Theoretical Studies On Prediction Of Structure And Physical Properties Of Magnesium Dicarbide Under High Pressure

The existing state of matter can be transformed due to high pressure, and the novel properties of materials under high pressure could be exploited, which broad-ens a new orientation and dimension for the research of condensed matter physics. Because of the high pressure, interatomic distance of matter can be expected to decrease distinctly inducing structural phase transitions; Further more, the over-lap degree of interatomic electron orbits could change under pressure, which would induce electronic transformations. As a consequence, every aspect of matter’s prop-erties could be changed dramatically under pressure.Carbon-based compounds exhibit unexpected structures and electronic behav-ior at high pressure arising from various bonding features of carbon, like sp, sp2 and sp3 bonds. Physical properties of carbide under high pressure research is particularly important. Here we report evolution of crystal structures and physical properties of MgC2 as predicted through ab-initio calculations in combination with an unbiased swarm structure search. Our results are listed as follow:(i) Initially, three pressure-induced structural transformations are unraveled, following the phase sequence of ambient-pressure P42/mnm (α-phase) â†' Cmcm (β-phase)â†' C2/m (γ-phase) â†' EuGe2-type P-3ml (δ-phase), where significant C-C bonding modifications from C-C dimer to quasi 1-dimension armchair-type chain, to polymerized ribbon and then to winkled quasi 2-dimension graphite sheet are evident. No imaginary frequencies are observed throughout the whole Brillouin zone, declaring that the three novel phases are dynamically stable at studied pressure region. The abrupt volume collapses of about 19.7% ,9.6% and 4.8% around phase transitions pressures, respectively, indicating the first-order natnre, of those phase transitions in MgC2.(ii) The predicted β-and γ-phases with sp2 C-C hybridization are metals, while the δ-phase characterized by a sp3 C-C hybridization is a narrow-gap semiconductor with a band gap of 0.667 eV. Strong electron-phonon couplings in the compressed β-and γ-phases are predicted with β-phase showing a high superconducting critical temperature of 11.2 K which is higher than superconducting critical temperature of γ-phase (7.1 K). The current results indicate that pressure is effective in tuning the crystal and electronic structures of MgC2, which is expected to have impact on physical properties for potential applications.