Simultaneous Fermi Level and Weighted Mobility Engineering in CaCuP-Based Thermoelectrics via Multi-Route Compositional Tuning

Ternary metal phosphides emerge as promising thermoelectric materials due to their earth-abundant constituents and inherently complex crystal structures, which favor low lattice thermal conductivity (κ_lat). Here, three routes (slight Ca excess, Zn²⁺, and La³⁺ substitution) are investigated to span a broad carrier concentration range, combined with a single parabolic band (SPB) model, confirming that each route shifts Fermi level (E_f) toward the theoretical optimum. Ca_1.05CuP maintains its weighted mobility (µ_W), delivering the highest power factor (≈1.83 mW·m⁻¹·K⁻²) and a zT of ≈0.45 at 823 K. By contrast, Zn- or La-substituted samples experienced modest µ_W reductions yet demonstrate that E_f can be tuned almost continuously by stoichiometric engineering. Collectively, these results establish host-cation stoichiometry control as a pathway for continuous E_f engineering and provide practical guidelines for designing phosphide thermoelectrics.