Effect of Deuteration on the Thermodynamic Properties of Dioxotetrafluoromolybdate(VI), (NH4)2MoO2F4

Thermal and dielectric studies of (ND4)2MoO2F4 crystals undergoing successive phase transitions at T1 = 272 K and T2 = 181 K showed that deuteration is accompanied by an increase in the chemical pressure in the crystal lattice (Δp ≈ 0.02 GPa), which shifts the Cmcm ↔ Pnma transformation for the first order to the tricritical point. The direct participation of ammonium groups in the mechanism of structural distortions is demonstrated by a decrease in the entropy of the high-temperature phase transition (ΔS1 = R ln 6.0). An external hydrostatic pressure leads to an expansion of the temperature interval of the intermediate antiferroelectric Pnma phase. The triple point on the T–p phase diagram, where the Cmcm, Pnma, amd Pnma* phases coexist, can be realized at a negative pressure of ptrp ≈ −0.8 GPa.