Relating 139La Quadrupolar Coupling Constants to Polyhedral Distortion in Crystalline Structures

Abstract

A broad series of crystalline lanthanum oxide-based materials has been investigated through high-field 139La solid state nuclear magnetic resonance (ssNMR) spectroscopy and ab initio density functional theory (DFT) calculations. The 139La NMR spectra of LaBGeO5, LaBSiO5, LaBO3, LaPO4·1.8H2O, La2(SO4)3·9H2O, and La2(CO3)3·8H2O are reported for the first time. Both newly reported and literature values of 139La quadrupolar coupling constants (CQ) are related to various quantitative expressions of polyhedral distortion, including sphericity (Σ) and ellipsoid span (ϵ). The compounds were separated into two groups based upon their polyhedral distortion behavior: compounds with the general formula LaMO3, where M is a trivalent cation; compounds with different general formulas. The 139La CQ of the LaMO3 family was found to correlate best with ϵ. The 139La CQ of non-LaMO3 compounds correlates adequately to ϵ but is better described by Σ. The 139La isotropic chemical shift (δisoCS) of the non-LaMO3 compounds is negatively correlated with the lanthanum coordination number; there is insufficient data from the LaMO3 compounds to draw conclusions relating to chemical shift. DFT calculations of NMR parameters prove to be a sensitive probe of the quality of input geometry, with predicted parameters agreeing with experiment except in cases where the crystal structure is suspect.