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Computational and experimental studies of sp3-materials at high pressure

Authors: José A. Flores Livas

Ref.: PhD Thesis, Université Claude Bernard - Lyon 1 (2012)

Abstract: We present experimental and theoretical studies of sp3 materials at high pressure, alkaline-earth-metal (AEM) disilicides, disilane (Si2H6) and carbon. First, we review the thermodynamic phase diagram of pressure and temperature of AEM disilicides. In particular, we study the case of a layered phase of BaSi2 which has an hexagonal structure with sp3 bonding of the silicon atoms. This electronic environment leads to a natural corrugated Si-sheets. We demonstrate experimentally and theoretically an enhancement of superconducting transition temperatures from 6 to 8.9 K when silicon planes flatten out in this structure. Our extensive ab initio calculations based on density-functional theory guided the experimental research and permit explain how electronic and phonon properties are strongly affected by changes in the buckling of silicon plans in AEM disilicides. Second, we investigated the crystal phases of disilane at the megabar range of pressure. A novel metallic phase of disilane is proposed by using crystal structure prediction methods. Enthalpically this structure is more favourable than previously proposed structures of disilane up to 280 GPa. The calculated transition temperatures yielding a superconducting Tc of around 20 K at 100 GPa and decreasing to 13 K at 220 GPa. These values are significantly smaller than previously predicted Tc and put serious drawbacks in the possibility of high-Tc superconductivity based on silicon-hydrogen systems. Third, we studied the sp3-carbon structures at high pressure through a systematic structure search. We found a new allotrope of carbon with Cmmm symmetry which we refer to as Z-carbon. This phase is predicted to be more stable than graphite for pressures above 10 GPa and is formed by sp3-bonds. It is a wide band-gap semiconductor and has a hardness comparable to diamond. Experimental and simulated XRD, Raman spectra suggest the existence of Z-carbon in micro-domains of graphite under pressure.