17 O NMR对过渡金属钙钛矿中p-d杂化的高灵敏度：大的各向异性化学屏蔽的第一性原理计算
High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding
摘要：
采用第一性原理嵌入团簇方法计算了典型过渡金属氧化物ABO_3钙钛矿晶体中氧的化学屏蔽张量σ。我们的主要发现是：（1）去屏蔽的σ_x ~ σ_y和屏蔽的σ_z分量（z沿Ti-O键）之间存在较大的σ各向异性;（2）在所有研究的体系中，各向同性σ_iso和单轴σ_ax分量作为B-O-B键不对称性的函数几乎呈线性变化。我们发现，各向异性和线性变化引起的大顺磁贡献sigma_x和sigma_y由于虚跃迁之间的O（2 p）和未占用的B（nd）的状态。计算得到的各向同性δ_iso和单轴δ_ax化学位移与最近BaTiO_3和SrTiO_3单晶~（17）O NMR测量结果符合得很好。在PbTiO_3和PbZrO_3中，δ_iso的计算值与NMR粉末谱测量值也符合得很好。在PbZrO_3中，对五个不同化学位置的delta_iso计算表明了实验结果的修正。在我们的计算中发现，σ对共价O（2 p）-B（nd）相互作用的强烈依赖性表明，17 O NMR光谱与第一性原理计算相结合，可以成为研究复杂钙钛矿合金中局部结构的特别有用的工具。
英文摘要：
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.
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