Thermoelectric, transport, and magnetic properties of the polaron semiconductor FexCr3-xSe4

The series of compounds FexCr3-xSe4, with 0 less than or equal tox less than or equal to3, has been prepared and various thermoelectric properties measured from 5 to 900 K. Unlike most thermoelectric materials that are band semiconductors with high carrier mobilities, FexCr3-xSe4 are polaronic semiconductors with Hall mobility values, over 100 times smaller. Nevertheless, FexCr3-xSe4 has a surprisingly high thermoelectric figure of merit-within a factor of 5 of state-of-the-art materials. Advantageous as a thermoelectric, FexCr3-xSe4 can be heavily doped both n- and p-type, switching at x=0.75. Although a nearly complete solid solution appears to exist in FexCr3-xSe4, a small but distinct discontinuity was found in the structural and physical properties at x=1.5. The electronic transport measurements are consistent with small polaron conductivity, both in the high-temperature activated region, and low-temperature phonon-assisted hopping region proportional to exp(-T-1/4). The magnetic properties are complex and only an approximate phase diagram is given. Only small discontinuities are observed in the transport data at the magnetic transitions. The thermal conductivity of the solid solutions are relatively low, with a glasslike temperature dependence. The thermoelectric figure of merit (ZT) peaks at compositions of x=0.9 for p-type and about x=0.5 for n-type. The highest ZT value of 0.15 was found in p-type samples at 525 K.