(NH4)3HfF7: Crystallooptical and calorimetric studies of a number of successive phase transitions

Abstract

Single crystals of (NH4)3HfF7 were grown. Polarising optical observations as well as measurement of the heat capacity and birefringence Δn(Т) were performed in the temperature range of 200–310 K. Reversible phase transitions at temperatures T0 =290 K, Т1 =280.5 K, Т2 ≈273 K, Т3 =266 K, Т4 =259 K, Т5 =231 K, Т6 =229 K were found. Observations in polarised light make it possible to suggest a sequence of changes in the symmetry groups for these transitions. The T-p phase diagram was studied and the temperature boundaries of the stability of the distorted crystalline phases were determined. A significant change of the entropy at successive phase transitions ΣΔS =10.6 J/mol K indicates a disordering of the initial cubic phase.