Optical and calorimetric studies of K2TaF7

The phase transition Pnma ↔ P21/c in K2TaF7 is a strong first-order transformation of the “proper” ferroelastic origin. A huge angle of rotation of the optical indicatrix ϕ ≈ 40° and strong pre-transition phenomena in the birefringence were observed. Shear deformation and transition parameter contribute to the photoelastic effect. Heat treatments lead to the correlated changes in temperature and enthalpy. Large transition entropy Δ S= R·ln14 is not consistent with an ordered structure model in phase Pnma. [Display omitted] •No phase transitions were found in K2TaF7 below room temperature.•The transformation Pnma ↔ P21/c is a “proper” ferroelastic of the strong first order.•A huge angle of rotation of the optical indicatrix ϕ ≈ 40° is observed.•Thermal cycling leads to correlated change in temperature and enthalpy of transition.•Large transition entropy ΔS = R·ln14 does not agree with the model of ordered structure. Optical and calorimetric experiments on K2TaF7 are performed in a wide temperature range. No features were found in the behavior of the birefringence Δnb(T), the angle of rotation of the indicatrix ϕ(T) and the heat capacity ΔCp(T) except for those associated with the Pnma↔P21/c phase transition. Structural transformation was characterized as strong first order “proper” ferroelastic accompanied by a huge angle ϕ ≈40° and strong pre-transition phenomena in Δnb(T). Two contributions to the anomalies of the optical properties were found associated with the photoelastic effect and the transition parameter related linearly to the spontaneous deformation. Thermal treatments cause correlated changes in temperature and enthalpy of the phase transition, which leads to the invariance of the large magnitude of the corresponding entropy ΔS = 22 J/mol·K which does not match the model with the absence of structural disorder in the Pnma phase.