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.