Thermal properties and phase transition in the fluoride, (NH4)(3)SnF7

Abstract

Calorimetric, dilatometric and differential thermal analysis studies were performed on (NH4)(3)SnF7 for a wide range of temperatures and pressures. Large entropy (delta S-0=22 J/mol IQ and elastic deformation (8 (Delta V/V)(0)=0.89\%) jumps have proven that the Pa-39 <-> Pm-3m phase transition is a strong first order structural transformation. A total entropy change of Delta S-0=32.5 J/mol K is characteristic for the order disorder phase transition, and is equal to the sum of entropy changes in the related material, (NH4)(3)TiF7, 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)(3)SnF7, contrary to (NH4)(3)TiF7, characterised by a negative baric coefficient. The effect of experimental conditions on the chemical stability of (NH4)(3)SnF7 was observed.