Physics and Chemistry of Minerals

, Volume 28, Issue 4, pp 277–284

The crystal chemistry of synthetic potassium-bearing neighborite, (Na1−xKx)MgF3

  • A. R. Chakhmouradian
  • K. Ross
  • R. H. Mitchell
  • I. Swainson
ORIGINAL PAPER

DOI: 10.1007/s002690100151

Cite this article as:
Chakhmouradian, A., Ross, K., Mitchell, R. et al. Phys Chem Min (2001) 28: 277. doi:10.1007/s002690100151

Abstract

 A series of fluoride perovskites related to neighborite was investigated using X-ray and neutron diffraction techniques, and Rietveld profile refinement of powder diffraction data. The series (Na1−xKx)MgF3 comprises orthorhombic (Pbnm, a ≈ \(\), b ≈ \(\), c ≈ 2ap, Z=4) perovskites in the compositional range 0 ≤ x ≤ 0.30, tetragonal perovkites (P4/mbm, a ≈ \(\), c ≈ ap, Z=2) in the range 0.40 ≤ x ≤ 0.46, and cubic phases (Pmm, Z=1) for x > 0.50. The structure of the orthorhombic neighborite is derived from the perovskite aristotype by rotation of MgF6 octahedra about the [110] and [001] axes of the cubic subcell. The degree of rotation, measured as a composite tilt Φ about the triad axis, varies from 18.2° at x=0 to 11.2° at x=0.30 (as determined from the fractional atomic coordinates). Orthorhombic neighborite also shows a significant displacement of Na and K from the “ideal” position (≤0.25 Å). The tetragonal members of the neighborite series exhibit only in-phase tilting about the [001] axis of the cubic subcell (φ) ranging from 4.5° to 4.8° (determined from the atomic coordinates). The solid solution (Na1−xKx)MgF3, shows a regular variation of unit-cell dimensions with composition from 3.8347 Å for the end-member NaMgF3 (reduced to pseudocubic subcell, ap) to 3.9897 Å for KMgF3. This variation is accompanied by increasing volumes of the A-site polyhedra, whereas the volume of MgF6 octahedra initially decreases (up to x=0.40), and then increases concomitantly with K content. The polyhedral volume ratio, VA/VB, gradually increases towards the tetragonal structural range, in agreement with diminishing octahedral rotation in the structure. The P4/mbm-type neighborite has an “anomalous” polyhedral volume ratio (ca. 5.04) owing to the critical compression of MgF6 polyhedra.

Key words Perovskite Neighborite Crystal structure Phase transition 

Copyright information

© Springer-Verlag Berlin Heidelberg 2001

Authors and Affiliations

  • A. R. Chakhmouradian
    • 1
  • K. Ross
    • 1
  • R. H. Mitchell
    • 1
  • I. Swainson
    • 2
  1. 1.Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada Tel.: 1-807-343-8329; Fax: 1-807-346-7853 e-mail: achakhmo@gale.lakeheadu.caCA
  2. 2.Neutron Program for Materials Research, Steacie Institute for Molecular Sciences, National Research Laboratory, Chalk River, Ontario K0J 1J0, CanadaCA

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