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Transport and thermoelectric properties of n-type Ruddlesden–Popper phase (Sr1−xGdx)3(Ti1−yTay)2O7 oxides

The transport and thermoelectric properties of (Sr1−xGdx)3(Ti1−yTay)2O7(x = 0.05, y = 0.05–0.15; x = 0.1, y = 0.05–0.1) compounds with Ruddlesden–Popper (RP) phase, prepared by a conventional solid-state reaction method plus spark plasma sintering (SPS), were investigated in the temperature range from 300 to 1000 K. The results indicate that the electrical resistivity ρ for all compounds increases with temperature, and has a relation ρ ∝ TM (i.e. the mobility μ ∝ T−M) with M = 1.58–1.92 at T ≳ 650 K, indicating that the phonon scattering is predominant. Basically, the absolute value of the Seebeck coefficient |S| increases almost linearly with temperature above ∼400 K, showing degenerated semiconducting behaviour. Moreover, both ρ and |S| decrease with the increase in both Ta and Gd content, which can be attributed to an increase in the carrier concentration. The lattice thermal conductivity κL of the compounds decreases monotonically with an increase in both Ta and Gd content due to mass-defect phonon scattering, and the lowest κL(κL = 3.3 W K−1m−1 at room temperature and 2.0 W K−1 m−1 at 1000 K) is achieved in (Sr0.9Gd0.1)3(Ti0.9Ta0.1)2O7. Among all the compounds investigated here, the largest dimensionless figure of merit ZT = 0.08 (at 1000 K) is obtained in the compound (Sr0.9Gd0.1)3(Ti0.95Ta0.05)2O7.