Study On Structural And Vibrational Properties In The Tetra-alkyl Compounds Of Group Ⅳ Elements Under Pressure

As an important challenge in modern physics, metallization of hydrogen has long been amajor driving force in high-pressure science and technology development, which originatesfrom the possible superconductivity with high T_c（＞200K） under sufficiently strongcompression. However, the experimental realization of metallic hydrogen in the condensedform has remained elusive due to much higher pressures （e.g.,＞400GPa）, which currentlyare a great challenge for hydrogen with high-pressure techniques. In2004, Ashcroft alreadyinvigorated this interest with the suggestion that hydrogen-dominant hydrides could also behigh-temperature superconductors in monatomic and molecular phases, providing analternative way to metallic hydrogen. By this argument, such covalent hydrides could exhibitmetallization at significantly reduced pressures compared with pure hydrogen due to chemicalprecompression. Group IVa hydrides were specifically suggested as potential candidates forthis material and many experimental and theoretical efforts are currently underway toinvestigate this prediction, such as SiH4, GeH4, SnH₄, and PbH4. However, very recentlyexperiments show the possible decomposition of SiH4under irradiation from x-ray and lasers,which result in a loss of superiority for these compounds to further investigate themetallization of bulk hydrogen. For achieving the metallic hydrogen at “low” pressure, it hasbeen extremely urgent to search other hydrogen-rich compounds on the group IVa hydrides.The tetra-alkyl compounds of group IVa elements have attracted much attention from thescientific community due to their highly symmetrical character. The skeletal vibrations oftheir molecules have been investigated using Raman and infrared spectra since the1930s. Inthe molecular structure of these compounds, the group IVa element is tetrahedrallycoordinated by methyl groups （CH₃）, making the molecule with the T_dsymmetry. This is thesame to the case on the hydrides of group IVa elements. The CH₃groups are expected to playan important role in understanding the interesting physical and chemical properties of thetetra-alkyl compounds of group IVa elements. At low temperature, the CH₃groups becomenonequivalent, which result from the different rotation of the CH₃groups throughintramolecular interactions. This phenomenon has been observed in other CH₃-bearingcompounds at high pressures. Upon compression, the rotation of CH₃groups have been restricted in the compounds, such asCH₃HgM （M=Cl, Br, I） and （CH₃）₂XM （X=Sn or Tl）.The CH₃groups display different rotational angles in cubic Si（CH₃）₄at0.58GPa. To ourknowledge, there is no information on these compounds at higher pressures, which providesthe possibility to perform the measurements furtherly. The major work of this thesis is toexplore the phase transformation of the compounds under pressure by Raman spectroscopy,determine the structures corresponding with the phases by synchrotron X-ray diffractionmeasurements, and attempt to search the metallization of the compounds under pressure bythe electrical transport measurements and the details are as follows:1. n-pentane, as one of chain alkanes and the allotropes of the neopentane, is selected toinvestigate the metallic hydrides, which provides an alternative way to metallic hydrogen. Theresults reveal it is harder to compress the compound and keep stable at high pressures. Until60GPa, we did not observe the metallization of the compound.2. tetramethylsilane （TMS）, is usually used as an internal standard for calibratingchemical shifts in¹H,¹³C, and²⁹Si NMR spectroscopy, but it is also applied to aviationindustry as a fuel sometimes. Because of its highly symmetrical character, the normalcoordinate analysis of the skeletal vibrations of the molecule of TMS has been reported in anumber of papers. Accord-ing to differential thermal analysis, TMS would form threepolymorphs. The α phase existed from the melting point down to159K; with further coolingthe β phase appeared and would be stable to20K. However, when the sample wassub-sequently heated from20to118K a new γ phase occurred. Further investigationillustrates that α phase with TMS as a plastic crystal is metastable, so is the β phase, butγphase is stable at low temperatures. Additionally, low-temperature crystallization at100Kresulted in a single crystal, and its crystal structure was determined to have the space group ofPnma, Z=4. Although TMS has been studied extensively, little information can be referencedabout the properties of TMS at higher pressure region, and only a high-pressure crys-talstructure of TMS in Pa-3, Z=8, at0.58GPa,296K has been reported