Diagnostic Imaging

Valence instability of europium in EuCo2Si2

“…Such disorder was suggested to be possible for EuNi 2 Si 2 [6]. A change of stoichiometry in EuNi 2−x Si 2+x and EuCu 2−x Si 2+x has a strong influence on the europium valence [33,34], so local exchange of silicon and cobalt in EuCo 2 Si 2 can have a similar influence. Europium ions in the clusters order magnetically at 73(1) K, this transition being of second order, and a second transition -presumably of first order -occurs at 21(2) K, possibly because of spin reorientation.…”

“…Only structural information is available for Eu 2 Si [29] and for ∼Eu 4 Si [10], ∼Eu 3 Si [30] and ∼Eu 7 Si 3 [10] even those data are unavailable. No magnetic information is available for EuCo 1.5 Si 2.5 , EuCo 0.33 Si 1.67 , EuCo 9 Si 2 and EuCo 9.8 Si 3.2 , but it seems doubtful that they were would order at 73(1) and at 21(2) K: EuCo 0.33 Si 1.67 and EuCo 9 Si 2 were demonstrated to have a dominant divalent valence state of europium [31,32], replacement of the transition metal by silicon stabilises divalent europium in EuNi 2−x Si 2+x [33] and in EuCu 2−x Si 2+x [34], so europium is probably (mostly) divalent in EuCo 1.5 Si 2.5 as well, thus one can compare them with similar isoelectronic gadolinium compounds of close stoichiometry (GdCoSi 3 [35] considered together with GdCo 2 Si 2 [11], GdCo 0.4 Si 1.6 [11] and GdCo 9 Si 2 [36]), which exhibit different magnetic behaviour. Although no information on the valence state of europium in EuCo 9.8 Si 3.2 is available, the reasoning presented above holds probably also for this compound, when compared with GdCo 9.5 Si 3.5 [37]-if europium is divalent, it is isolectronic with gadolinium, and if it is trivalent, it does not order magnetically for sure and it is very doubtful for eventual magnetic moments on cobalt to order at such low temperatures in such a cobalt-rich compound.…”