A powder neutron diffraction study of the crystal structure of the fluoroperovskite NaMgF₃ (neighborite) from 300 to 3.6 K

Abstract

The cell dimensions and crystal structures of the fluoroperovskite NaMgF₃ (neighborite), synthesized by solid state methods, have been determined by powder neutron diffraction and Rietveld refinement over the temperature range 300–3.6 K using Pt metal as an internal standard for calibration of the neutron wavelength. These data show that Pbnm NaMgF₃ does not undergo any phase transitions to structures of lower symmetry with decreasing temperature. The cell dimensions and atomic coordinates together with polyhedron volumes and distortion indices are given for Pbnm NaMgF₃ at 25 K intervals from 300 to 3.6 K. Decreases in the a and c cell dimensions reach a saturation point at 50 K, whereas the b dimension becomes saturated at 150 K. The distortion of the structure of Pbnm NaMgF₃ from the aristotype cubic $Pm\ifmmode\expandafter\bar\else\expandafter\=\fi{3}m$ structure is described in terms of the tilting of the MgF₆ octahedra according to the tilt scheme a ⁻ a ⁻ c ⁺. With decreasing temperature the antiphase tilt (a ⁻) increases from 14.24° to 15.39°, whereas the in-phase tilt (c ⁺) remains effectively constant at ∼10.7°. Changes in the tilt angles are insufficient to cause changes in the coordination sphere of Na that might induce a low temperature phase transition. The structure of Pbnm NaMgF₃ is also described in terms of normal mode analysis and displacements of the condensed normal modes are compared with those of Pbnm KCaF₃.