D SC and Hot Stage Optical Microscopic Observation on the Phase Transition in NaBF₄

Abstract

The crystallographic phase transition in NaBF₄ was studied using DSC and optical microscopy. The transition was obtained with a hysteresis as large as 30 K. The heat of transition gave a higher ΔH value during cooling cycle. The optical microsopic study brought out two facts. First the high temperature form of NaBF₄ is not isotropic as has already been wrongly reported in literature. Second the crystal, which was colourless to start with at RT, on heating exhibited fast changing “Pol” colours close to the transition temperature. This is explained in terms of anomalous variation in both refractive index and thermal expansion coefficient close to the transition.