High-pressure crystal chemistry of neighborite, NaMgF₃: An angle-dispersive diffraction study using monochromatic synchrotron X-radiation

The crystal structure of neighborite (NaMgF₃) is determined at high pressure using monochromatic synchrotron X-ray powder diffraction data with Rietveld structural refinement methods. The previously assumed regularity of the MgF₆ octahedra at high pressure in the perovskite structure is demonstrated to be valid. The pressure-induced dimensional changes are expressed empirically as a combination of compression of the Mg-F bond length and tilting of the MgF₆ octahedral framework. We demonstrate that the dominant compression mechanism involves shortening of the octahedral Mg-F bond, which contributes about 80% to the overall compression, with the remaining 20% due to an increase in octahedral tilting. Linear compressibilities are consistent with single-crystal elasticity data, with the significant anisotropy observed being directly related to the tilting freedom of the octahedra along each axis of the unit cell.