High-pressure phases of a hydrogen-rich compound: Tetramethylgermane

The vibrational and structural properties of a hydrogen-rich group IVa hydride, Ge(CH${}_3$)${}_4$, are studied by combining Raman spectroscopy and synchrotron x-ray diffraction measurements at room temperature and at pressures up to 30.2 GPa. Both techniques allow the obtaining of complementary information on the high-pressure behaviors and yield consistent phase transitions at 1.4 GPa for the liquid to solid and 3.0, 5.4, and 20.3 GPa for the solid to solid. The four high-pressure solid phases are identified to have the cubic, orthorhombic, monoclinic, and monoclinic crystal structures with space groups of $\mathit{Pa}$-3 for phase I, Pnma for phase II, $P$2${}_1$/$c$ for phase III, and $P$2${}_1$ for phase IV, respectively. These transitions are suggested to result from the changes in the inter- and intramolecular bonding of this compound. The softening of some Raman modes on CH${}_3$ groups and their sudden disappearance indicate that Ge(CH${}_3$)${}_4$ might be an ideal compound to realize metallization and even high-temperature superconductivity at modest static pressure for laboratory capability.

Figure 1

The configurations of TMGe with respect to ideally tetrahedral $T$${}_d$ (-43$m$) symmetry. (a) Cup model of TMGe shown to illustrate almost spherically shaped molecules in a close-packed stacking and (b) ball-and-stick model of TMGe manifested that one bond distance C-H, one Ge-C, one Ge-C-H/C-Ge-C angle could describe the molecular geometry.

Figure 2

Representative Raman spectra of TMGe in the full spectral regions at ambient conditions upon compression to 30.2 GPa. The red arrows indicate the sudden changes of Raman modes.

Figure 3

Comparison of observed and fitted Raman spectra of TMGe with respect to overlapped modes of (a) $\nu$${}_1$ and $\nu$${}_2$ at ambient conditions, (b) $\nu$${}_4$ with a shoulder at 19.7 GPa, (c) $\nu$${}_1$${}_0$ splitting with pressure up to 16.8 GPa, (d) $\nu$${}_9$ with a shoulder peak at 25.5 GPa. Black lines indicate observed Raman spectra; red lines through black lines indicate sum curve of the fit; the individual bands of the fit are represented by green lines in the lower part.

Figure 4

Pressure dependence of the frequencies of TMGe for the observed modes in all regions at room temperature. The vertical dashed lines at near 1.4, 3.0, 5.4, and 16.8–20.3 GPa indicate the proposed phase boundaries.

Figure 5

Synchrotron x-ray ($\lambda =0.4067$ Å) diffraction patterns of TMGe during the pressurization from ambient conditions to 30.0 GPa. Red asterisk marked to illustrate the signals from the gasket material tungsten.

Figure 6

X-ray powder diffraction patterns of liquid TMGe at pressures of 1.0 (a), 6.4 (b), 14.5 (c), 17.5 (d), and 20.8 (e) GPa. The refined lattice parameters for the corresponding space groups are given respectively. The open circles represent the measured intensities and the red lines the results of profile refinements by the best LeBail fit with each space group. The positions of the Bragg reflections are marked by vertical lines and the difference profiles are shown at the bottoms (blue lines). The $R$ values are $R_p=0.7\%$, $R_{wp}=1.0\%$ for the fitting at 1.0 GPa, $R_p=0.8\%$, $R_{wp}=1.6\%$ at 6.4 GPa, $R_p=0.4\%$, $R_{wp}=0.7\%$ at 14.5 GPa, $R_p=0.3\%$, $R_{wp}=0.6\%$ at 17.5 GPa, and $R_p=0.3\%$, $R_{wp}=0.5\%$ at 20.8 GPa.

Figure 7

Pressure dependence of the lattice parameters’ corresponding space groups for each phase; the vertical dashed lines denote the phase boundaries.

Figure 8

Volume per formula unit change of TMGe with pressure. The solid lines demonstrate the fitting data of phases to the Birch-Murnaghan equation of state and the vertical dashed lines denote the phase boundaries.