Anomalous behaviour of thermodynamic properties at successive phase transitions in (NH4)3GeF7

Heat capacity and thermal expansion of (NH4)3GeF7 show anomalous behaviour associated with the succession of nonferroelectric phase transitions P4/mbm↔Pbam↔P21/c↔Pa-3. Hydrostatic pressure leads, first, to disappearance of the monoclinic P21/c phase at the triple point (ptrp = 0.3 GPa) and, second, to strong broadening and narrowing of the orthorhombic Pbam and cubic Pa-3 phases, respectively. The entropy changes at T1 and T2 agree well with a model of structural distortions. Reconstructive transformation into Pa-3 phase is accompanied by large values of the entropy, ΔS3 = 13.5 J/mol K, and volume strain, δ(ΔV/V0) ≈ − 1%.184