93Nb and 17O NMR chemical shifts of niobiophosphate compounds

Abstract

Niobiophosphate compounds with a large range of niobium and oxygen environments were studied with 93Nb and 17O solid-state NMR. 93Nb isotropic chemical shift of pure niobate Nb(ONb) 6, pure phosphate Nb(OP) 6 and mixed phosphate–niobate Nb(OP) x (ONb) (6− x ) (1⩽ x⩽5) sites were measured at a high magnetic field (18.8 T). 93Nb chemical shifts were found to be sensitive to the variation of local charge on Nb, but not to the local bond geometry (i.e. crystallographic site and edge or corner connectivity). A systematic shift to high field of the 93Nb chemical shift is measured when x increases. Then, 17O NMR spectra of a series of enriched samples provided the chemical shift and quadrupolar parameters for 4 types of oxygen environment (P–O–P, P–O–Na, P–O–Nb and Nb–O–Nb). Finally, Nb–O–Nb sites were characterized by a large 17O chemical shift anisotropy.