  Compounds of Sr1-xLaxFe2As2 were synthesized by solid state reaction at 1273
K, under pressures of 2 or 3 GPa. The Sr1-xLaxFe2As2 phase was dominant up to x
= 0.5, and superconductivity was observed at $x \ge 0.2$. A maximum critical
temperature Tc 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 Tc 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 Tc or superconductivity range, compared with differences in carrier
polarity.
