PURPOSE: To grow a flawless and large single crystal of LiNdP4O12 by having a seed crystal into a metal of liNdP4O12 and a flux composed of specified ratios of Li2O and P2O5 and cooling this melt under stereoisotermal conditions.

CONSTITUTION: A mixture (the weight ratio of the LindP4O2 and the flux is the satd. value of the LiNdP4O12 in the flux at a seeding temp.) composed of the LiNdP4O12 and the flux (the weight ratio of the Li2O and the P2O5 is 0.7: to 2:1) composed of the Li2O and the P2O5 is put into a crucible and is melted at a tmep. from 750 to 1100°C, more preferably 1050°C. This crucible is hung in a growth furnace of nearly the isothermal conditions (the max. temp. difference between the arbitrary two points of the melt is about ≤4°C, more preferably ≤2°C) over the entire part of the melt. The heating then maintained for about 4 hours at a temp. form 950 to 1000°C while the melt is kept stirred and, thereafter, the temp. of the growth furnace is lowered by 30 to 50°C and the seed crystal of the LiNdP4O12 is hung at the terminal of a central revolving shaft. The temp. of the furnace is lowered at a rate of ≤5°C/hour, more preferably 0.1 to 20°C/day to grow the single crystal. this process is applicable to the production of the single crystal of XB2O4 (X is Ba, Sr, Ca) as well.