Structural and thermoelectric properties of GdxSr2-xCoO4 layered perovskites

Gd_xSr_2-xCoO₄-layered perovskites with varying gadolinium content (x = 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) were synthesised using a conventional solid-state reaction method in air. X-ray diffraction (XRD) analysis indicated that all samples were single phase with a tetragonal I4/mmm K₂NiF₄-type structure. Rietveld refinements showed that the lattice parameters and the in-plane Co–O bond length decreased with increasing Gd³⁺ content, suggesting stronger structural distortion. The X-ray photoelectron spectroscopy (XPS) analysis indicated that cobalt ions on the surface of Gd_xSr_2-xCoO₄ had Co³⁺ and Co⁴⁺mixed valence states and many oxygen vacancies. The thermoelectric properties of Gd_xSr_2-xCoO₄ (x = 0.7, 0.8) were also investigated. The temperature dependencies of the electrical conductivity (σ) and Seebeck coefficient (S) up to 750 °C for Gd_0.7Sr_1.3CoO₄ showed maximum and minimum values at approximately 550 °C. Below 550 °C with increasing Gd³⁺ content, S increased, whereas σ and the thermal conductivity (κ) decreased, leading to an increase in the figure of merit (ZT). A maximum ZT value of 0.0039 for Gd_0.8Sr_1.2CoO₄ was obtained at 400 °C. The variations of σ, S and κ with x and temperature are discussed.