Thermoelectric properties of Sr–Ir–O compounds prepared by spark plasma sintering

Abstract

Sr–Ir–O compounds in various molar ratios of Ir to Sr (RIr/Sr) from 0.2 to 1.2 were prepared by solid-state reaction and compacted by spark plasma sintering (SPS). Single phase SrIrO3 was obtained at RIr/Sr = 1.0. Sr2IrO4 and Sr4IrO6 with a trace of second phase were obtained at RIr/Sr = 0.5 and 0.25, respectively. The electrical conductivity (σ) of Sr–Ir–O compounds increased with increasing RIr/Sr. The σ at RIr/Sr < 0.8 showed semiconducting behavior, whereas at RIr/Sr > 1.0, it was metallic. The σ of SrIrO3 (2.45 × 104 S m−1 at room temperature) was 1–2 orders higher than that of Sr2IrO4 (2.2 × 102 S m−1 at room temperature) and 3–6 orders higher than that of Sr4IrO6 (2 × 10−2 S m−1 at room temperature). The activation energy (Ea) of σ for Sr2IrO4 was 0.055 eV lower than that of Sr4IrO6, suggesting a hopping conduction at low temperatures. The Seebeck coefficient (S) of all specimens was p-type. The S decreased with increasing RIr/Sr, with Sr4IrO6 indicating the highest value, i.e., 200 μV K−1, at 650 K. The S of SrIrO3 increased from 2 to 28 μV K−1 from 450 to 1023 K. The κ was around 0.6 to 2.5 W m−1 K−1. The thermal conductivity (κ) at RIr/Sr = 0.7 showed the lowest value, i.e., around 0.6 W m−1 K−1. The highest dimensionless figure-of-merit (ZT) was 0.013 at RIr/Sr = 0.8 at 1023 K.