Valence instability of europium in EuCo2Si2

Paweł Maślankiewicz ,  Jacek Szade 

University of Silesia, August Chełkowski Institute of Physics, Department of Solid State Physics, Uniwersytecka 4, Katowice 40-007, Poland

Abstract

EuM2X2 intermetallics (where M = transition metal and X = Si, Ge or a mixture thereof) are well known for phenomena of intermediate or unstable valence of europium. Among these compounds, EuCo2Si2 was studied by Mössbauer spectroscopy [1] and X-ray absorption spectroscopy [2], which revealed an effective valence of europium close to +3. We have synthesised polycrystalline samples of EuCo2Si2 by multiple induction melting in a levitation crucible under a protective atmosphere of argon. A surplus of europium was necessary to compensate evaporation losses. As-cast samples were basically single-phase (ThCr2Si2-type orthorhombic structure, a = 3.9217(8) Å, c = 9.834(2) Å). Bulk methods such as magnetic susceptibility and comparison of lattice parameters with those of other RECo2Si2 (RE = rare earth) compounds suggest that most of europium in our EuCo2Si2 is trivalent, while X-ray photoelectron spectroscopy measurements performed on samples cleaned by argon ion etching in ultra-high vacuum indicate a divalent oxidation state of europium. This discrepancy can be attributed to local chemical changes induced by Ar+ sputtering or to a difference between valence of bulk and surface europium. A weak ferromagnetic-like transition was detected at 74 K.

[1] B. Perscheid, I. Nowik, G. Wortmann, G. Schmiester, G. Kaindl and I. Felner, Z. Phys. B, Condens. Matter 73, 511 (1989).

[2] N.D. Samsonova, L.D. Finkel’shtein and E.M. Levin, Fiz. Tverd. Tela 24, 3711 (1982).

    N.D. Samsonova, L.D. Finkel’shtein and E.M. Levin, Sov. Phys. - Solid State 24, 2116 (1982).

 

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Presentation: poster at E-MRS Fall Meeting 2005, Symposium B, by Paweł Maślankiewicz
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-17 13:23
Revised:   2009-06-07 00:44