Effect of A-site cation radius on ordering of BX₆ octahedra in (K,Na)MgF₃ perovskite

We present a structural model for (K,Na)MgF₃ perovskite using results from high-resolution synchrotron X-ray powder diffraction and nuclear magnetic resonance (NMR) spectroscopy. (K,Na)MgF3 perovskite is found to transition from orthorhombic (Pbnm) to tetragonal (P4/mbm) to cubic (Pm3̄m) as potassium concentration is increased. These phase transitions are not accompanied by a discontinuity in pseudo-cubic unit-cell volume and occur close to compositions (K_0.37Na_0.63)MgF₃ and (K_0.47 Na_0.53)MgF₃, respectively. ¹⁹F NMR spectra indicate that the Na⁺ and K⁺ cations do not occupy the A cation site at random and end-member local environments are favored for all compositions. Based on results from both X-ray diffraction and NMR, we propose that diffuse diffraction is the result of strain between coexisting regions of different octahedra (MgF₆) tilts brought about by the ionic radius mismatch of Na⁺ di and K⁺ cations. We suggest A-site cations group with like cations as neighbors to reduce excess volume and total strain.