Thermal propertiesandphasetransitioninthe fluoride, (NH4)3SnF7

Abstract

Calorimetric, dilatometric and differential thermal analysis studies were performed on (NH4)3SnF7 for a wide range of temperatures and pressures. Large entropy (δS0 = 22 J/mol•K) and elastic deformation (δ(ΔV/V) 0 = 0.89%) jumps have proven that the Pa-3 ↔ Pm-3m phase transition is a strong first order structural transformation. A total entropy change of ΔS0 = 32.5 J/mol•K agrees well with the model associated with the complete ordering of structural units in the Pa-3 phase, which are disordered in the Pm-3m phase, and is equal to the sum of entropy changes in the related material, (NH4)3TiF7, undergoing transformation between the two cubic phases through the intermediate phases. Hydrostatic pressure decreases the stability of the high temperature Pm-3m phase in (NH4)3SnF7, contrary to (NH4)3TiF7, characterised by a negative baric coefficient. The effect of experimental conditions on the chemical stability of (NH4)3SnF7 was observed.