Pressure-induced structural transitions in europium to 92 GPa

Synchrotron x-ray diffraction experiments have been carried out on europium metal at ambient temperature to pressures as high as 92 GPa (0.92 Mbar). Following the well-known bcc-to-hcp transition at 12 GPa, a mixed-phase region is observed from 18 to 66 GPa until finally a single orthorhombic (Pnma) phase persists from 66 to 92 GPa. These results are compared to predictions from density functional theory calculations. Under pressure the relatively large molar volume $V_{\mathrm{mol}}$ of divalent Eu is rapidly diminished, equaling or falling below $V_{\mathrm{mol}}(P)$ for neighboring trivalent lanthanides above 15 GPa. The present results suggest that above 15 GPa Eu is neither divalent nor fully trivalent to pressures as high as 92 GPa.

Figure 1

X-ray diffraction images of the Eu sample and Pt pressure marker at 4 GPa (top, bcc phase), 14 GPa (center, hcp phase) with λ$=$0.41493 Å beam and 2 s exposure time, and at 92 GPa (bottom, orthorhombic phase) with λ$=$0.36229 Å beam and 15 s exposure time.

Figure 2

Results of the random structure and genetic algorithm search at Cornell University showing the enthalpies of possible crystal structures of Eu relative to the bcc phase as a function of pressure up to 100 GPa. The DFT calculation predicts a structure sequence from bcc→hcp→$C$2/$c$→Fdd2→Pnma→$C$2/$c$→hcp. (a) bcc (horizontal line), (b) hcp (open circle), (c) C2/c (open triangle), (d) Fdd2 (diamond), (e) Pnma (solid triangle).

Figure 3

Results of density function theory calculations at the University of Nevada showing the enthalpies of possible crystal structures of Eu metal relative to that for the bcc phase as a function of pressure to 100 GPa: (a) bcc (square), (b) hcp $P{6}_3/\mathit{mmc}$ (triangle), (c) orthorhombic Pnma (diamond), (d) hcp $P{6}_3/\mathit{mmc}$ (triangle).

Figure 4

Density of phonon states of Eu versus energy for structures in Fig. 3 at different pressures.

Figure 5

Representative high-pressure x-ray diffraction spectra of Eu (black lines, wavelength λ$=$0.41493 Å) from 4 to 35 GPa with Rietveld full-profile refinements (red lines) for bcc and hcp phases. The tickmarks in the 4 GPa and 14 GPa plots correspond to positions of diffraction peaks of Eu. Below the tickmarks are the difference plots between calculated and observed spectra. Pt peaks are identified by asterisks in all spectra.

Figure 6

Selected x-ray diffraction spectra of Eu (black lines, wavelength λ$=$0.4256 Å) including the refinements (red lines) at 55 and 75 GPa showing the sluggish transition from $C$2/$c$ to Pnma. In the plot for 55 GPa, the tickmarks correspond to the positions of diffraction peaks from Eu's Pnma phase (upper) and $C$2/$c$ phase (lower). In the plot for 75 GPa, tickmarks show the peak positions from Eu's Pnma phase. The blue lines below the tickmarks show the difference plots between fits and data. Asterisks indicate peak positions from Pt. The letter “g” marks peaks from Re gasket.

Figure 7

For Eu pressure dependence of (a) lattice parameters and (b) ratio of lattice parameters above 12 GPa. In the pressure range 12–35 GPa, the lattice parameters are obtained from the hcp phase, and 35–92 GPa from the orthorhombic phase. The agreement of the c/a values from this study (solid circles) with those from Ref. 5 (open triangles) is reasonable.

Figure 8

Equation of state at ambient temperature for Eu to 92 GPa pressure from present studies compared to earlier work by Takemura and Syassen,[5] Grosshans and Holzapfel,[6] and McWhan, Souers, and Jura.[29] The V(P) fit in the bcc phase is obtained using the third-order Birch-Murnaghan equation[30] (see text).

Figure 9

Comparison of pressure-dependent molar volume of trivalent Nd,[31] Sm,[32] Gd,[33] and Tb[34] to present results for Eu. Dashed line is calculation for divalent Eu by Johansson and Rosengren[2].