Evidence of a mixed-valence state for europium in the misfit layer compound [(EuS)1.5]1.15NbS2 by means of a superspace structural determination, Mossbauer spectroscopy, and magnetic measurements

The complete structure of the misfit layer compound [(EuS)(1.5)](1.15)NbS2 is described as a (3+1)-dimensional intergrowth compound with two modulated subsystems (v = 1, 2, for NbS2 and EuS, respectively) which have the (a(v2)*, a(v3)*) reciprocal-lattice plane in common. A superspace embedding follows from four reciprocal-lattice vectors given by a(1)* = a(11)*, a(2)* = a(12)*, a(3)* = a(23)* and a(4)* = a(21)*. The symmetry is given by the superspace group G(s)=Fm2m (alpha 00) with modulation wave vector q = a(21)* = alpha a(11)* (alpha = 0.5768(1)). The structure of this composite compound is built up of an alternated sequence of two different types of slabs (i.e., NbS2 (H-part) and EuS (e-part)), stacked along the c direction. The e-part is a three-atom-thick layer (EU2S2-Eu1S1-Eu2S2), The NbS2 subsystem has cell parameters a(11) = 3.3241(4)Angstrom, a(12) = 5.7841(5)Angstrom, and a(13) = 14.816(2)Angstrom, with space group Cm2m. The EuS subsystem has lattice parameters a(21) = 5.7628(5)Angstrom, a(22) = 5.7841(5) Angstrom, and a(23) = 29.632(4) Angstrom, with space group Fm2m. Refinements on 1721 unique reflections converged to R = 3.58% (R-w = 4.75%) on a modulated structure model with 50 parameters. The largest modulation amplitudes occur on the outer Eu2 atoms. A bond-valence calculation using results from this structure determination shows that Eu atoms on the exterior sides (Eu2) of the EuS slab are in the +11 oxidation state while inner Eu atoms (Eu1) are at the +III state. Mossbauer and magnetic measurements confirm the mixed-valence (Eu2+/Eu3+) state of this compound. A ferromagnetic long-range order appears below 7 K. (C) 1999 Academic Press.