  We have synthesized AgSn1-xBixSe2 polycrystalline samples to investigate the
effect of partial substitution of mixed-valence Sn by Bi3+ to the
superconductivity of the valence-skip superconductor AgSnSe2. The
Bi-substituted AgSn1-xBixSe2 were obtained up to x = 0.2, but an insulating
phase Ag2SnBi2Se5 with a NaCl-type structure showed up above x = 0.3. The
superconducting transition temperature increased from 4.5 K (x = 0) to 5.0 K (x
= 0.1) by Bi substitution. The enhancement of superconductivity by the
suppression of the valence-skip states of Sn suggests that the valence-skip
states of Sn are not positively linked to the pairing mechanisms of
superconductivity in the AgSnSe2 system.
