  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 ~ 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 17O 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 17O NMR spectroscopy,
coupled with first principles calculations, can be an especially useful tool to
study the local structure in complex perovskite alloys.
