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

Abstract

A first principles embedded cluster approach is used to calculate O chemical shielding tensors, sigma, in prototypical transition metal oxide ABO3 perovskite crystals. Our principal findings are 1) a large anisotropy of sigma between deshielded sigmax ~ sigmay and shielded sigmaz components (z along the Ti-O bond); 2) a nearly linear variation, across all the systems studied, of the isotropic sigmaiso and uniaxial sigmaax 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 sigmax and sigmay due to virtual transitions between O(2p) and unoccupied B(nd) states. The calculated isotropic deltaiso and uniaxial deltaax chemical shifts are in good agreement with recent BaTiO3 and SrTiO3 single crystal 17O NMR measurements. In PbTiO3 and PbZrO3, calculated deltaiso are also in good agreement with NMR powder spectrum measurements. In PbZrO3, deltaiso 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.