  We have investigated the substitution effect of Eu on the superconductivity
in La2-xEuxO2Bi3Ag0.6Sn0.4S6. Recently, we reported an observation of
superconductivity at 0.5 K in a layered oxychalcogenide La2O2Bi3AgS6. The Sn
doping at the Ag site was found to raise the superconducting transition
temperature, Tc to 2.5 K in La2O2Bi3Ag0.6Sn0.4S6. To further improve the
superconducting properties, we have partially substituted Eu for the La site to
increase the chemical pressure in La2-xEuxO2Bi3Ag0.6Sn0.4S6 (x = 0.1 to 0.6).
With the increase in Eu concentration, x, the lattice constant a was found to
shrink, while the lattice constant c was marginally shortened, which suggests
that the chemical pressure induced by the Eu doping is uniaxial along the
a-axis. Tc was observed to increase with increasing x up to x = 0.4, further
decreasing for higher Eu concentrations of x = 0.5 and 0.6. From the magnetic
susceptibility and resistivity measurements, the bulk nature of
superconductivity has been observed for x = 0.1 to 0.5 with Tc = 2.5 to 4.0 K,
respectively. The upper critical field (Bc2) was noted to be 3.5 T for x = 0.4,
which also has the highest Tc.
