Crystals were grown from the melt and characterized. It was found that there were many heavy twins and characteristic voids. The twins curved in the crystals, and the voids were inclusions of N gas that originated from the atmosphere in the growth chamber. The voids were elongated in the growth direction, and their length could attain 0.5mm.

K.Terashima, M.Takena: Journal of Crystal Growth, 1991, 110[3], 623-5