The crystal chemistry of synthetic potassium-bearing neighborite, [Na_(1-x)K_x)MgF_3

摘要

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 (Na_(1-x)K_x)MgF_3 comprises orthorhombic (Pbnm, a ≈ 2~(1/2)a_p, b ≈ 2~(1/2)a_p, c ≈ 2a_p, Z = 4) perovskites in the compositional range 0 ≤ x ≤ 0.30, tetragonal perovkites (P4/mbm, a ≈ 2~(1/2)a_p, c ≈ a_p, Z = 2) in the range 0.40 ≤ x ≤ 0.46, and cubic phases (Pm(3-bar)m, Z = 1) for x > 0.50. The structure of the orthorhombic neighborite is derived from the perovskite aristotype by rotation of MgF_6 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 A). 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 (Na_(1-x)K_x)MgF_3, shows a regular variation of unit-cell dimensions with composition from 3.8347 A for the end-member NaMgF_3 (reduced to pseudocubic subcell, a_p) to 3.9897 A for KMgF_3. This variation is accompanied by increasing volumes of the A-site polyhedra, whereas the volume of MgF_6 octahedra initially decreases (up to x = 0.40), and then increases concomitantly with K content. The polyhedral volume ratio, V_A/V_B, 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 MgF_6 polyhedra.