Electrical transport properties of F-doped LaFeAsO oxypnictide

A series of LaFeAsO_1-xF_x (x = 0-0.225) oxyarsenides have been synthesized by a solid-state reaction method in order to optimize electrical transport properties through appropriate F doping. Both electrical resistivity and Seebeck coefficient of undoped LaFeAsO show an anomaly at about 150 K, which is related to a structural phase and/or spin-density-wave (SDW) transition. Seebeck coefficient seems to be determined by two competitive factors: it is enhanced by suppressing the structural phase and/or SDW transition, and reduced by increasing carrier concentration. Seebeck coefficient is significantly enhanced just after suppressing the anomaly, and the maximum Seebeck coefficient reached -142 μV/K for the sample with F doping x = 0.075 from -58 μV/K for undoped LaFeAsO, and then decreased with further increasing carrier concentration through F doping. Meanwhile, the electrical resistivity is decreased with increasing F doping, resulting in a maximum power factor value of 1.2 mW/mK² at 80 K for polycrystalline LaFeAsO _0.85F_0.15 sample_, this value is the same order as that of the best low temperature thermoelectric Bi₈₈Sb ₁₂ compounds, and could be significantly higher in single crystals.