Science, Editors’ Choice, em 3/08/2012 Volume 337
Many chemical elements display different valence states in the different compounds that they form. Some, like rare earths and actinides, can also have a mixed valence state within a single compound if the difference in energies between the competing states is small. Often, the valence state is deduced from structural data by assuming a simple dependence on the bond lengths. Souza-Neto et al. demonstrate that there is a big discrepancy between the results obtained from this bond-valence model and the direct measurements of the valence state of Eu in the compound EuO under pressure. At about 44 GPa, this oxide undergoes a transition from an NaCl-like to a CsCl-like structure, wherein the two phases coexist; above 59 GPa, only the CsCl phase remains. The authors found a mixture of Eu2+and Eu3+ in the intermediate-pressure regime, and a near recovery of a homogeneous Eu2+ valence state above 59 GPa, despite a 7% volume collapse associated with the structural transition; the bond-valence model based on structural data predicts a much higher valence. The results support probing of both structure and electronic states in mixed-valence systems. — JS Phys. Rev. Lett. 109, 026403 (2012).