Single crystals were grown by sublimation in closed ampoules of fused silica at temperatures of between 1300 and 1400K. It was observed that the crystals preferentially developed {110} faces. The free corner of three {110} faces appeared to be the preferred formation site of a twin nucleus. Repeated twin formation led to predominant growth in the <111> direction. In the presence of SiS2, {100} and {111} faces appeared near to equilibrium, and twins formed on {111} faces; thus resulting in preferred growth in <112> directions. The observations were explained in terms of an idealized atomic arrangement of growth steps.

E.Schönherr, M.Freiberg: Journal of Crystal Growth, 1999, 197[3], 455-61