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

Abstract

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 Pmm. 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.