High-pressure synthesis of the indirectly electron-doped iron pnictide superconductor Sr_1-x La_x Fe₂ As₂ with maximum T_c =22 K

Compounds of Sr_1-x La_x Fe₂ As₂ were synthesized by solid-state reaction at 1273 K, under pressures of 2 or 3 GPa. The Sr_1-x La_x Fe₂ As₂ phase was dominant up to x=0.5 and superconductivity was observed at x≥0.2. A maximum critical temperature T_c of 22 K and a maximum shielding volume fraction of ∼70% were obtained at x=0.4. This is the first experimental demonstration of electron-doped superconductivity in 122-type iron pnictides, where electrons were doped through aliovalent substitution at the site of the alkaline-earth metal. The optimal T_c was slightly higher than that for the Co-doped (directly electron-doped) samples (19 K) and much lower than that for the hole-doped case (37 K). The bulk superconductivity range was narrower than that for the Co-substituted case and both ranges were much narrower than that for the hole-doped case. These observations revealed that differences in the electron-doping mode (direct or indirect) did not have a prominent effect on the optimal T_c or superconductivity range, compared with differences in carrier polarity.