31P MAS NMR spectroscopy with 93Nb decoupling and DFT calculations: A structural characterization of defects in a niobium-phosphate phase

International audienceThe present study has investigated the structure of four niobium phosphates compounds using 31P MAS NMR spectroscopy. Niobium 93Nb decoupling, applied during 31P NMR acquisition led to a resolution enhancement by a factor of 2–3, which allowed distinguish phosphorous sites separated by 1 ppm or less. The assignment of 31P spectra has been completed by use of first-principles calculations derived from the original XRD structures. One of the compounds, Na3.04Nb7P4O29, contains a sodium vacancy, clearly revealed in the 31P MAS spectrum obtained with 93Nb decoupling. A series of structural models were proposed to account for the sodium distribution in the structure of Na3.04Nb7P4O29 and the assignment was possible using DFT calculation and a statistical analysis of the NMR parameters