Structure and thermoelectric properties of Ca _2-xSr _xFeMoO ₆ (0 ≤ x ≤ 0.3) double-perovskite oxides

The thermoelectric properties of double perovskite-type oxides Ca ₂FeMoO ₆ are investigated in terms of Sr substitution at the A site of the oxides. The electrical conductivity, σ, of Ca _2-xSr _xFeMoO ₆ (0 ≤ x ≤ 0.3) showed a metallic behavior, decreasing monotonically from ca. 10 ³ S cm ^-1 at room temperature to ca. 10 ² S cm ^-1 at 1250 K. At room temperature, although the σ values of the oxides increased with increasing substitution level, x, the σ values maintained almost the same values at high temperature range of 1000-1250 K. The absolute values of the Seebeck coefficient, S, for the samples at x < 0.3 abruptly increase at around 1000 K. The Rietveld refinement of the XRD patterns of the oxides indicated that the anti-site defects in the oxides decreased with increasing Sr concentration. The power factor, S ²σ, of the oxides largely increased with increasing temperature; the S ²σ value of Ca ₂FeMoO ₆ was ca. 0.35 mW mK ^-2 at 1200 K, the largest value of all the samples in this study. Although the thermal conductivity, κ, of the oxides generally decreased from ca. 3.5 to 4.5 W mK ^-1 at room temperature to ca. 2-3 W mK ^-1 at 1050 K, the sample at x = 0.3 showed the lowest and most T-independent κ values, implying that the relative increase in the temperature is independent from the phonon-impurity scattering caused by the A-site substitution. The power factor of the oxides increased above 900 K; thereby, the dimensionless figure of merit, ZT = (S ²σ/κ)T, significantly increased at the same temperature range. The largest ZT value of 0.15 was observed for Ca ₂FeMoO ₆, Ca _1.9Sr _0.1FeMoO ₆, and Ca _1.8Sr _0.2FeMoO ₆ at 1250 K.