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 (κlat). 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 (Ef) toward the theoretical optimum. Ca1.05CuP maintains its weighted mobility (µW), 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 µW reductions yet demonstrate that Ef can be tuned almost continuously by stoichiometric engineering. Collectively, these results establish host‐cation stoichiometry control as a pathway for continuous Ef engineering and provide practical guidelines for designing phosphide thermoelectrics.