Mixed-valence state of europium in the misfit layer compound (EuS)1.173NbS2

A new misfit layer compound (EuS)(1.173)NbS2 was synthesized, and its structure was solved :in a (3+1)D superspace using the superspace group Xm2m(alpha00) with X referring to a pseudo F centering. The cell parameters are a(s) = 3.3251(9) Angstrom, bs = 5.7432(9) Angstrom, and c(s) = 22.939(5) Angstrom (V-s = 438.1(3) Angstrom) with a modulation vector q = 0.5867(2) a(s)*. This compound is built from a regularly alternated stacking of [EuS] slabs (NaCl structure type with two atomic layers) and [NbS2] slabs (NbS2 structure type). Mossbauer and XPS spectroscopies as well as magnetic measurements clearly indicate a mixed-valence state for europium with a ratio EU2+/EU3+ close to 40:60. Bond valence calculations taking into account the modulation of the Eu-S distances ascertain the mixed valence state of europium and indicate that Eu2+ and Eu3+ ions occupy the same crystallographic site. Moreover, Mossbauer spectra demonstrate that a charge hopping between Eu2+ and Eu3+ takes place above room temperature. The misfit layer (EuS)(1.173)NbS2 can be considered as an inhomogeneous mixed-valence compound. Resistivity measurements show that (EuS)(1.173)NbS2 behaves as a metal and at 3.5 K a strong decrease of the resistivity is observed conjointly with a ferromagnetic transition.