B-site selective doping in Ba2Ca1-xLaxMoO6-δ: Synergistic optimization of phonons and electrons for thermoelectrics

B-site La doping co-tunes electron-phonon transport in Ba 2 CaMoO 6 double perovskite. • La doping brings distortion, fine grains and dislocations to lower lattice κ. • Aliovalent La doping yields Mo mixed valence vacancies for degeneracy transition. • x = 0.1 sample delivers 68.7% higher ZT than A-site doped materials. This work reports the B-site selective La 3+ doping in the double-perovskite oxide Ba 2 Ca 1-x La x MoO 6-δ (x = 0, 0.05, 0.1, 0.15, 0.2) and its synergistic regulation of phonon and electron transport for thermoelectric optimization. The ceramics were synthesized through a combination of the conventional solid-state reaction and high temperature reduction. XRD, Rietveld refinement, and TEM confirm the presence of a pure cubic Fm-3m phase up to x = 0.15. However, when x = 0.2, it exceeds the solid solubility limit and forms a La-rich secondary phase. The substitution of La 3+ at Ca 2+ sites induces lattice contraction, octahedral tilting, and high-density dislocations, which effectively strengthen phonon scattering and reduce the lattice thermal conductivity. XPS verifies charge compensation through the formation of Mo 6+ /Mo 4+ mixed valence and oxygen vacancies. Electrical transport measurements reveal an n-type semiconducting behavior. La doping increases the carrier concentration and optimizes the weighted mobility, with a semiconductor to degenerate transition occurring at x ≥ 0.15. The x = 0.1 sample achieves the maximum power factor of 45.2 μW m −1  K −2 at 1073 K and the lowest thermal conductivity of 0.93 W m −1  K −1 , resulting in a peak ZT of ∼0.05 at 1073 K, which is improved by 68.7% compared to A-site doped counterparts. These results demonstrate that B-site selective doping enables the synergistic optimization of electrical and thermal transport, providing a new defect engineering strategy for high temperature oxide thermoelectrics.