Directly Probing the Metal Center Environment in Layered Zirconium Phosphates by Solid-State91Zr NMR

Abstract

Layered zirconium phosphates (ZrPs) and their derivatives potentially have many important applications. In the past, these materials have been mainly characterized by X-ray diffraction and 31P MAS NMR. Their metal centers have not been directly probed by solid-state NMR spectroscopy. In this work, we present solid-state 91Zr NMR spectra acquired at several magnetic fields for several representative layered zirconium phosphates including α-Zr(HPO4)2·H2O, γ-Zr(PO4)(H2PO4)·2H2O, Zr(NH4PO4)2·H2O, and Zr2(NaPO4)4·6H2O. The NMR interaction tensors were extracted from the spectra. The results indicate that the 91Zr spectra are sensitive not only to the relatively small distortion in ZrO6 polyhedron but also to the difference in the geometry of Zr(OP)6 units in these materials. We show that 91Zr quadrupolar coupling constants (CQ) correlate well with several angular distortion parameters reflecting the deviation from a perfect ZrO6 octahedron such as distortion index, shear strain, and mean O−Zr−O angle. The relationships between CQ and structural parameters related to the Zr(OP)6 unit including the mean Zr−P distance and Zr−O−P angle also appear to exist. The theoretical calculations at both restricted Hartree−Fock and density functional levels were performed on model clusters to establish the relationships of various structural parameters with 91Zr EFG tensors, and the calculation results are consistent with the empirical correlations. For the related layered zirconium phosphates whose structures are unknown or poorly described, we have shown that 91Zr NMR can be used to directly obtain structural information on the local environment around the metal centers, which is complementary to that obtained from powder XRD and 31P MAS NMR, as demonstrated by a novel meso-lamellar ZrP as an example.