Defects in Doped LaGaO3 Anionic Conductors: Linking NMR Spectral Features, Local Environments, and Defect Thermodynamics

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• 作者：Fr茅d茅ric Blanc ; Derek S. Middlemiss ; Zhehong Gan ; Clare P. Grey
• 刊名：Journal of the American Chemical Society
• 出版年：2011
• 出版时间：November 9, 2011
• 年：2011
• 卷：133
• 期：44
• 页码：17662-17672
• 全文大小：1041K
• 年卷期：v.133,no.44(November 9, 2011)
• ISSN：1520-5126

文摘

Doped lanthanum gallate perovskites (LaGaO₃) constitute some of the most promising electrolyte materials for solid oxide fuel cells operating in the intermediate temperature regime. Here, an approach combining experimental multinuclear NMR spectroscopy with density functional theory total energy and GIPAW NMR calculations yields a comprehensive understanding of the structural and defect chemistries of Sr- and Mg-doped LaGaO₃ anionic conductors. The DFT energetics demonstrate that Ga鈥揤_O鈥揋a (V_O = oxygen vacancy) environments are favored (vs Ga鈥揤_O鈥揗g, Mg鈥揤_O鈥揗g and Mg鈥揙鈥揗g鈥揤_O鈥揋a) across a range y = 0.0625, 0.125, and 0.25 of fractional Mg contents in LaGa_1鈥?i>yMg_yO_3鈥?i>y/2. The results are interpreted in terms of doping and mean phase formation energies (relative to binary oxides) and are compared with previous calculations and experimental calorimetry data. Experimental multinuclear NMR data reveal that while Mg sites remain six-fold coordinated across the range of phase stoichiometries, albeit with significant structural disorder, a stoichiometry鈥揹ependent minority of the Ga sites resonate at a shift consistent with Ga^V coordination, demonstrating that O vacancies preferentially locate in the first anion coordination shell of Ga. The strong Mg鈥揤_O binding inferred by previous studies is not observed here. The ¹⁷O NMR spectra reveal distinct resonances that can be assigned by using the GIPAW NMR calculations to anions occupying equatorial and axial positions with respect to the Ga^V鈥揤_O axis. The disparate shifts displayed by these sites are due to the nature and extent of the structural distortions caused by the O vacancies.