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

First principles calculations are used to show that O chemical shielding tensors, $\hat{\sigma}$, are a sensitive indicator of local structure in transition metal ABO$_3$ perovskites, due to their strong dependence on covalent O(2p)-B($n$d) interactions.\footnote{Pechkis et al., JCP {\bf 131}, 184511 (2009); references therein.} This 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. Our principal findings are 1) a large anisotropy between deshielded $\sigma_{x}\simeq\sigma_{y}$ and shielded $\sigma_{z}$ components; 2) a nearly linear variation of isotropic $\sigma_{\mathrm{iso}}$ and uniaxial $\sigma_{\mathrm{ax}}$ components, as a function of the B-O-B bond asymmetry, across all the systems studied; 3) the demonstration 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($n$d) states. 4) Very good agreement with recent BaTiO$_3$ and SrTiO$_3$ single crystal $^{17}$O NMR measurements of isotropic $\delta_{\mathrm{iso}}$ and uniaxial $\delta_{\rm ax}$ chemical shifts, and good agreement with PbTiO$_3$ and PbZrO$_3$ powder spectrum $\delta_{\mathrm{iso}}$ measurements.\footnotemark[2]