High sensitivity of O17 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, σ, in prototypical transition metal oxide ABO3 perovskite crystals. Our principal findings are 1) a large anisotropy ofσ between deshielded σx ≃ σy and shielded σz components (z along the Ti-O bond); 2) a nearly linear variation, across all the systems studied, of the isotropic σiso and uniaxial σax 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 σx and σy due to virtual transitions between O(2p) and unoccupied B(nd) states. The calculated isotropic δiso and uniaxial δax chemical shifts are in good agreement with recent BaTiO3 and SrTiO3 single crystal 17 O NMR measurements. In PbTiO3 and PbZrO3, calculated δiso are also in good agreement with NMR powder spectrum measurements. In PbZrO3, δiso calculations of the five chemically distinct sites indicate a correction of the experimental assignments. The strong dependence ofσ on covalent O(2p)-B(nd) interactions seen in our calculations 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.

Ab initio27Al NMR chemical shifts and quadrupolar parameters for Al2O3phases and their precursors

“…In the literature there have been several attempts to correlate features in NMR spectra with local structural details such as coordination numbers 22 , local atomic bondlength and angular distortion 57 , shortest bondlength 58 , Mulliken charge population 22 etc.…”

“…However, the internal distribution within a given coordination number does not correlate with local geometric descriptors such as local bondlength, angular distortions 57 , or shortest bondlength 58 . To explain this we performed a simple test in the non-spinel model B for γ-alumina by displacing an oxygen atom while keeping the rest fixed and we observed that the NMR parameters were affected in a region extending up to the third coordination shell, thus demonstrating the sensitivity of NMR to non-local structural details.…”

“…In the literature there have been several attempts to correlate features in NMR spectra with local structural details such as coordination numbers 22 , local atomic bondlength and angular distortion 57 , shortest bondlength 58 , Mulliken charge population 22 etc. Our calculations confirm the dependence of chemical shifts on aluminium coordination numbers, such that 4-, 5-and 6-coordinated aluminium sites show chemical shifts in well separated ranges: the lower the coordination number the larger is the chemical shift.…”

Applications of nuclear shielding

“…426 A first principles embedded cluster approach is used to calculate O chemical shielding tensors, s, in prototypical transition metal oxide ABO3 perovskite crystals. 427 The DFT B3LYP/SBKJC method is used to calculate the gas-phase optimised geometries of the glycolate oxoperoxo vanadium(V) complexes [V 2 O 2 (OO) 2 (gly) 2 ] 2À , [V 2 O 3 (OO)(gly) 2 ] 2À and [VO(OO)(gly)(H 2 O)] À . The 51 V, 17 O, 13 C and 1 H chemical shifts were calculated for the theoretical geometries in all-electron DFT calculations at the UDFT-IGLO-PW91 level and were subsequently compared with the experimental chemical shifts in solution.…”

Recent Advances in Nuclear Shielding Calculations

“…Another good example of embedded cluster approach is the study of the17 O shielding dependence on structural parameters, B-O bond length and B-O-B angle, in ABO 3 transition metal perovskite crystals 166. The authors use a cluster embedded in point charges to model the long range Coulomb interactions in the ABO 3 materials.…”