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

Bogdanov, Evgeniy V.; Pogoreltsev, Evgeniy I.; Gorev, Mikhail V.; Kartashev, Andrey V.; Laptash, Natalia M.; Flerov, Igor N. Source: JOURNAL OF SOLID STATE CHEMISTRY, 276 152-158; 10.1016/j.jssc.2019.04.029 AUG 2019

Heat capacity, thermal dilatation, permittivity and sensitivity to external pressure of ${({NH}_{4})}_{3} {SiF}_{7}$ were studied. Due to the absence of cubic phases $P m \bar{3} m$ and $P a \bar{3}$ , a strong decrease in the total entropy change $\sum Δ S_{i} = 19$  J/mol K associated with four successive transformations $P 4 / m b m \leftrightarrow P b a m \leftrightarrow P 2_{1} / c 11 \leftrightarrow P \bar{1} \leftrightarrow P 12_{1} / c 1$ was found in silicate in comparison with other double fluoride salts ${({NH}_{4})}_{3} {MeF}_{7}$ (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 $P 2_{1} / c 11$ . 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.