is particularly important for the precise position of the cutting edge to be clearly seen by the user. The high and uniform nitrogen content of the material of the current invention makes it uniformly dark even in relatively thin slices suitable for cutting blades. It is generally beneficial for such blades to have a longest dimension of at least 3 mm, preferably 4 mm, preferably 5 mm, preferably 6 mm, preferably 6.5 mm. Thin slices cut from the large diamond samples grown using the method of the current invention therefore have the required dimensions and optical properties for this application. It is further found that the near-cubic morphology (i.e. CMI of greater than 6) of the stones enables blades to be produced with a higher yield than from stones with lower CMI values.

Designer Anvils

Diamond is often used as a material for anvils for research involving generation of high pressures and temperatures. In such work it is desirable for the material under investigation to be thermally insulated so that high temperatures may be more easily generated and maintained. It is well known that the high thermal conductivity of diamond is reduced as the nitrogen content increases. It is therefore possible to modify and control the thermal conductivity of diamond by doping it with nitrogen, thereby producing designer diamond anvils suited to particular experiments. Diamond anvils are generally produced with a shape approximately the same as that of a round brilliant gemstone and when they are produced from HPHT synthesized material it may be beneficial for them to be produced from a single growth sector so that they have uniform thermal conductivity characteristics. The method of the current invention can be used to produce large diamond crystals with a high and

controlled nitrogen concentration (for a given growth sector) and in such a way that a large round brilliant anvil can be cut from the central <100> sector that forms above the seed crystal. Such a round brilliant anvil has a controlled and uniform thermal conductivity that is beneficial in experiments involving generation of high pressures and temperatures. The uniform nitrogen is also critical to minimize and strain variation due to variations for example in lattice parameter which might limit the ultimate pressures the anvils can be used at in experimentation. Given the shape of a round brilliant and how it fits into an octahedron, it is clear that being able to synthesise stones of high CMI (i.e. octahedra) will be advantageous to the fabrication of such anvils. (See Figure 9.)

In preference this requires an N concentration greater than 100 ppm, preferably greater than 200 ppm, preferably greater than 300 ppm, preferably greater than 400 ppm, preferably greater than 500 ppm, preferably greater than 1000 ppm.

The material will have a variation in N (δN as defined above) of less than 100% over a volume greater than 1 mm 3 , preferably greater than 1.5 mm 3 , preferably greater than 3 mm 3 , preferably greater than 4 mm 3 , preferably greater than 5 mm 3 , preferably greater than 6 mm 3 , preferably greater than 7 mm 3 , preferably greater than 8 mm 3 , preferably greater than 10 mm 3 , preferably greater than 12 mm 3 , preferably greater than 15 mm 3 , preferably greater than 18 mm 3 , preferably greater than 20 mm 3 .

The material will have a variation in N (defined above) of less than 75% over a volume greater than 1 mm 3 , preferably greater than 1.5 mm 3 , preferably greater than 3 mm 3 , preferably greater than 4 mm 3 , preferably greater than 5 mm 3 , preferably greater than 6 mm 3 , preferably greater than 7 mm 3 , preferably greater than 8 mm 3 , preferably greater than 10 mm 3 , preferably greater than 12 mm 3 , preferably greater than 15 mm 3 , preferably greater than 18 mm 3 , preferably greater than 20 mm 3 .

The material will have a variation in N (defined above) of less than 50% over a volume greater than 1 mm 3 , preferably greater than 1.5 mm 3 , preferably greater than 3 mm 3 , preferably greater than 4 mm 3 , preferably greater than 5 mm 3 , preferably greater than 6 mm 3 , preferably greater than 7 mm 3 , preferably greater than 8 mm 3 ,

preferably greater than 10 mm 3 , preferably greater than 12 mm 3 , preferably greater than 15 mm 3 , preferably greater than 18 mm 3 , preferably greater than 20 mm 3 .

The material will have a variation in N (defined above) of less than 40% over a volume greater than 1 mm 3 , preferably greater than 1.5 mm 3 , preferably greater than 3 mm 3 , preferably greater than 4 mm 3 , preferably greater than 5 mm 3 , preferably greater than 6 mm 3 , preferably greater than 7 mm 3 , preferably greater than 8 mm 3 , preferably greater than 10 mm 3 , preferably greater than 12 mm 3 , preferably greater than 15 mm 3 , preferably greater than 18 mm 3 , preferably greater than 20 mm 3 .

The material will have a variation in N (defined above) of less than 30% over a volume greater than 1 mm 3 , preferably greater than 1.5 mm 3 , preferably greater than 3 mm 3 , preferably greater than 4 mm 3 , preferably greater than 5 mm 3 , preferably greater than 6 mm 3 , preferably greater than 7 mm 3 , preferably greater than 8 mm 3 , preferably greater than 10 mm 3 , preferably greater than 12 mm 3 , preferably greater than 15 mm 3 , preferably greater than 18 mm 3 , preferably greater than 20 mm 3 .

