This study investigates the electron transport properties of mother NiSi3P4 and Ga-substituted NiSi3−xGaxP4 (x = 0.125 and 0.25), both experimentally and theoretically. The experimental solubility limit of NiSi3−xGaxP4 is x ≈ 0.25. For x = 0, the ρ vs T curve shows a −log T dependence below 70 K because the conduction holes are localized by Anderson localization originating from the random potential generated by Ni defects. For Ga-substituted NiSi3−xGaxP4 (x = 0.125 and 0.25), the experimental electrical resistivity ρ, Seebeck coefficient S, and Hall coefficient RH decrease with increasing x. These results indicate that the hole concentration p increases upon Ga substitution. With increasing x, the power factor PF increases dramatically with decreasing ρ0 and increasing hole concentration p due to Ga substitution. For x = 0.25, PF reaches the high value of 0.1 mW−2 m−1 K−1 at 300 K. To calculate the density of states and decompose the E–k relations for x = 0 and 0.125, the degenerate Ni-3d, P-3p hybrid orbital dominates near the top of the valence band. At x = 0.125, the chemical potential is lower than that at x = 0, indicating that hole doping occurs through Ga substitution. The theoretical maximum PF is less than 1.2 mW−2 m−1 K−1 at 300 K, which originates from the degenerate Ni-3d, P-3p hybrid orbital.

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