Variations of titanium interactions in solid state NMR-correlations to local structure

Padro, D. and Jennings, V. and Smith, M.E. and Hoppe, R. and Thomas, P.A. and Dupree, R. (2002) Variations of titanium interactions in solid state NMR-correlations to local structure. Journal of Physical Chemistry B, 106 (51). pp. 13176-13185. ISSN 1089-5647

Abstract

Direct solid state NMR observation of titanium opens up new possibilities for characterization of many chemical problems. Here, solid state 47,49Ti NMR spectra are reported for a large variety of crystalline ternary and quaternary titanates. A wide range of values for the NMR interaction parameters is observed, with chemical shifts covering ∼300 ppm and quadrupole interactions (CQ) up to 24 MHz. The isotropic chemical shift (δiso) ranges for TiO6, TiO5, and TiO4 deduced from these samples suggest that NMR can help to determine coordinations in materials where it is unknown, although the strong overlap of these ranges may not allow unequivocal identification in all cases. For TiO6 units, there is a good correlation of the shear strain of the local TiO6 unit to CQ. Similar correlations are found for the much rarer TiO4 coordination. The correlation of CQ is generalized through the concept of a distortion index of the local structural unit, which applies to all TiOx units, including TiO5. Static NMR data from several samples (e.g., PbTiO3, Sr2TiO4) show unequivocally that for some compounds titanium has a significant chemical shift anisotropy, which needs to be included for accurate simulations of static titanium NMR spectra. The electric field gradient determined by NMR does not agree with values previously obtained by perturbed angular correlation. Calculations of the quadrupole parameters using the WIEN97 code, based on the linearized augmented plane wave method, have been carried out for some perovskite- and ilmenite-structured titanates and agree well with the values determined from NMR (i.e., within a few percent), except for the ilmenite form of CdTiO3 where the current structure appears unreliable. The 43Ca NMR spectrum is reported for CaTiO3 showing that CQ = 2.15 ± 0.03 MHz, η = 0.70 ± 0.07, and δiso = 17.2 ± 1.0 ppm.