Process for producing boron nitride of cubic system

A process for producing boron nitride of the cubic system, which comprises the steps of:

• maintaining a mixture of (i) a finely divided lithium nitride (Li.sub.3 N) or metallic lithium powder, (ii) a finely divided powder of at least one alkaline earth metal nitride (Ca.sub.3 N.sub.2), barium nitride (Ba.sub.3 N.sub.2), metallic calcium and metallic barium, and (iii) hexagonal boron nitride (BN), at a temperature of 800.degree. to 1,300.degree. C. in an atomsphere of an inert gas or ammonia thereby to react the ingredients (i), (ii) and (iii) with each other in a molten state to produce a reaction product comprising a compound represented by the formula:
• LiMBN.sub.2
• wherein M is calcium or barium;
• cooling the reaction product to be thereby solidified;
• mixing the reaction product with a hexagonal boron nitride powder;
• and
• maintaining the thus-obtained mixture in a high-temperature and high-pressure region in which boron nitride of the cubic system is thermodynamically stable.