Time-resolved, high-temperature X-ray powder diffraction of BaB₂O₄

Abstract

The study of the phase transition of barium borate (BaB₂O₄) in which the low temperature phase (β -phase) is well known as a nonlinear optical crystal have been carried out by a newly developed high temperature X-ray powder diffraction apparatus, where a time-resolved high temperature X-ray powder diffraction apparatus using a PSPC(Position Sensitive Propotional Counter) is used. The β - to α -phase transition is not detected on heating a powdered sample of β -BaB₂O₄ crystal to the molten state. On the other hand, the α -phase is partially transformed into the β -phase on heating above 900°C, and then the originated β -phase is retransformed into the α -phase above 1000°C. The fusion occurs at about 1 100°C. It is attributable to the lower activation energy of the β -to α -phase transition in the coexistence of the α -phase compared with in the isolated existence of β -phase. The thermal expansion of the α -phase is anisotropic along the a- with c-axis whereas that of the β -phase is isotropic.