Thermoelectric Properties of LaFePO_1-<i>x</i>F_<i>x</i>and LaFeAsO_1-<i>x</i>F_<i>x</i>–Possibility of the Hidden Mass Enhancement of LaFeAsO_1-<i>x</i>F_<i>x</i>–

Abstract

We investigated thermoelectric properties for polycrystalline oxypnictide LaFePO 1- x F x and LaFeAsO 1- x F x . The temperature ( T ) and F-doping dependences of resistivity (ρ) 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 ρ and S for LaFePO 1- x F x , the S for LaFeAsO 1- x F x for 0< x ≤0.14 take relatively large maximum absolute values of -60 to -150 µV/K around 90–180 K, while the T dependence of ρ dramatically changes and the ρ monotonically decreases with the F-doping, leading to a large power factor of about 37 µW/K 2 cm at 53 K and a dimensinless thermoelectric figure of merit ( Z T ) of about 0.08 at 110 K for LaFeAsO 0.89 F 0.11 . The non-monotonic behavior of S with the relatively large absolute value and the reported T dependence of Hall coefficient for LaFeAsO 1- x F x seem to evidence an instability of an orbital order accompanying a stripe-type AF order even in the over F-doping region for LaFeAsO 1- x F x and indicate a possibility of a hidden mass enhancement of LaFeAsO 1- x F x .