(NH4)(3)HfF7: Crystallooptical and calorimetric studies of a number of successive phase transitions

Single crystals of (NH₄)₃HfF₇ were grown. Polarising optical observations as well as measurement of the heat capacity and birefringence Δn(Т) were performed in the temperature range of 200–310 K. Reversible phase transitions at temperatures T₀ = 290 K, Т₁ = 280.5 K, Т₂ ≈ 273 K, Т₃ = 266 K, Т₄ = 259 K, Т₅ = 231 K, Т₆ = 229 K were found. Observations in polarised light make it possible to suggest a sequence of changes in the symmetry groups for these transitions: ⟵Т₀→ cub. ⟵Т₁→ mmm (1) ⟵Т₂→ mmm (2) ⟵Т₃→ mmm (3) ⟵Т₅→ 2/c. The T-p phase diagram was studied and the temperature boundaries of the stability of the distorted crystalline phases were determined. A significant change of the entropy at successive phase transitions ΣΔS = 10.6 J/mol K indicates a disordering of the initial cubic phase.