Synchrotron x-ray spectroscopy studies of valence and magnetic state in europium metal to extreme pressures

In order to probe the changes in the valence state and magnetic properties of Eu metal under extreme pressure, x-ray absorption near-edge spectroscopy, x-ray magnetic circular dichroism, and synchrotron Mössbauer spectroscopy experiments were carried out. The Mössbauer isomer shift exhibits anomalous pressure dependence, passing through a maximum near 20 GPa. Density functional theory has been applied to give insight into the pressure-induced changes in both Eu's electronic structure and Mössbauer isomer shift. Contrary to previous reports, Eu is found to remain nearly divalent to the highest pressures reached (87 GPa) with magnetic order persisting to at least 50 GPa. These results should lead to a better understanding of the nature of the superconducting state found above 75 GPa and of the sequence of structural phase transitions observed to 92 GPa.

Figure 1

(a) XANES spectra at ambient temperature for Eu at selected pressure to 87 GPa; (b) ab initio simulation of XANES spectra FEFF8 under pressure to 75 GPa (see text). $E_{\mathrm{o}}$ is the absorption threshold for the L${}_3$ absorption edge.

Figure 2

(a) DFT simulations in LDA+U scheme for Eu showing change in electron occupancy Δ$n$ under pressure for $s$, $p$, $d$, $f$, and interstitial electrons. The simulations were based on the crystal structure data from Ref. 8: bcc from 0 to 11 GPa, hcp from 11 to 35 GPa, Pnma (orthorhombic) from 36 to 88 GPa. Middle and lower panels: PBE simulations for ferromagnetic bcc and antiferromagnetic orthorhombic Pnma structures, respectively. For (a), (b), and (c) panels the electron occupancy $n$ at $P$ $=$ 0 equals (2.07, 2.00, 1.99) for $s$; (5.54, 5.27, 5.27) for $p$; (0.12, 0.21, 0.21) for $d$; (6.74, 6.60, 6.60) for $f$; and (2.53, 2.93, 2.92) for interstitial electrons, respectively.

Figure 3

DFT simulations in the LDA+U scheme for Eu show (a) XANES spectra FDMNES in bcc and hcp phases, where $E_{\mathrm{F}}$ is the Fermi energy; (b) total magnetic moment per Eu ion in units of Bohr magneton μ${}_{\mathrm{B}}$.

Figure 4

(a) Normalized XANES spectra and (b) corresponding XMCD amplitudes at 3.3 and 21.4 GPa.

Figure 5

XMCD amplitude of Eu metal under pressure to 60 GPa. Structural phase boundaries from Ref. 8.

Figure 6

Magnetic field dependence of the XMCD amplitude at 4.6 K for a Al-Eu-Al foil sample outside the DAC at ambient pressure and at 21.4 GPa in the DAC.

Figure 7

Ambient temperature synchrotron Mössbauer spectra of ${}^{151}$Eu under pressures in three experimental runs to (a) 25 GPa, (b) 35.6 GPa, and (c) 72.6 GPa in the time domain (left column) and in the corresponding energy domain (right column). Numbers in parentheses give order of measurement. In the left column filled black circles show experimental data with error bars, and the red (dark gray) lines show fits to data. In the right column black lines show resonant absorption from Eu sample and the red dash-dotted lines indicate position of zero IS. In (a) blue (medium gray) lines represent the absorption from Eu${}_2$O${}_3$ and purple (light gray) line from EuS as reference. In (b) and (c) blue (medium gray) lines are from EuS as reference.

Figure 8

IS of ${}^{151}$Eu at ambient temperature and two low temperatures under pressure to 73 GPa in comparison with previous studies (Refs. 13 and 15).

Figure 9

Synchrotron Mössbauer spectra for Eu metal at low temperatures and high pressures. The oscillations at 27.7 GPa and 11 K, 20.2 GPa and 11 K, and 19.7 GPa and 81 K show Eu orders magnetically, while at 18.6 GPa and 115 K the absence of oscillations shows Eu does not order.

Figure 10

Contact density Δ${\rho }_0$ calculated in DFT as a function of (a) volume and (b) pressure for Eu in the bcc and Pnma phase for both ferromagnetic and antiferromagnetic order. The contact density Δ${\rho }_0$ in units electrons/(Bohr radius)${}^3$ is shown relative to the contact density of the ferromagnetic bcc phase at zero pressure. The right ordinate shows the linear transformation of the contact density to obtain the isomer shift.

Figure 11

Comparison between the percentage change in the occupation of the valence $s$ orbitals of Eu Δ$n_s$ and the change in contact density Δ${\rho }_0$ [in units electrons/(Bohr radius)${}^3$] of the valence electrons for the ferromagnetic bcc Eu phase as a function of pressure. A significant fraction of the change in contact density can be directly attributed to the change in $s$-valence occupation.