Thermoelectric properties of the electron-doped perovskites Sr1-xCaxTi1-yNbyO3

We have investigated thermoelectric (TE) properties for single crystals of perovskites Sr1-xCaxTi1-yNbyO3 for 0 <= x <= 0.4 and 0 <= y <= 0.03 below room temperature (RT). We found that SrTi0.99Nb0.01O3 shows a large power factor at low temperature (PF = 50 mu W/K-2 cm at 100 K similar to 90 mu W/K-2 cm at 50 K) and the largest dimensionless TE figure-of-merit below 40 K (ZT similar to 0.07) among the ever-reported materials. Such a large low-temperature TE response around a carrier concentration of 10(20) cm(-3) is due to a distinct electron-phonon interaction, which could relate to the superconducting state. We also found that the Ca2+ substitution for Sr2+ increases ZT at 300 K for Sr1-xCaxTi0.97Nb0.03O3 from 0.08 to 0.105. The enhancement of ZT around RT originates both in a large reduction of a thermal conductivity due to an introduced randomness into the crystal structure and in an unexpected enhancement of a Seebeck coefficient.