INVESTIGATION OF THE HIGH-TEMPERATURE POLYMORPHIC TRANSFORMATION IN CHRYSOBERYL

The melting behavior of chrysoberyl, BeAl2O4, was studied by different ial thermal analysis. At 1853 +/- 10 degrees C, chrysoberyl undergoes a first-order structural transition. The melting point of the high-tem perature form, beta-BeAl2O4, is 1867 +/- 5 degrees C. Under certain co nditions, the alpha-phase may be formed directly from the melt at 1847 +/- 4 degrees C. Crystals of beta-BeAl2O4 have the shape of long hexa gonal prisms and a density 20-25% lower than that of alpha-chrysoberyl . The beta-phase is stable in a fairly narrow temperature range from 1 826 to 1867 degrees C and accommodates only small amounts of chromium, titanium, and vanadium-typical isomorphous substituents for aluminum in alpha-chrysoberyl. The melt density, evaluated from the crystal-to- liquid volume ratio, is 2.753 +/- 0.007 g/cm(3) at 1867 degrees C.