Characterization of Na5P3O10 polymorphs by 23Na MAS, 23Na MQMAS, and 31P MAS NMR spectroscopy

The two anhydrous polymorphs of Na5P3O10 have been characterized by Na-23 and P-31 MAS NMR spectroscopy, The Na-23 multiple-quantum (MQ) MAS NMR spectrum of the low-temperature form (phase II) displays three resonances for which the quadrupole coupling parameters and isotropic chemical shifts have been accurately determined from the MAS NMR spectra of the central transition. Thereby, discrepancies between recently reported Na-23 MQMAS spectra of this phase and the crystal structure have been clarified. The Na-23 resonances observed for the low-temperature form are assigned to the crystallographically nonequivalent Na sites in the crystal structure using point-monopole calculations of the electric-field gradient tensors. Three Na-23 resonances have also been observed for the high-temperature form (phase I), with two signals having very similar quadrupolar couplings and isotropic chemical shifts indicating similar coordination environments for the corresponding Na sites, in disagreement with the reported single-crystal structure. Point-monopole calculations of the electric-field gradient tensors based on the crystal structure fail to reproduce the experimental values. The P-31 chemical shielding anisotropies, obtained from P-31 MAS NMR spectra, show that the terminal P atoms of the P3O105- ions have a negative shielding anisotropy parameter (delta(sigma) = delta(iso) - delta(33)) in agreement with similar observations reported for diphosphates. This characteristic feature has been used in the assignment of the three P-31 resonances observed for the hexahydrate Na5P3O10. 6H(2)O.