Structure of (NH4)3GaF6 investigated by multinuclear magic-angle spinning NMR spectroscopy in comparison with Rietveld refinement

The structure of ammonium gallium cryolite (NH4)(3)GaF6 was investigated by F-19 and Ga-69,Ga-71 magic-angle spinning (MAS) NMR in comparison with X-ray powder diffraction followed by Rietveld refinement. In agreement with previous thermodynamic measurements, NMR experiments on (NH4)3GaF6 support the model of rigid GaF6 octahedra. At high spinning speeds (30 kHz), the scalar coupling between the six equivalent F-19 nuclei and Ga-69,Ga-71 can be directly observed in the powder spectra. The coupling constants are J(19)F(69)Ga = 197 Hz and J(19)F(71)Ga = 264 Hz. To explain the Ga-71 spectra recorded at 3 kHz a small distribution of quadrupolar frequencies has to be included. The spread of the spinning sidebands hints to a largest nu(Q) value of 28 kHz for Ga-71. This can be explained by the occurrence of highly symmetric GaF6 octahedra, which are tilted against the surrounding atoms. In addition, the incomplete motional excitation does not average out the quadrupolar effects. NMR findings are in discrepancy to those of Rietveld refinement. As result it appears that X-ray diffraction is not sensitive enough to deliver proper results.