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.