Concurrent La and A-site vacancy doping modulates the thermoelectric response of SrTiO3: experimental and computational evidence
journal contributionposted on 2018-01-03, 09:41 authored by Feridoon Azough, Samuel S. Jackson, Dursun Ekren, Robert Freer, Marco Molinari, Stephen Yeandel, Pooja GoddardPooja Goddard, Stephen C. Parker, David Hernandez Maldonado, Demie M. Kepaptsoglou, Quentin M. Ramasse
To help understand the factors controlling the performance of one of the most promising n-type oxide thermoelectric SrTiO3, we need to explore structural control at the atomic level. In Sr1–xLa2x/3TiO3 ceramics (0.0 ≤ x ≤ 0.9), we determined that the thermal conductivity can be reduced and controlled through an interplay of La-substitution and A-site vacancies and the formation of a layered structure. The decrease in thermal conductivity with La and A-site vacancy substitution dominates the trend in the overall thermoelectric response. The maximum dimensionless figure of merit is 0.27 at 1070 K for composition x = 0.50 where half of the A-sites are occupied with La and vacancies. Atomic resolution Z-contrast imaging and atomic scale chemical analysis show that as the La content increases, A-site vacancies initially distribute randomly (x < 0.3), then cluster (x ≈ 0.5), and finally form layers (x = 0.9). The layering is accompanied by a structural phase transformation from cubic to orthorhombic and the formation of 90° rotational twins and antiphase boundaries, leading to the formation of localized supercells. The distribution of La and A-site vacancies contributes to a nonuniform distribution of atomic scale features. This combination induces temperature stable behavior in the material and reduces thermal conductivity, an important route to enhancement of the thermoelectric performance. A computational study confirmed that the thermal conductivity of SrTiO3 is lowered by the introduction of La and A-site vacancies as shown by the experiments. The modeling supports that a critical mass of A-site vacancies is needed to reduce thermal conductivity and that the arrangement of La, Sr, and A-site vacancies has a significant impact on thermal conductivity only at high La concentration.
The authors gratefully acknowledge the support and the provision of funding from EPSRC for this work (EP/H043462, EP/I036230/1, EP/L014068/1, EP/L017695/1, EP/I03601X/1, and EP/K016288/1).