  We report the effect of Se substitution on the crystal structure and
superconductivity of BiCh2-based (Ch: S, Se) layered compounds
Eu0.5Ce0.5FBiS2-xSex (x = 0-1). Crystal structure analysis showed that both
lattice constants, a and c, increased with increasing x, which is different
from the related La-doped system, such as Eu0.5La0.5FBiS2-xSex. This is due to
Se substitution for both in-plane and out-of-plane Ch sites in the present
Ce-doped system. Zero resistivity was observed for x = 0.2-1 above 2 K.
Superconducting properties of Eu0.5Ce0.5FBiS2-xSex was investigated by magnetic
susceptibility, and the highest superconducting transition temperature of 3.5 K
was obtained for x = 0.6 with a large shielding volume fraction. The emergence
of bulk superconductivity and metallic conductivity can be qualitatively
described in terms of increased in-plane chemical pressure effect. A magnetic
anomaly below 8 K, probably because of ferromagnetic order of magnetic moment
of Ce3+ ions, coexists with bulk superconductivity in BiCh2 layer. Since the
effect of Se substitution on the magnetic transition temperature is ignorable,
we suggest that the coupling between the magnetic order at the (Eu,Ce)F layer
and superconductivity at the Bi(S,Se)2 layer is weak.
