High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding.

A first principles embedded cluster approach is used to calculate O chemical shielding tensors, sigma, in prototypical transition metal oxide ABO(3) perovskite crystals. Our principal findings are (1) a large anisotropy of sigma between deshielded sigma(x) approximately sigma(y) and shielded sigma(z) components (z along the Ti-O bond); (2) a nearly linear variation, across all the systems studied, of the isotropic sigma(iso) and uniaxial sigma(ax) components, as a function of the B-O-B bond asymmetry. We show that the anisotropy and linear variation arise from large paramagnetic contributions to sigma(x) and sigma(y) due to virtual transitions between O(2p) and unoccupied B(nd) states. The calculated isotropic delta(iso) and uniaxial delta(ax) chemical shifts are in good agreement with recent BaTiO(3) and SrTiO(3) single crystal (17)O NMR measurements. In PbTiO(3) and PbZrO(3), calculated delta(iso) are also in good agreement with NMR powder spectrum measurements. In PbZrO(3), delta(iso) calculations of the five chemically distinct sites indicate a correction of the experimental assignments. The strong dependence of sigma on covalent O(2p)-B(nd) interactions seen in our calculations indicates that (17)O NMR spectroscopy, coupled with first principles calculations, can be an especially useful tool to study the local structure in complex perovskite alloys.