Effects of Octahedral Tilting on Band Structure and Thermoelectric Power Factor of Titanate Perovskites: A First-Principles Study on SrTiO3

Doped SrTiO3 and other perovskite structured titanates are attracting interest as n-type thermoelectric materials due to their relatively high thermoelectric power factor, low toxicity, and modest cost. Taking SrTiO3 as an example, the effects of octahedral tilting on the electronic band structure and thermoelectric power factor of titanate perovskites have been studied from first-principles calculations. By utilizing Glazer's notation, six representative tilt systems, including three out-of-phase (a0a0c–, a0b–b–, and a–a–a–) and three in-phase tilt systems (a0a0c+, a0b+b+, and a+a+a+), were investigated. It is found that out-of-phase tilting improves the optimum power factor as compared to the cubic aristotype, while in-phase tilting marginally lowers the optimum power factor. The largest increase in power factor (∼100%) is obtained in the one-tilt system a0a0c– at a tilt angle of 15°, which can be achieved with an energy cost of only 44 kJ mol–1 per formula unit. These findings agree with the experimental evidence that increased power factors are found in a0a0c– and a–a–a– tilt systems of titanate perovskites. The predicted increase of Seebeck coefficient as a function of tilt angle in the a–a–a– tilt system of SrTiO3 is also consistent with the experimental increase of Seebeck coefficient in a–a–a– titanates of La0.55K0.45TiO3 and La0.5Na0.5Ti0.9Nb0.1O3. Our simulations provide valuable insights into tuning the thermoelectric power factor of titanate perovskites by controlling octahedral tilting.