Polymorphism, structures and phase transformation of K₃[SO₄]F

The phase K₃[SO₄]F was found to be dimorphic, undergoing a reversible tetragonal (β) to cubic (α) phase transition at 585°C. The room-temperature polymorph, β-K₃[SO₄]F, is isostructural with Ba₃SiO₅, as confirmed by Rietveld refinement of X-ray powder diffraction data: a = 7.2961(5), c = 10.854(1) Å, R = 0.0689 and R_wp = 0.0907. The α form is cubic, a ≈ 5.43 Å, with probable space group Pm3m. Partial Rietveld refinement suggested that it has an anti-perovskite structure. Lattice parameter measurements as a function of temperature showed no discontinuity at the α-β transition, which is therefore suggested to be a displacive second-order transition.