Local‐distortion regulation of small‐polaron transport: A design degree of freedom for thermoelectric oxides
Yang Zhang, Chengpeng Dong, Huimin Chen, Yihua Zhang, Guyang Peng, Yuxuan Yang, Lu Chen, Minghua Yang, Zhihao Zhao, Tong Song, Chaoliang Zhang, Tianle Xie, Kangjin Zhou, Kun Wang, Yushan Guo, Zhi Cao, Faming Cai, Fei Li, Haijun Wu, Jun Sun, Xiangdong DingAbstract
Oxide‐based thermoelectric materials offer low cost, low pollution, and exc ellent intrinsic thermal stability, showing great promise for high‐temperature power generation. However, their large metal–oxygen electronegativity contrast often limits thermoelectric performance and co mplicates electron–phonon decoupling. Here, we investigate dopant‐specific local distortion as a design degree of freedom for regulating small‐polaron trapping and transport in CaMnO 3 (CMO). Ce‐ and La‐doped CMO were designed with comparable nominal electron doping and similar Hall carrier concentrations but distinctly different local strain backgrounds. The large Ce–Ca radius mismatch ( R mismatch = 18%) generates pronounced and spatially concentrated distortion, which is associated with stronger small‐polaron trapping, a higher hopping activation energy, and a larger mobility penalty. In contrast, the nearly matched La and Ca ionic radii ( R mismatch < 1%) produce a milder and more broadly distributed distortion landscape, enabling lower‐barrier polaron hopping and a more favorable balance between the polaron‐related Seebeck contribution and carrier mobility. Atomic‐resolution STEM further reveals nanometer‐scale coherent Mn displacement fields spatially correlated with dopant enrichment and local lattice distortion, which may provide additional phonon‐scattering centers together with conventional defect‐ and strain‐related scattering. Consequently, although La‐CMO exhibits higher lattice thermal conductivity than Ce‐CMO, its improved electrical transport leads to increases of approximately 27% in PF avg and 17% in zT avg . These results establish the local trapping environment of small polarons as an experimentally accessible design degree of freedom for optimizing polaronic thermoelectric oxides.