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 tha n or equal to3, has been prepared and various thermoelectric properties mea sured from 5 to 900 K. Unlike most thermoelectric materials that are band s emiconductors with high carrier mobilities, FexCr3-xSe4 are polaronic semic onductors 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 thermoelectri c, 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 physic al properties at x=1.5. The electronic transport measurements are consisten t with small polaron conductivity, both in the high-temperature activated r egion, and low-temperature phonon-assisted hopping region proportional to e xp(-T-1/4). The magnetic properties are complex and only an approximate pha se diagram is given. Only small discontinuities are observed in the transpo rt 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-ty pe and about x=0.5 for n-type. The highest ZT value of 0.15 was found in p- type samples at 525 K.