Assigning Single-Crystal NMR Data to Crystallographic Sites for Symmetry-Affected Interaction Tensors: A Case Study of Sinhalite, MgAlBO4, by 11B and 27Al NMR Spectroscopy and DFT Calculations

The natural mineral sinhalite, MgAlBO4, was investigated by means of single-crystal NMR spectroscopy. In its orthorhombic space group, the aluminium and boron atoms occupy Wyckoff positions 4b and 4c, but the specific site symmetry of the boron atoms situated on a mirror plane leads two only two instead of four magnetically inequivalent atoms per site. This structural feature also restricts the corresponding NMR interaction tensors and reduces known assignment ambiguities regarding the NMR resonances of 27Al and 11B to distinct atomic positions. Comparing the experimentally determined eigenvectors of the quadrupolar coupling tensors with the results of DFT calculations for the electric field gradients further condensed the remaining options and delivered the full quadrupolar coupling tensors for both nuclides in the crystal structure of sinhalite.