Reentrant valence transition in EuO at high pressures: beyond the   bond-valence model
  The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at $\approx$ 45 GPa is accompanied by a reentrant Eu valence transition into a $\emph{lower}$ valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure. 
N. M. Souza-Neto and J. Zhao and E. E. Alp and G. Shen and S. V.   Sinogeikin and G. Lapertot and D. Haskel
cond-mat.str-el cond-mat.mtrl-sci