The material will have a variation in N (defined above) of less than 20% over a volume greater than 1 mm 3 , preferably greater than 1.5 mm 3 , preferably greater than 3 mm 3 , preferably greater than 4 mm 3 , preferably greater than 5 mm 3 , preferably greater than 6 mm 3 , preferably greater than 7 mm 3 , preferably greater than 8 mm 3 , preferably greater than 10 mm 3 , preferably greater than 12 mm 3 , preferably greater than 15 mm 3 , preferably greater than 18 mm 3 , preferably greater than 20 mm 3 .

Gemstones

The material of the current invention may also find applications as a gemstone in jewellery. If the material is used in its as-grown form, such gemstones will have a dark colour which may be considered desirable according to personal preferences and fashions. Alternatively the colour of the material may be altered by one or more post-growth treatments. Gemstones produced from material produced by the method of the current invention may either comprise more than one growth sector but when uniformity of colour is desired it is considered that it is beneficial for their majority volume to comprise a single growth sector. This is particularly the case when the colour has been lightened by post-growth treatment. Using the method of

the current invention, a large and uniformly coloured round brilliant can be produced from the central <100> sector that is formed above the seed crystal.

Further the low metal content of the stones of this invention is beneficial to the appearance of the stone.

The cutting of gemstones from high CMI stones (i.e. octahedra) may be geometrically more efficient that cutting them from low CMI stones (i.e. cubes).

Substrates for the Synthesis of Single Crystal Diamond by Chemical Vapour Deposition (CVD)

Material with large {100} and <100> edges is a preferred material for use as substrates for the synthesis of single crystal diamond by CVD. In general the requirements for substrates can be summarised by:

1. Large as possible (for specific products and to reduce costs etc.). The size should be greater than 3 mm x 3 mm, preferably greater than 3.5 mm x 3.5 mm, preferably greater than 4 mm x 4 mm, preferably greater than 4.5 mm x 4.5 mm, preferably greater than 5 mm x 5 mm, preferably greater than 5.5 mm x 5.5 mm, preferably greater than 6 mm x 6 mm, preferably greater than 8 mm x 8 mm, preferably greater than 10 mm x 10 mm, preferably greater than 15 mm x 15 mm.

2. Cubic morphology with a CMI of less than 5, preferably CMI less than 4, preferably CMI less than 3, preferably CMI less than 2.

3. Low cost (i.e. the more substrates that can be produced/HPHT synthesis run the better), i.e. the morphology of the HPHT stone should be dominated only by {100} and {111} growth faces.

4. Uniform in strain, while minimizing strain i.e. large amounts of metallic inclusions are not desired as reported in some of the prior art (I. Kiflawi, H. Kanda, S. C. Lawson. Diamond Relat. Mater., 11 (2002), p204). While high levels of nitrogen and low metallic inclusions can lead to some strain, the overall benefit of having a uniform nitrogen concentration associated with the material of this invention outweighs the disadvantages.

5. Single growth sector.

Specifically it has been found that material of this invention offers:

The change in nitrogen (defined above), δN over an area of 2 mm x 2 mm of less than 90%, preferably δN over an area of 2 mm x 2 mm of less than 80%, preferably δN over an area of 2 mm x 2 mm of less than 70%, preferably δN over an area of 2 mm x 2 mm of less than 60%, preferably δN over an area of 2 mm x 2 mm of less than 50%; preferably δN over an area of 2 mm x 2 mm of less than 40%.

δN over an area of 3 mm x 3 mm of less than 90%, preferably δN over an area o3 mm x 3 mm of less than 80%, preferably δN over an area of 3 mm x 3 mm of less than 70%, preferably δN over an area of 3 mm x 3 mm of less than 60%, preferably δN over an area of 3 mm x 3 mm of less than 50%; preferably δN over an area of 3 mm x 3 mm of less than 40%.

δN over an area of 3.5 mm x 3.5 mm of less than 90%, preferably δN over an area of 3.5 mm x 3.5 mm of less than 80%, preferably δN over an area of 3.5 mm x 3.5 mm of less than 70%, preferably δN over an area of 3.5 mm x 3.5 mm of less than 60%, preferably δN over an area of 3.5 mm x 3.5 mm of less than 50%; preferably δN over an area of 3.5 mm x 3.5 mm of less than 40%.

δN over an area of 4 mm x 4 mm of less than 90%, preferably δN over an area of 4 mm x 4 mm of less than 80%, preferably δN over an area of 4 mm x 4 mm of less than 70%, preferably δN over an area of 4 mm x 4 mm of less than 60%, preferably δN over an area of 4 mm x 4 mm of less than 50%; preferably δN over an area of 4 mm x 4 mm of less than 40%.

