Heat capacity, thermal expansion and sensitivity to hydrostatic pressure of (NH4)3SiF7 at successive structural phase transitions

Abstract

Heat capacity, thermal dilatation, permittivity and sensitivity to external pressure of (NH4)3SiF7 were studied. Due to the absence of cubic phases Pm3m and Pa3, a strong decrease in the total entropy change dSi = 19 J/molK associated with four successive transformations P4=mbm - Pbam - P21=c11 - P1 - P121/c1 was found in silicate in comparison with other double uoride salts (NH4)3MeF7 (Me: Ge, Ti, Sn). Using analysis of the excess heat capacity in the framework of the thermodynamic theory, the entropies associated with each individual phase transition were determined. In accordance with the entropic parameters, the complete ordering of the structural elements occurs in the monoclinic phase P21=c11. Further change in symmetry is associated with small entropy changes which prove insignicant displacement of structural units. A T-p phase diagram was constructed and good agreement was found between measured and calculated baric coefficients.