Crystals which had been grown from the melt by using the Bridgman method were typified by the presence of defects such as grain boundaries and packing defects of twinning type. Microstructural and X-ray analyses showed that optical non-uniformity was due mainly to coherent twinned interlayers that were related to a wurtzite-sphalerite phase transition during cooling; which was incomplete on (00•1)W||(111)S planes. The Te favorably affected the optical uniformity and degree of structural perfection. This was due mainly to the sphalerite structure being completely stabilized during growth, and to the absence of twinning-type packing defects.

L.V.Atroshchenko, L.P.Galchinetskii, S.N.Galkin, V.D.Ryzhikov, V.I.Silin: Journal of Crystal Growth, 1999, 197[3], 475-9