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 $\approx 45\mathrm{GPa}$ is accompanied by a reentrant Eu valence transition into a 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.

Figure 1

Pressure-volume dependence of EuO up to 92 GPa obtained from Rietveld refinements of XRD data. The inset panel shows a modest iso-structural volume collapse at about 35 GPa ($B1$ phase). From 44 to 59 GPa a mixture of NaCl ($B1$) and CsCl ($B2$) phases is present. Above 59 GPa a CsCl structural phase is homogeneous over the sample volume. Two schematic figures show the local coordination in NaCl and CsCl phases.

Figure 2

Eu $L_3$ XANES measurements on EuO up to 80 GPa. The bottom panel compares spectra for ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ ions in EuO and ${\mathrm{Eu}}_2{\mathrm{O}}_3$, respectively. An $\approx 8\mathrm{eV}$ threshold energy difference unequivocally distinguishes the two valence states. The top panel shows XANES spectra in the 4 to 40 GPa pressure range where fractional [$P<14\mathrm{GPa}$] and mixed valence [$14<P<40\mathrm{GPa}$] states are observed, with translated spectra showing the reentrant valence behavior at higher pressures [45–60 GPa] coincident with the volume collapse transition. The 4 GPa spectrum was taken upon pressure release indicating reversibility of the electronic transition.

Figure 3

Synchrotron Mössbauer spectra of EuO under pressure, collected using an ${\mathrm{Eu}}_2{\mathrm{O}}_3$ sample at ambient pressure as reference, are shown in the time domain in the left panel together with fits to the data. Fit results were used to simulate spectra in the energy domain shown in the right panel. Red peaks correspond to the ${\mathrm{Eu}}_2{\mathrm{O}}_3$ reference located after the diamond anvil cell, blue peaks correspond to the $B1$ phase of EuO, and green peaks to the $B2$ phase. A complete data set for all pressures is given with the Supplemental Material [15].

Figure 4

Europium valence determined by XANES, NFS, and bond-valence-parametrization is shown in the left axis. The IS determined by NFS is shown in the right axis. Valence from NFS is obtained after volume correction based on the $B1$ phase compressibility [25]. XANES and NFS results are in good agreement but at odds with the bond-valence-parametrization method. Colors differentiate the four different valence regimes as described in the text.