Stable cubic crystal structures and optimized thermoelectric performance of SrTiO3-based ceramics driven by entropy engineering

Abstract

SrTiO₃-based perovskites show great potential as n-type thermoelectric (TE) materials, but maintaining the good electrical properties while reducing lattice thermal conductivity is challenging. Entropy engineering could be a promising way to synergistically regulate the electrical and thermal transport in order to improve the thermoelectric performance of SrTiO₃-based ceramics. In this work, single cubic structured Sr_0.9−xBa_xLa_0.1Ti_0.9Nb_0.1O_3−δ (x = 0.1, 0.2, 0.3, 0.4, 0.5) is designed by the calculations of Goldschmidt's tolerance factor, ionic radius difference and the detailed thermodynamic process and successfully synthesized by a solid-state reaction. The single cubic structure and homogenous element distribution are confirmed by Rietveld refinement of XRD and EDS mapping. Oxygen deficiency and high crystal symmetry lead to good electrical properties derived by the effect of configurational entropy, which results in a high power factor, PF = 602.1 μW m⁻¹ K⁻². The enhanced point defect and local disorder scattering is the dominant factor to significantly reduce the lattice thermal conductivity, especially for large mass fluctuation driven by the increase of point defects, which is a typical result of entropy effects. As a result, the lowest lattice thermal conductivity of 2.93 W m⁻¹ K⁻¹ at 923 K is achieved for the Sr_0.4Ba_0.5La_0.1Ti_0.9Nb_0.1O_3−δ sample. By the synergistic optimization of entropy engineering, a highest zT value, 0.15, is achieved for the x = 0.2 sample at 973 K. The estimated corresponding optimized configurational entropy ranges from 9 to 10 J mol⁻¹ K⁻¹. Besides, Vickers hardness of 7–11 GPa is also measured for all samples. Henceforth, entropy engineering is a promising method for optimizing the thermoelectric and mechanical performance of SrTiO₃-based ceramics, which opens a new way to design thermoelectric oxide materials with good electrical properties and low lattice thermal conductivity.