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

A series of fluoride perovskites related to neighborite was investigated us ing X-ray and neutron diffraction techniques, and Rietveld profile refineme nt of powder diffraction data. The series (Na1-xKx)MgF3 comprises orthorhom bic (Pbnm, a approximate to root 2a(p), b approximate to root 2a(p), c appr oximate to 2a(p), Z = 4) perovskites in the compositional range 0 less than or equal to x less than or equal to 0.30, tetragonal perovkites (P4/mbm, a root 2a(p),, c approximate to a(p), Z = 2) in the range 0.40 less than or equal to x less than or equal to 0.46, and cubic phases (Pm (3) over barm, Z =1) for x > 0.50. The structure of the orthorhombic neighborite is derive d from the perovskite aristotype by rotation of MgF6 octahedra about the [1 10] and [001] axes of the cubic subcell. The degree of rotation, measured a s a composite tilt Phi about the triad axis, varies from 18.2 degrees at s = 0 to 11.2 degrees at s = 0.30 (as determined from the fractional atomic c oordinates). Orthorhombic neighborite also shows a significant displacement of Na and K from the "ideal" position (less than or equal to0.25 Angstrom) . The tetragonal members of the neighborite series exhibit only in-phase ti lting about the [001] axis of the cubic subcell (Phi) ranging from 4.5 degr ees to 4.8 degrees (determined from the atomic coordinates). The solid solu tion (Na1-xK,)MgF3, shows a regular variation of unit-cell dimensions with composition from 3.8347 Angstrom for the end-member NaMgF3 (reduced to pseu docubic subcell, a(p)) to 3.9897 Angstrom for KMgF3. This variation is acco mpanied by increasing volumes of the A-site polyhedra, whereas the volume o f MgF6 octahedra initially decreases (up to = 0.40), and then increases con comitantly with K content. The polyhedral volume ratio, V-A/V-B, gradually increases towards the tetragonal structural range, in agreement with dimini shing octahedral rotation in the structure. The P4/mbm-type neighborite has an "anomalous" polyhedral volume ratio (ca. 5.04) owing to the critical co mpression of MgF6 polyhedra.