Structure of (NH4)3GaF6 Investigated by Multinuclear Magic-Angle Spinning NMR Spectroscopy in Comparison with Rietveld Refinement
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- 作者：Thoralf Krahl ; Mike Ahrens ; Gudrun Scholz ; Detlef Heidemann ; Erhard Kemnitz
- 刊名：Inorganic Chemistry
- 出版年：2008
- 出版时间：January 21, 2008
- 年：2008
- 卷：47
- 期：2
- 页码：663 - 670
- 全文大小：251K
- 年卷期：v.47,no.2(January 21, 2008)
- ISSN：1520-510X
文摘
The structure of ammonium gallium cryolite (NH4)3GaF6 was investigated by 19F and 69,71Ga magic-angle spinning(MAS) NMR in comparison with X-ray powder diffraction followed by Rietveld refinement. In agreement with previousthermodynamic measurements, NMR experiments on (NH4)3GaF6 support the model of rigid GaF6 octahedra. Athigh spinning speeds (30 kHz), the scalar coupling between the six equivalent 19F nuclei and 69,71Ga can bedirectly observed in the powder spectra. The coupling constants are J19F69Ga = 197 Hz and J19F71Ga = 264 Hz.To explain the 71Ga 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 
Q value of 28 kHz for 71Ga. This can be explained by theoccurrence of highly symmetric GaF6 octahedra, which are tilted against the surrounding atoms. In addition, theincomplete motional excitation does not average out the quadrupolar effects. NMR findings are in discrepancy tothose of Rietveld refinement. As result it appears that X-ray diffraction is not sensitive enough to deliver properresults.
Q value of 28 kHz for 71Ga. This can be explained by theoccurrence of highly symmetric GaF6 octahedra, which are tilted against the surrounding atoms. In addition, theincomplete motional excitation does not average out the quadrupolar effects. NMR findings are in discrepancy tothose of Rietveld refinement. As result it appears that X-ray diffraction is not sensitive enough to deliver properresults.