Optical and calorimetric studies of K2TaF7

Optical and calorimetric experiments on K₂TaF₇ are performed in a wide temperature range. No features were found in the behavior of the birefringence Δn_b(T), the angle of rotation of the indicatrix ϕ(T) and the heat capacity ΔC_p(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 Δn_b(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.