A structural phase-transition in K (Mg_1-xCu_x) F₃ perovskite

A complete solid-solution series between cubic (Pm 3 m) KMgF₃ and tetragonal (I 4/mcm) KCuF₃ was synthesized at 730-735°C in an inert atmosphere. X-ray powder-diffraction at room temperature shows that the transition between the cubic and tetragonal perovskite structures in the series K (Mg_1-xCu_x) F₃ occurs at x ∼ 0.6. Rietveld structure-refinements were done for selected compositions. In the cubic phase, all parameters are linear with composition up to the transition point. At the transition point, there is a strong discontinuity in the cell volume; this is strongly anisotropic with expansion along the a axes and contraction along the c axis due to a pronounced axial elongation of the (Mg, Cu) F₆ octahedron that increases with increasing Cu content. The phase transition is first-order, with a discontinuity of ≈ 2% in the symmetry-breaking strain at x_c. It is proposed that the phase transition in K (Mg, Cu) F₃ is due to the onset of the cooperative Jahn-Teller effect. Compositional relationships for lattice vibrations in this solid solution were established using thin-film infrared spectroscopy. A phase transition occurring above 60 mole % KCuF₃ is indicated by the appearance of one of the two modes expected for the tetragonal phase; the weaker mode is not resolved below 80 mole % KCuF₃. Modes common to both structures vary smoothly and continuously across the binary; however, frequencies do not depend linearly on composition, nor is mode-softening discernable. Two-mode behaviour is observed only for the bending motion of the cubic phase, because this peak alone has non-overlapping end-member components.