so far. High-pressurebehavior of tetramethylsilane was investigated by Raman scattering measurements atpressures up to142GPa and room temperature. Our results revealed the phase transitions at0.6,9.0, and16.0GPa from both the mode frequency shifts with pressure and the changes ofthe full width half maxima of these modes. These transitions were suggested to result from the changes in the inter-and intramolecular bonding of this material. We also observed two otherpossible phase transitions at49–69GPa and96GPa. No indication of metallization intetramethylsilane was found with stepwise compression to142GPa. We also performed thesynchrotron X-ray diffraction measurements on the copound with the pressure up to30GPa.The results by fitting are Pnma space group at4.2GPa, P2₁/c space group at9.9GPa via acoexisted phase, P2/m space group at18.2GPa.3. Tetramethylgermane （TMGe）, as one of heavier group IVa hydrides, belongs to a classof nonpolar molecular compounds. At low temperature, only one mod-ification of TMGe wasobserved in the temperature range15–300K. Although the crystal structures of TMGe werepredicted by global lattice-energy minimizations using force-field methods, no high-pressurephases have been determined experimentally. The high-pressure behaviors of TMGe areinvestigated by combining Raman scattering and synchrotron x-ray diffraction （XRD）techniques up to30.2GPa using the diamond anvil cells （DAC）. Both techniques allow theobtaining of complementary information on the high-pressure behaviors and yield consistentphase transitions at1.4GPa for the liquid to solid and3.0,5.4, and20.3GPa forthe solid tosolid. The four high-pressure solid phases are identified to have the cubic, orthorhombic,monoclinic,and monoclinic crystal structures with space groups of Pa-3for phase I, Pnma forphase II, P2₁/c for phase III, and P2₁for phase IV, respectively. These transitions aresuggested to result from the changes in the inter-and intramolecular bonding of thiscompound. The softening of some Raman modes on CH₃groups and their suddendisappearance indicate that Ge（CH₃）₄might be an ideal compound to realize metallization andeven high-temperature superconductivity at modest static pressure for laboratory capability.4. Very recently experiment shows the possible decomposition of SiH4under irradiationfrom x-rays and lasers, which makes the metal-lization of SiH4extremely intangible and thestability of group IVa hydrides unprecedentedly important. Excitingly, Si（CH₃）₄, one of thetetra-alkyl hydrides of group IVa elements, was found no decomposition up to142GPa，although it remains unknown to metallize. Above96GPa, the sudden disappearing of originalvibrational modes and appearing of the softening behavior from the new Raman modes makesthe metallization of tetramethylsilane more complex. In addition, it is suggested that thehomologous hydrides with heavier group IVa atoms would yield lower metallization pressure, due to the weaker chem-ical bonds which be dissociated under pressure. Study on Ge（CH₃）₄by the Raman spectroscopy verified that high-pressure behaviors of CH₃groups do appearearlier with the compressed process despite uncertain to metallize. Therefore, theinvestigation of heavier group IVa hydrides is in great demand. The vibrational and structuralproperties of a hydrogen-rich group IVa hydride, Sn（CH₃）₄, have been investigated bycombining Raman spectroscopy and synchrotron x-ray diffraction measurements at roomtemperature and at pressures up to49.9GPa. Both techniques allow the obtaining ofcom-plementary information on the high-pressure behaviors and yield consistent phasetransitions at0.9GPa for the liquid to solid and2.8,10.4,20.4, and32.6GPa for the solid tosolid. The foregoing solid phases are identified to have the orthorhombic, tetragonal,monoclinic crystal structures with space groups ofPmmm for phase I, P4/mmm for phase II,P2/m for phase III, respectively. The phases IV and V coexist with phase III, resulting incomplex analysis on the possible structures.These transitions suggest the variation in the inter-and intra-molecular bonding of this compound.5. Study on the high-pressure behaviors of these compounds, it remains unknown tometalize. However, rich high-pressure behaviors, especially the softening of modes, reveal thepossibility to metalize, whereas the pressure to undergo metallization might be higher.Therefore, it is extremely urgent to perform the measurements at higher pressures. In view ofthe recent study on pure hydrogen under pressure, it is analyzed for the possibly to undergometallization, which illustrates we have achieved a semi-metalic hydrogen-rich meterail.Furthermore, we explain the failure to perform the high-pressure electrical transportmeasurements.