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Relating defect chemistry and electronic transport in the double perovskite Ba₁₋ₓGd₀.₈La₀.₂₊ₓCo₂O₆₋δ (BGLC)

Vøllestad, Einar, Schrade, Matthias, Segalini, Julie, Strandbakke, Ragnar, Norby, Truls
Journal of materials chemistry A 2017 v.5 no.30 pp. 15743-15751
cobalt, differential scanning calorimetry, electrical properties, enthalpy, heat production, models, oxidation, oxides, oxygen, temperature, thermogravimetry
Rare earth double perovskites comprise a class of functional oxides with interesting physiochemical properties both for low- and high-temperature applications. However, little can be found relating electrical properties with equilibrium thermodynamics of non-stoichiometry and defects. In the present work, a comprehensive and generally applicable defect chemical model is developed to form the link between the defect chemistry and electronic structure of partially substituted BGLC (Ba₁₋ₓGd₀.₈La₀.₂₊ₓCo₂O₆₋δ, 0 ≤ x ≤ 0.5). The equilibrium oxygen content of 4 different compositions is determined as a function of pO₂ and temperature by thermogravimetric analysis, and combined with defect chemical modelling to obtain defect concentrations and thermodynamic parameters. Oxidation enthalpies determined by TG-DSC become increasingly exothermic (−50 to −120 kJ mol⁻¹) with increased temperature and oxygen non-stoichiometry for all compositions, in excellent agreement with the thermodynamic parameters obtained from the defect chemical model. All compositions display high electrical conductivities (500 to 1000 S cm⁻¹) with shallow pO₂-dependencies and small and positive Seebeck coefficients (3 to 15 μV K⁻¹), indicating high degree of degeneracy of the electronic charge carriers. The complex electrical properties of BGLC at elevated temperatures is rationalized by a two-band conduction model where highly mobile p-type charge carriers are transported within the valence band, whereas less mobile “n-type” charge carriers are located in narrow Co 3d band.