SINGLE-CRYSTAL HIGH-PRESSURE STUDIES OF NA3SCF6

The crystal structure of Na3ScF6 was investigated by high-pressure sin gle-crystal X-rag diffraction techniques in the range from ambient con ditions to 68.2 kbar, Pressure was measured by the ruby fluorescence m ethod, The single-crystal structure determination at 27.9 kbar resulte d in the cell parameters a = 5.484(1) Angstrom, b = 5.732(1) Angstrom, c = 7.948(2) Angstrom, and beta = 90.96(3)degrees and the same space group symmetry, P2(1)/n (Z = 2), as at ambient conditions, The 19 stru ctural parameters needed to describe a model with isotropic displaceme nt parameters were refined by least-squares techniques, For the 143 mo st significant observed unique reflection amplitudes, the R value beca me 0.047, When the pressure was increased from ambient to 27.9 kbar, t he change in the geometry and the relative change in the volume of the coordination polyhedron around the six-coordinated Sc3+ ion were negl igible, Significant changes of the coordination volumes around the Na ions were observed, The volume decreases for the six-coordinated and the eight-coordinated Na+ ions were similar, viz., 7 and 8%, The layer ed stacking of the larger, low-charged F- and Na+ ions along [103] off ers an explanation for the pattern which determines the changes of the unit cell parameters with pressure, The relative change in unit cell volume versus pressure in the range from ambient to 68.2 kbar was fit to the Birch type equation of state, The isothermal bulk modulus at am bient pressure and its pressure derivative were determined to be B-0 = 46.0(7) GPa and B-0' = 3.3(2), respectively. (C) 1998 Academic Press.