Thermal phase transformations in LaGaO₃ and LaAlO₃ perovskites: An experimental and computational solid-state NMR study

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• 作者：Fré ; dé ; ric Blanc^a ; ^b ; Derek S. Middlemiss^a ; ^b ; Lucienne Buannic^a ; John L. Palumbo^a ; Ian Farnan^c ; Clare P. Grey^a ; ^b ; ^{cpg27@cam.ac.uk}
• 关键词：Multinuclear solid state NMR ; GIPAW calculations ; LaGaO3 ; LaAlO3 ; Perovskites
• 刊名：Solid State Nuclear Magnetic Resonance
• 出版年：2012
• 期刊代码：83_09262040
• 类别：ce
• 出版时间：April, 2012
• 卷：42
• 期：Complete
• 页码：87-97
• 文件大小：1263 K

摘要

Multinuclear ⁷¹Ga, ⁶⁹Ga, ²⁷Al and ¹⁷O NMR parameters of various polymorphs of LaGaO₃ and LaAlO₃ 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 ⁷¹Ga (and ⁶⁹Ga) and ²⁷Al NMR in the LaGaO₃ and LaAlO₃ phases, respectively, with heating to 1400 K. The orthorhombic-to-rhombohedral phase transformation occurring in LaGaO₃ at approximately 416 K is clearly observed in the ⁷¹Ga/⁶⁹Ga 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 GaO₆ octahedral units and a reduction in the degree of octahedral tilting. The experimental and theoretical ⁷¹Ga, ⁶⁹Ga, ²⁷Al and ¹⁷O 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.