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

Abstract

The vibrational and structural properties of a hydrogen-rich Group IVa hydride, Ge(CH3)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 Pa-3 for phase I, Pnma for phase II, P 21/c for phase III, and P 21 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 CH3 groups and their sudden disappearance indicate that Ge(CH3)4 might be an ideal compound to realize metallization and even high-temperature superconductivity at modest static pressure for laboratory capability.

High-Pressure Phases of a S-Based Compound: Dimethyl Sulfide

“…For example, the pressure-induced restricted rotation of NH 3 /CH 3 groups and the locked NH 3 /CH 3 positions in the compounds of dihydrogen bonding molecule of NH 3 BH 3 , CH 3 HgM, (M = Cl, Br, I) and (CH 3 ) 2 XM, (X = Sn or Tl) , can be reflected by the softening of NH 3 /CH 3 vibrational mode in the Raman spectra. This anomalous phenomenon could also be found in the high-pressure experiments of the hydrogen-rich compounds, X­(CH 3 ) 4 (X = Si, Ge, and Sn). ,, For the Si­(CH 3 ) 4 , this softening behavior existed with the pressure up to 9.0 GPa, while it is 1.4 and 0.9 GPa for Ge­(CH 3 ) 4 and Sn­(CH 3 ) 4 , respectively. Compared with these results, DMS is very easy to undergo phase transition via active rotation of the CH 3 groups to freezing in a position only at 3.6 GPa in view of such small molecule relatively.…”

“…From above we have found evidence for three phase transitions at approximately 1. nearly all the modes exhibit the blue-shift with increasing pressure, whereas the ν 6 and ν 7 modes show a softening behavior (the negative pressure coefficients of both modes are listed in Table 1), which is regarded as a typical characteristic of rotational motions of the CH 3 group. 33−35 This phenomenon has also been observed in pressure-induced investigation on X(CH 3 ) 4 (X = Si, Ge and Sn), 27,30,31 where equivalent CH 3 groups would not remain stable and rotate by certain angles due to pressure-induce increase of intra-and intermolecular interaction. Additionally, it is found that each mode has very different pressure coefficients, as shown in Table 1, and CH 3 stretch modes exhibit the most intensive pressure effects in liquid phase.…”

“…This anomalous phenomenon could also be found in the highpressure experiments of the hydrogen-rich compounds, X-(CH 3 ) 4 (X = Si, Ge, and Sn). 27,30,31 For the Si(CH 3 ) 4 , this softening behavior existed with the pressure up to 9.0 GPa, while it is 1.4 and 0.9 GPa for Ge(CH 3 ) 4 and Sn(CH 3 ) 4 , respectively. Compared with these results, DMS is very easy to undergo phase transition via active rotation of the CH 3 groups to freezing in a position only at 3.6 GPa in view of such small molecule relatively.…”

“…Fortunately, an exception was found in tetramethylsilane (Si­(CH 3 ) 4 ), which is also one of the Group IVa hydrides and yet shows no decomposion at pressures up to 142 GPa . Although it remains unknown whether Si­(CH 3 ) 4 could be metalized or not, our previous work by Raman spectroscopy revealed that new Raman modes, which appeared at around 100 GPa, underwent a dramatic softening and characterized Si­(CH 3 ) 4 as semimetallic state possibly as in the case of hydrogen at high pressures. , Even more interestingly, the CH 3 groups within such meterials were proved to be a motivating factor both for maintaining stability under compression and for the softening behaviors of those modes. ,− Moreover, the CH 3 group also shows various interesting behaviors at high pressures, including the restriction in the rotation of the CH 3 group in some compounds (such as CH 3 Hg M (M = Cl, Br, I) and (CH 3 ) 2 XM (X = Sn or Tl) , ) and the different rotational angles in cubic Si­(CH 3 ) 4 at 0.58 GPa etc. Additionally, the recent breakthrough in the discovery of superconductivity above recorded high 190 K in H–S compound indicates the possibility of high-temperature superconductivities in hydrogen-dominant hydrides.…”

High-pressure structural properties of tetramethylsilane

“…However, the corresponding result shows inconsistency among the lattice parameters b/a, c/a, and angle β , which will happen if P2 1 /c symmetrical Ge(CH 3 ) 4 (TMGe) appears when compression almost possesses the same lattice parameters as TMS (Table 1). [52] Considering the application of external condition, nonhydrostatic situation of crystallized TMS could be a reasonable explanation for the abnormity, which leads to the lattice distortion compared with SnBr 4 , SiCl 4 , and SiBr 4 to undergo the structural change depending on temperature. [45,50,53] Whereas the SiH 4 under applied pressure is still kept as an excellent crystal with space group P2 1 /c as SnBr 4 , SiCl 4 , and SiBr 4 behave, suggesting that different pressure activities exist in both Si(CH 3 ) 4 compound and Ge(Ch 3 ) 4 compound.…”