Correlation between 31P NMR and X-ray diffraction data in the refinement of the atomic positions of the thorium phosphate-diphosphate crystal structure

In the framework of the long term repository of radwastes, the thorium phosphate-diphosphate, Th4(PO4)4P2O7, appears as a promising material for the immobilization of tetravalent actinides. In the way of a better characterization of this matrix, the average crystal structure was reconsidered on the basis of both 31P NMR and X-ray diffraction experiments. The NMR 1D and 2D results obtained suggested the existence of two local configurations in the same proportion for the diphosphate group. In correlation with this position disorder of the P2O7 groups, the electronic environments of the phosphorus atoms in the PO4 tetrahedra are different, involving the splitting of each resonance line observed by NMR. The local crystal structure was refined and the results obtained are in good agreement with the structural data of phosphate compounds.