Journal
AMERICAN MINERALOGIST
Volume 90, Issue 10, Pages 1522-1533Publisher
MINERALOGICAL SOC AMER
DOI: 10.2138/am.2005.1693
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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 (Pm (3) over barm) 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 (K0.37Na0.63)MgF3 and (K0.47Na0.53)MgF3, respectively. F-19 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+ and K+ cations. We suggest A-site cations group with like cations as neighbors to reduce excess volume and total strain.
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