Thermal phase transformations in LaGaO(3) and LaAlO(3) perovskites: an experimental and computational solid-state NMR study.

Multinuclear (71)Ga, (69)Ga, (27)Al and (17)O NMR parameters of various polymorphs of LaGaO(3) and LaAlO(3) perovskites were obtained from the combination of solid-state MAS NMR with solid-state DFT calculations. Some of the materials studied are potential candidate electrolyte materials with applications in intermediate temperature solid oxide fuel cells (ITSOFCs). Small variations in the local distortions of the subject phases are experimentally observed by (71)Ga (and (69)Ga) and (27)Al NMR in the LaGaO(3) and LaAlO(3) phases, respectively, with heating to 1400 K. The orthorhombic-to-rhombohedral phase transformation occurring in LaGaO(3) at approximately 416 K is clearly observed in the (71)Ga/(69)Ga NMR spectra and is associated with a significant increase in the quadrupolar coupling constant (QCC). Thereafter a gradual decrease in QCC is observed, consistent with increased motion of the GaO(6) octahedral units and a reduction in the degree of octahedral tilting. The experimental and theoretical (71)Ga, (69)Ga, (27)Al and (17)O NMR parameters (including isotropic and anisotropic chemical shift parameters, quadrupolar coupling constants, and associated asymmetries) of the low and high temperature polymorphs are compared. In general, the calculated values display good agreement with experimental data, although some significant deviations are identified and discussed.