Transport and thermoelectric properties of Sr3(Ti0.95R0.05)2O7 (R = Ta, Nb, W) oxides

Abstract

The Sr3(Ti0.95R0.05)2O7 (R = Ta, Nb, W) polycrystalline compounds were fabricated, and their transport and thermoelectric properties were investigated. The results indicate that at T > 300 K electrical resistivity ρ for all the doped compounds increases monotonically with temperature, and basically can be described by a relation ρ ∝ TM at T > ∼650 K, with M = 1.39, 1.66, and 1.77 for R = Ta, Nb, and W, respectively, implying that at the high temperatures the acoustic phonon scattering dominates the scattering process. Although the resistivity ρ of Sr3(Ti0.95Ta0.05)2O7 exhibits a metallic-like behavior at the temperature as low as 5 K, a transition from metallic state (dρ/dT > 0) to semiconductor-like state (dρ/dT < 0) was observed at a critical low temperature ∼41 K and ∼79 K for R = Nb and W, respectively. At T < ∼22 K, ∼57 K, and ∼80 K, a relation of σ ∝ T1/2 (here conductivity σ = 1/ρ) holds for the doped compounds with R = Nb, Ta, and W, respectively, suggesting that at the low temperatures the main transport mechanism is electron-electron interaction due to the presence of disorder induced by the dopants. The thermoelectric figure of merit (ZT) for Ta-doped compound increases more steeply with increasing temperature among the three compounds and reaches 0.066 at 1000 K.