Superconducting properties of a mixed anion layered compound, Ca and F co-doped LaFeAsO with T c = 31.5 K

Polycrystalline Ca and F co-doped LaFeAsO samples (La1-xCaxFeAsO1-yFy) were synthesized by solid-state reactions. Samples with nominal composition: (y - x) <= 0.05 did not show superconductivity at temperatures T> 42 K, although an anomalous kink of the rho-T curves, which is related to crystallographic phase transitions, was observed at T similar to 150 K. Samples with nominal composition: (y - x) >= 0.15 did exhibit superconductivity. The superconducting transition temperature (T-c) of co-doped LaFeAsO with nominal composition: (x, y) = (0.25, 0.50) was determined to be T-c = 31.5 K, which is higher than the highest T-c value observed for LaFeAs1-xFx materials synthesized under ambient pressure. (C) 2019 The Japan Society of Applied Physics