Valence instability of europium in EuCo2Si2

EuCo2Si2 intermetallic compound was synthesised and characterised by X-ray powder diffraction, electric resistivity, magnetic susceptibility and X-ray photoelectron spectroscopy. No sign of magnetic ordering could be depicted from the temperature dependence of electrical resistivity, whereas two ferromagnetic-like transitions were observed in the temperature dependence of the magnetic susceptibility: at 73(1) and at 21(2) K. This magnetic behaviour could be attributed to small clusters containing divalent europium ions originating from local chemical disorder embedded in a matrix containing trivalent europium ions (effective europium valence was found to be 2.94(1)). X-ray photoelectron spectroscopy study indicates a surface valence transition and a dramatic change of europium valence caused by Ar+ sputtering, probably due to local disorder introduced.