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, M?ssbauer Spectroscopy, and Magnetic Measurements

The complete structure of the misfit layer compound [(EuS)1.5]1.15NbS2 is described as a (3+1)-dimensional intergrowth compound with two modulated subsystems (nu=1, 2, for NbS2 and EuS, respectively) which have the (a*nu 2, a*nu 3) 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 Gs=Fm2m (alpha00) 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 (Q-part)), stacked along the c direction. The Q-part is a three-atom-thick layer (Eu2S2-Eu1S1-Eu2S2). The NbS2 subsystem has cell parameters a11=3.3241(4) ?, a12=5.7841 (5) ?, and a13=14.816(2) ?, with space group Cm2m. The EuS subsystem has lattice parameters a21=5.7628(5) ?, a22= 5.7841(5) ?, and a23=29.632(4) ?, with space group Fm2m. Refinements on 1721 unique reflections converged to R= 3.58% (Rw=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 +II oxidation state while inner Eu atoms (Eu1) are at the +III state. M?ssbauer and magnetic measurements confirm the mixed-valence (Eu2+/Eu3+) state of this compound. A ferromagnetic long-range order appears below 7 K.