Thermoelectric enhancement in A-site deficient high-entropy perovskite (Sr0.25Ca0.25La0.25Ba0.25)1-xTiO3±δ ceramics by fine manipulating cation vacancies

SrTiO3-based perovskite structured oxides have attracted much attention as an n-type thermoelectric material over a wide temperature range. An enhancement in performance requires strategies that simultaneously optimize electron and phonon transports. In this work, a series of A-site cation vacancies were introduced into a high-entropy system to fabricate A-site deficient (Sr0.25Ca0.25La0.25Ba0.25)1-xTiO3±δ(0＜x ≤ 0.125) samples which optimize configuration entropy ranges 10.08–11.24 J mol−1K−1 and simultaneously manipulate electronic and phonon transports properties. Most importantly, the direct evidence of positron annihilation and iDPC-STEM results unambiguously confirms these A-site cation vacancies. Consequently, carrier mobility was improved by 16.5 times which significantly contributed to an increase in the electrical conductivity. The low lattice thermal conductivity of 2.54 W m−1K−1 was obtained at 1073 K which was significantly reduced by 21.4% compared to the majority values of SrTiO3-based ceramics. The maximum ZT = 0.24 was attained for the x = 0.100 sample with an enhancement of 27.8% than that of stoichiometric ceramic due to the synergistic effect of higher PF and relatively low κ. This study reveals that in a distorted, high-entropy perovskite structure, multiple band degeneracy is advantageous for enhancing the electrical characteristics due to the delocalized electrons that result from enhanced crystal symmetry. In the meanwhile, anharmonicity and localized phonons inhibited the propagation of transverse phonons resulting in a markedly reduced lattice thermal conductivity. This feasible strategy of defect engineering combined with entropy manipulation demonstrates that it is not necessary to simply pursue a much higher configuration entropy in TE ceramics to optimal ZT values.