Thermoelectric properties of polycrystalline NiSi3P4

The Hall and Seebeck coefficients, electrical resistivity, and thermal conductivity of polycrystalline NiSi₃P₄ were characterized from 2 to 775 K. Undoped NiSi₃P₄ behaves like a narrow gap semiconductor, with activated electrical resistivity ρ below room temperature and a large Seebeck coefficient of ∼400 μV/K at 300 K. Attempts to substitute boron for silicon resulted in the production of extrinsic holes, yielding moderately doped semiconductor behavior with ρ increasing with increasing temperature above ∼150 K. Hall carrier densities are limited to approximately 5 × 10¹⁹ cm^-3 at 200 K, which would suggest the solubility limit of boron is reached if boron is indeed incorporated into the lattice. These extrinsic samples have a Hall mobility of ∼12 cm²/V/s at 300 K, and a parabolic band equivalent effective mass of ∼3.5 times the free electron mass. At 700 K, the thermoelectric figure of merit zT reaches ∼0.1. Further improvements in thermoelectric performance would require reaching higher carrier densities, as well as a mechanism to further reduce the lattice thermal conductivity, which is ∼5 W/m/K at 700 K. Alloying in Ge results in a slight reduction of the thermal conductivity at low temperatures, with little influence observed at higher temperatures.