  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.
