Heat capacity, thermal expansion and sensitivity to hydrostatic pressure of 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 Pm¯ 3m and Pa¯3, a strong decrease in the total entropy changeP∆Si = 19 J/mol·K associated with four successive transformations P4/mbm ↔ Pbam ↔ P21/c11 ↔ P¯ 1 ↔ P121/c1 was found in silicate in comparison with other double fluoride 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 en tropic 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 insignificant displacement of structural units. A T−p phase diagram was constructed and good agreement was found between measured and calculated baric coefficients.