Phase formation of BaB2O4 from melts in graphite crucible

Conditions for formation of the low temperature beta phase of BaB2O4 from nominally pure melts in graphite and Pt crucibles were investigated along with phase transformation kinetics. In order to obtain the beta phase crystals from supercooled metastable melts in a graphite crucible, it was found necessary to first superheat the melts almost 100 degrees C. The nucleation-growth behavior was different when a Pt crucible was used. This difference can be explained by the presence of graphite particles in the melts in the graphite crucible acting as nucleation sites. It was also found that bulk or powdered beta phase crystals transformed to the high temperature alpha phase when heated for only 5 min above 1000 degrees C (the equilibrium transformation temperature is 925 degrees C). Some differences were found in the behavior of the beta to alpha transformation of bulk and powdered crystals, and were believed related to thermal stress.