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, Mössbauer Spectroscopy, and Magnetic Measurements
✍ Scribed by L. Cario; A. Lafond; P. Palvadeau; C. Deudon; A. Meerschaut
- Publisher
- Elsevier Science
- Year
- 1999
- Tongue
- English
- Weight
- 273 KB
- Volume
- 147
- Category
- Article
- ISSN
- 0022-4596
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✦ Synopsis
The complete structure of the mis5t layer compound [(EuS) 1.5 ] 1.15 NbS 2 is described as a (3؉1)-dimensional intergrowth compound with two modulated subsystems ( ؍ 1, 2, for NbS 2 and EuS, respectively) which have the (a* 2 , a* 3 ) reciprocallattice 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 ( 00) with modulation wave vector q ؍ a* 21 ؍ a* 11 ( ؍ 0.5768(1)). The structure of this composite compound is built up of an alternated sequence of two di4erent types of slabs (i.e., NbS 2 (H-part) and EuS (Q-part)), stacked along the c direction. The Q-part is a three-atom-thick layer (Eu2S2-Eu1S1-Eu2S2). The NbS 2 subsystem has cell parameters a 11 ؍ 3.3241(4) A s , a 12 ؍ 5.7841(5) A s , and a 13 ؍ 14.816(2) A s , with space group Cm2m. The EuS subsystem has lattice parameters a 21 ؍ 5.7628(5) A s , a 22 ؍ 5.7841(5) A s , and a 23 ؍ 29.632(4) A s , with space group Fm2m. Re5nements on 1721 unique re6ections 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 ؉II oxidation state while inner Eu atoms (Eu1) are at the ؉III state. MoK ssbauer and magnetic measurements con5rm the mixed-valence (Eu 2؉ /Eu 3؉ ) state of this compound. A ferromagnetic long-range order appears below 7 K.