Kirk Ross MSc thesis abstract

This study presents an X-ray diffraction analysis and Rietveld structural refinement of selected synthetic fluoroperovskite-type compounds including the Na_1-xK_xMgF₃ solid solutions series in addition to synthetic analogues of cryolite (Na₂NaAlF₆) and simmonsite (Na₂LiAlF₆).

The Na_1-xK_xMgF₃ solid solution series is comprised of three structurally distinct perovskite phases. In order of increasing potassium they are: orthorhombic (Pbnm, a=5.3609(1), b=5.4862 (1), c=7.6661(1), Z=4) in the x=0-0.35 compositional range, tetragonal (P4/mbm, a=5.444(3), c=3.9217(3), Z=2) in the x=0.40~0.46 compositional range and cubic (Pm3m, a=3.9903, Z=1) in the x=0.50-1.0 compositional range. The orthorhombic and tetragonal members are derived from the cubic aristotype by octahedral rotation, a^-a^-c⁺ and a⁰a⁰c⁺ respectively. Introduction of potassium into the Na 4c crystallographic site results in a decrease of octahedral rotation and an overall reduction of structural distortion from f = 4.42 for the neighborite end member to  f = 5.0 for cubic members. This is accompanied by an increase in the pseudocubic cell dimension in addition to a reduction in A-site cationic displacement.  Angular and bond length distortion of the MgF₆ octahedron are at a maximum in the x=0.20 intermediate member of the series and decrease linearly with additional potassium.  These effects are a direct result of cations approaching special positions in the unit cell as potassium is introduced into the neighborite structure.