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

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We investigated thermoelectric properties for polycrystalline oxypnictide LaFePO1-xFx and LaFeAsO1-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 LaFeAsO1-xFx for 0<x\leq 0.14 take relatively large maximum absolute values of -60 to -150 μV/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 μW/K2 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 LaFeAsO1-xFx seem to evidence an instability of an orbital order accompanying a stripe-type AF order even in the over F-doping region for LaFeAsO1-xFx and indicate a possibility of a hidden mass enhancement of LaFeAsO1-xFx.
2011-04-15