δN over an area of 4.5 mm x 4.5 mm of less than 90%, preferably δN over an area of 4.5 mm x 4.5 mm of less than 80%, preferably δN over an area of 4.5 mm x 4.5 mm of less than 70%, preferably δN over an area of 4.5 mm x 4.5 mm of less than

60%, preferably δN over an area of 4.5 mm x 4.5 mm of less than 50%; preferably δN over an area of 4.5 mm x 4.5 mm of less than 40%.

δN over an area of 5 mm x 5 mm of less than 90%, preferably δN over an area of 5 mm x 5 mm of less than 80%, preferably δN over an area of 5 mm x 5 mm of less than 70%, preferably δN over an area of 5 mm x 5 mm of less than 60%, preferably δN over an area of 5 mm x 5 mm of less than 50%; preferably δN over an area of 5 mm x 5 mm of less than 40%.

δN over an area of 6 mm x 6 mm of less than 90%, preferably δN over an area of 6 mm x 6 mm of less than 80%, preferably δN over an area of 6 mm x 6 mm of less than 70%, preferably δN over an area of 6 mm x 6 mm of less than 60%, preferably δN over an area of 6 mm x 6 mm of less than 50%; preferably δN over an area of 6 mm x 6 mm of less than 40%.

Other Application Areas

The inventors believe that the properties of the material of the invention, in particular the low and uniform strain and the ability to cut large single sector plates without missing corners from the crystals could make this material suitable for use as an x- ray optical material in synchrotrons and other demanding areas.

In infrared and other optical applications, it is often advantageous to have material with low and uniform strain (and hence birefringence). Where uniformity of optical properties is important it is advantageous for a high proportion of the diamond material in the finished product to have grown on surfaces with the same crystallographic orientation. This requirement therefore gives a further reason for wanting to be able to synthesise crystals which have morphologies that are as close to cubic or octahedral as possible.

The following Figures have and/or will be referred to in this description.

Figure 1 : Definition of the crystal morphology index for crystals grown uniformly in three dimensions.

Figure 2: When growing from a seed up into a half-space, only the top half of the circumscribing cube is apparent, but the same construction can be applied to the view in plan from above. Here is a cmi 3 crystal but seen here at a low angle to illustrate the effect.

Figure 3: Plan view from above (seed at bottom). Entries in the table match the entries in Table 1.

Figure 4: At intermediate levels of nitrogen, 50 to 100 ppm, there is rounding and curvature of crystal faces, leading to a loss of useful mass.

Figure 5: Diamonds grown using the teachings of this invention showing higher nitrogen content (darker colour) and better shape

Figure 6: Diamonds grown using the prior art and showing a lower nitrogen level, (lighter colour) and less useful shape, with more rounding and minor faces.

Figure 7: The beneficial action of nitrogen is illustrated. As the nitrogen concentration in the cubic sector (central goblet shaped region) is decreasing, the relative sizes of the cubic and octahedral sectors changes in the direction of a more octahedral, high CMI crystal.

Figure 8: Showing material of the invention grown at high temperature.

Figure 9: Showing how a RBC gem (anvil) can be cut from a single growth sector from a low CMI stone.

The following are non-limiting examples of the implementation of this invention

EXAMPLE 1

A sample of two kilograms of cobalt powder of purity 99% and containing 25 ppm of nitrogen as received from the supplier was treated in a commercial nitriding furnace at 750 0 C in a stream of dry ammonia at one to two inches of water gauge positive

pressure for 10 hours. After this time the nitrogen level was measured again by the same method and found to have increased to 592 ppm.

A component of 11.2% by weight percentage of this powder was added to the cobalt-iron solvent metal catalysts used in this diamond synthesis process. The synthesis process was carried out at typical conditions of temperature, 125O 0 C to 1300 0 C and 5 to 6 GPa. A representative sub-sample of the diamonds grown in this process is shown in Figure 5. These were characterized by FTIR. The average nitrogen concentration within the central {100} sector was 630 ppm, throughout the whole stone it was 490 ppm. The average CMI of the stones produced in this synthesis run was 2.8.

When the pre-treated cobalt powder was omitted and the same process of diamond growth was carried out, the diamond crystals were as shown in Figure 6. This material based on FTIR measurements contained an average nitrogen concentration of 100 ppm. The average CMI of the stones produced in this synthesis run was 4.8.

EXAMPLE 2

Example 1 was repeated with the exception the synthesis temperature was increased by 100 0 C, this produced the material shown in Figure 8. The average nitrogen concentration of the most octahedral stones was 652 ppm. The average CMI was 6.5.