Thermoelectric Properties of LaFePO1-xFx and LaFeAsO1-xFx-Possibility of the Hidden Mass Enhancement of LaFeAsO1-xFx

We investigated thermoelectric properties for polycrystalline oxypnictide LaFePO1-xFx and LaFePO1-xFx. The temperature (T) and F-doping dependences of resistivity (rho) and Seebeck coefficient (S) are quite different between these compounds. In contrast to a monotonic T dependence and an absence of large F-doping dependences of rho and S for LaFePO1-xFx, the S for LaFePO1-xFx for 0 < x <= 0.14 take relatively large maximum absolute values of -60 to -150 mu V/K around 90-180 K, while the T dependence of rho dramatically changes and the rho monotonically decreases with the F-doping, leading to a large power factor of about 37 mu W/K-2 cm at 53 K and a dimensinless thermoelectric figure of merit (ZT) of about 0.08 at 110 K for LaFeAsO0.89F0.11. The non-monotonic behavior of S with the relatively large absolute value and the reported T dependence of Hall coefficient for LaFePO1-xFx seem to evidence an instability of an orbital order accompanying a stripe-type AF order even in the over F-doping region for LaFePO1-xFx and indicate a possibility of a hidden mass enhancement of LaFePO1-xFx.