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
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  and X-ray absorption spectroscopy , 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.
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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