Crystal structure, oxygen content and conductivity of Sr1-xGdxCoO3-δ

Abstract

Solid solutions Sr1–xGdxCoO3–δ (0.0≤x ≤ 0.4) were synthesized through a glycerol-nitrate technique. The final anneals were performed at 1100 °C in air for 120 h. All the samples were slowly cooled to room temperature at a rate of about 100 °C/h; alternatively, they were quenched to room temperature at a cooling rate of about 500°/min. The structural parameters of all the samples were refined via Rietveld analysis. All the single-phase oxides slowly cooled to room temperature in air possess a tetragonal 2ap × 2ap × 4ap superstructure (sp. gr. I4/mmm). Only Sr0.9Gd0.1CoO3–δ undergoes a complete order-disorder type phase transition: heating it up to 1100 °C in air transforms it from an ordered tetragonal 2ap × 2ap × 4ap structure to a disordered cubic structure. The super-structural lines for Sr0.9Gd0.1CoO3–δ noticeably decreased at 1100 °C, but are still present in the XRD pattern. The structure of Sr1–xGdxCoO3–δ with x = 0.3 and 0.4 at 1100 °C remains almost unchanged. The oxygen content in Sr1–xGdxCoO3–δ at room temperature ranges from δ = 2.57 in the sample with x = 0.1 to δ = 2.75 in the sample with x = 0.4. The variations in oxygen content, electrical conductivity and the Seebeck coefficient in the Sr1–xGdxCoO3–δ oxides were measured in air versus temperature. The temperature dependences of conductivity for Sr1-xGdxCoO3-δ exhibit maximum (~550 S/cm) at 300–400 °C. The conductivity values depend weakly on the Gd content in Sr1–xGdxCoO3–δ. The temperature dependences of the Seebeck coefficient (S) for all oxides are represented by smooth curves with minimum. The positive S values at room temperature (12–36 μV/K) decreased down to zero or even tiny negative values (– 3 μV/K) as the temperate rose within the range of 300–800 °C and they then rose slightly to 2–14 μV/K at 1100 °C.