We synthesized single phase ${\text{La}}_{0.85}{\text{Sr}}_{0.15}{\text{FeAsO}}_{1-\delta }$ samples and systematically studied the effect of oxygen deficiency in this system. It is found that partial substitution of Sr for La induces the hole carrier evidenced by positive thermoelectric power (TEP) but no bulk superconductivity is observed. The superconductivity can be realized by annealing the as-grown sample in vacuum to produce the oxygen deficiency. With increasing oxygen deficiency, the superconducting transition temperature $\left(T_c\right)$ increases and the maximum $T_c$ reaches 26 K—the same as that observed in the ${\text{LaFeAsO}}_{1-x}{\text{F}}_x$ system. TEP changes the sign from positive for the nonsuperconducting as-grown sample to negative for the superconducting samples with oxygen deficiency, while $R_H$ keeps negative for all samples. It suggests that the dominated carrier in ${\text{La}}_{0.85}{\text{Sr}}_{0.15}{\text{FeAsO}}_{1-\delta }$ is electronlike.

• Received 10 June 2008

DOI:https://doi.org/10.1103/PhysRevB.78.092503