THE CRYOLITE STRUCTURE OF NA3SCF6 AND THE TILTING OF OCTAHEDRA IN ISOSTRUCTURAL SODIUM HEXAFLUOROMETALLATES NA3MF6

X-ray studies at single crystals of Na3ScF6 confirmed the monoclinic c ryolite type structure of this compound: a = 559.5, b = 580.2, c = 811 .6 pm, beta = 90.72-degrees, Z = 2, space group P2(1)/n; R1 = 0.021 fo r 512 symmetry independent reflections. The octahedra of [ScF6] (avera ge Sc-F = 200.7 pm), as well as those of [NaF6] (Na1-F = 229.1 pm) lin ked to them, are tilted by about 20-degrees with respect to the axes o f the perovskite-like pseudocell. This tilting of octahedra is discuss ed in comparison with other cryolites and with orthorhombic perovskite s NaMF3; there results a correlation between tilt angle and tolerance factor t < 0.88 of these compounds, the [NaF8] coordination of which i nvariably exhibits a constant mean value of Na2-F = 231.5 +/- 1 pm for the four shortest distances.