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

¹ Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York 11794, U.S.A.
² Department of Chemistry and Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
³ Department of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794, U.S.A.

Correspondence: ^* E-mail: chmartin{at}ic.sunysb.edu

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 (Pmm) 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_0.47Na_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⁺ and K⁺ cations. We suggest A-site cations group with like cations as neighbors to reduce excess volume and total strain.

This article has been cited by other articles:

				C. D. Martin, W. A. Crichton, H. Liu, V. Prakapenka, J. Chen, and J. B. Parise Rietveld structure refinement of perovskite and post-perovskite phases of NaMgF3 (Neighborite) at high pressures American Mineralogist, October 1, 2006; 91(10): 1703 - 1706. [Abstract] [Full Text] [PDF]