Pressure-induced amorphization and collapse of magnetic order in the type-I clathrate Eu${}_8$Ga${}_{16}$Ge${}_{30}$

We investigate the low temperature structural and electronic properties of the type-I clathrate Eu${}_8$Ga${}_{16}$Ge${}_{30}$ under pressure using x-ray powder diffraction (XRD), x-ray absorption near-edge structure (XANES), and x-ray magnetic circular dichroism (XMCD) techniques. The XRD measurements reveal a transition to an amorphous phase above 18 GPa. Unlike previous reports on other clathrate compounds, no volume collapse is observed prior to the crystalline-amorphous phase transition which takes place when the unit cell volume is reduced to 81$\%$ of its ambient pressure value. Fits of the pressure-dependent relative volume to a Murnaghan equation of state yield a bulk modulus $B_0=65\pm 3$ GPa and a pressure derivative $B_0^{\prime }=3.3\pm 0.5$. The Eu $L_2$-edge XMCD data shows quenching of the magnetic order at a pressure coincident with the crystalline-amorphous phase transition. This information along with the persistence of an Eu${}^{2+}$ valence state observed in the XANES spectra up to the highest pressure point (22 GPa) indicates that the suppression of XMCD intensity is due to the loss of long range magnetic order. When compared with other clathrates, the results point to the importance of guest ion-cage interactions in determining the mechanical stability of the framework structure and the critical pressure for amorphization. Finally, the crystalline structure is not found to recover after pressure release, resulting in an amorphous material that is at least metastable at ambient pressure and temperature.

Figure 1

Eu${}_8$Ga${}_{16}$Ge${}_{30}$ structural representation. (a) Unit cell of the clathrate type-I Eu${}_8$Ga${}_{16}$Ge${}_{30}$. The Eu atoms are shown at their two sites (Eu1 and Eu2) and inside the dodecahedral ($X_{20}$, red polyhedra) and tetrakaidecahedral ($X_{24}$, blue polyhedra) cages, respectively. The $X_{20}$ and $X_{24}$ polyhedra are shown in detail in (b) and (c), respectively. In (b) we show Eu1 at the 2$a$ and Eu2 at the 6$d$ Wyckoff positions. In (c) we show Eu1 at the 2$a$ and Eu2 at the 24$k$ Wyckoff positions. The Ga/Ge atoms (green and blue colors) are distributed at the 6$c$, 16$i$, and 24$k$ crystallographic positions.

Figure 2

XRD patterns of type-I clathrate Eu${}_8$Ga${}_{16}$Ge${}_{30}$ at 10 $=$K. Panels (a)–(h) show the diffraction patterns for increasing applied pressures. Panel (i) shows the diffraction pattern when the pressure is released after the sample reaches the crystal-to-amorphous transformation. The first three intense Bragg peaks (around $8^{\circ }$–$9^{\circ }$) correspond to the (222), (320), and (321) reflections, respectively. The peaks marked with an asterisk are due to Au powder.

Figure 3

Panel (a) shows selected powder XRD patterns in the 2$\theta$ range from 8${}^{\circ }$ to 10.4${}^{\circ }$ and panel (b) shows the pressure-volume dependence up to 18 GPa normalized by the volume at lower pressure ($V_0\sim 1221.3$ Å${}^3$ and $P_0\sim 1$ GPa). The solid line in panel (b) represents the result of fitting by a Murnaghan equation of state.

Figure 4

Eu $L_{2,3}$ XANES and XMCD signal for the type-I clathrate Eu${}_8$Ga${}_{16}$Ge${}_{30}$ measured at 10 K under an applied magnetic field of 0.5 T.

Figure 5

Eu $L_2$-edge (a) XANES and (b) XMCD spectra for the type-I clathrate Eu${}_8$Ga${}_{16}$Ge${}_{30}$ measured at 10 K and with an external applied magnetic field of 0.5 T at different pressures. The dashed vertical line marks the expected position of the Eu${}^{3+}$ features. The XANES and XCMD spectra at 2.0 GPa were measured after releasing the pressure.

Figure 6

Eu $L_2$-edge XMCD signal and hysteresis loops in type-I clathrate Eu${}_8$Ga${}_{16}$Ge${}_{30}$ obtained at $T=10$ K for different applied pressures. The panel (a) shows the XMCD integrated intensity as a function of pressure and the panel (b) shows hysteresis loops for some selected pressure values.