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A first principles embedded cluster approach is used to calculate O chemicalshielding tensors, sigma, in prototypical transition metal oxide ABO_3perovskite crystals. Our principal findings are 1) a large anisotropy of sigmabetween deshielded sigma_x ~ sigma_y and shielded sigma_z components (z alongthe 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 ofthe B-O-B bond asymmetry. We show that the anisotropy and linear variationarise from large paramagnetic contributions to sigma_x and sigma_y due tovirtual transitions between O(2p) and unoccupied B(nd) states. The calculatedisotropic delta_iso and uniaxial delta_ax chemical shifts are in good agreementwith recent BaTiO_3 and SrTiO_3 single crystal 17O NMR measurements. In PbTiO_3and PbZrO_3, calculated delta_iso are also in good agreement with NMR powderspectrum measurements. In PbZrO_3, delta_iso calculations of the fivechemically distinct sites indicate a correction of the experimentalassignments. The strong dependence of sigma on covalent O(2p)-B(nd)interactions seen in our calculations indicates that 17O NMR spectroscopy,coupled with first principles calculations, can be an especially useful tool tostudy the local structure in complex perovskite alloys.

journal of chemical physics online/the journal of chemical physics/journal of chemical physics

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