Kirk Ross MSc thesis abstract

Thesis Title: 
X-Ray Diffraction and Rietveld Structural Refinement of Selected Fluoroperovskites

This study presents an X-ray diffraction analysis and Rietveld structural refinement of selected synthetic fluoroperovskite-type compounds including the Na1-xKxMgF3 solid solutions series in addition to synthetic analogues of cryolite (Na2NaAlF6) and simmonsite (Na2LiAlF6).

The Na1-xKxMgF3 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.

Synthetic cryolite (Na2NaAlF6) and synthetic simmonsite (Na2LiAlF6) investigated exhibited similar structural characteristics such as 1:1 B-site cationic ordering and an a+b-b- octahedral rotation scheme. Synthetic cryolite exhibited the greatest structural and octahedral distortion (f = 4.07, ΔB' = 0.016, δB' = 11.75) and octahedral rotation (ΦB' = 18.6) as compared to simmonsite (f = 4.52, ΔB' = 0.06, δB' = 2.74 and ΦB' = 14.96). These ordered monoclinic perovskites (P2(1/n), a≈b≈√2ap, c≈2ap) are derived from the cubic aristotype by octahedral rotation and B-site cationic ordering.
A copy of the thesis can be downloaded here