Thermoelectric Properties of Polycrystalline NiSi3P4

The Hall and Seebeck coefficients, electrical resistivity and thermal conductivity of polycrystalline NiSi3P4 were characterized from 2 to 775K. Undoped NiSi3P4 behaves like a narrow gap semiconductor, with activated electrical resistivity \rho below room temperature and a large Seebeck coefficient of ~400uV/K at 300K. Attempts to substitute boron for silicon resulted in the production of extrinsic holes, yielding moderately-doped semiconductor behavior with \rho increasing with increasing temperature above ~150\,K. Hall carrier densities are limited to approximately 5x10^{19}/cm^3 at 200K, 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 ~12cm^2/V/s at 300K, 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 ~5W/m/K at 700K. Alloying in Ge results in a slight reduction of the thermal conductivity at low temperatures, with little influence observed at higher temperatures.