Abstract
A complete solid-solution series between cubic (Pm 3 m) KMgF3 and tetragonal (I4/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 (Mg1−xCux) 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 xC. It is proposed that the phase transition in K (Mg, Cu) F3 is due to the onset of the cooperative Jahn-Teller effect.
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