Quan Zhang

A new superhard carbon allotrope: tetragonal C64

A novel superhard carbon allotrope C64 is predicted which is composed of C28 cages. It is porous and exhibits distinct topologies including zigzag 5, 6, 8, 10 and 12-fold carbon rings. The elastic constants and phonon calculations reveal that C64 is mechanically and dynamically stable at ambient pressure. The hardness of C64 is 60.2xa0GPa. The tensile and shear strength calculations indicate that the lowest tensile and shear strengths have the almost same value of 48.1xa0GPa. For the electronic properties, the band structure calculations show that C64 is a quasi-direct band gap semiconductor with a band gap of 1.32xa0eV.

Cubic C3N: A New Superhard Phase of Carbon-Rich Nitride

Using the particle swarm optimization technique, we proposed a cubic superhard phase of C3N (c-C3N) with an estimated Vicker’s hardness of 65 GPa, which is more energetically favorable than the recently proposed o-C3N. The c-C3N is the most stable phase in a pressure range of 6.5–15.4 GPa. Above 15.4 GPa, the most energetic favorable high pressure phase R3m-C3N is uncovered. Phonon dispersion and elastic constant calculations confirm the dynamical and mechanical stability of c-C3N and R3m-C3N at ambient pressure. The electronic structure calculations indicate that both c-C3N and R3m-C3N are indirect semiconductor.

Crystal Structures and Mechanical Properties of Ca2C at High Pressure

Recently, a new high-pressure semiconductor phase of Ca2C (space group Pnma) was successfully synthesized, it has a low-pressure metallic phase (space group C2/m). In this paper, a systematic investigation of the pressure-induced phase transition of Ca2C is studied on the basis of first-principles calculations. The calculated enthalpy reveals that the phase transition which transforms from C2/m-Ca2C to Pnma-Ca2C occurs at 7.8 GPa, and it is a first-order phase transition with a volume drop of 26.7%. The calculated elastic constants show that C2/m-Ca2C is mechanically unstable above 6.4 GPa, indicating that the structural phase transition is due to mechanical instability. Both of the two phases exhibit the elastic anisotropy. The semiconductivity of Pnma-Ca2C and the metallicity of C2/m-Ca2C have been demonstrated by the electronic band structure calculations. The quasi-direct band gap of Pnma-Ca2C at 0 GPa is 0.86 eV. Furthermore, the detailed analysis of the total and partial density of states is performed to show the specific contribution to the Fermi level.

Mechanical and Electronic Properties of P42/mnm Silicon Carbides

Abstract Two new phases of Si8C4 and Si4C8 with the P42/mnm symmetry are proposed. Using first principles calculations based on density functional theory, the structural, elastic, and electronic properties of Si8C4 and Si4C8 are studied systematically. Both Si8C4 and Si4C8 are proved to be mechanically and dynamically stable. The elastic anisotropies of Si8C4 and Si4C8 are studied in detail. Electronic structure calculations show that Si8C4 and Si4C8 are indirect semiconductors with the band gap of 0.74 and 0.15 eV, respectively.

Stability, elastic anisotropy, and electronic properties of Ca2C3 *

The systematic investigations of the mechanical, elastic, and electronic properties, and stability of the newly synthesized monoclinic C2/m-Ca2C3 are performed, based on the first-principles calculations. Ca2C3 is found to be mechanically and dynamically stable only from 0 GPa to 24 GPa. The elastic anisotropy studies show that Ca2C3 exhibits the elastic anisotropy increasing with the augment of pressure. Furthermore, using the HSE06 hybrid functional, the electronic properties of Ca2C3 under pressure are calculated. The structure can be regarded as a quasi-direct band gap semiconductor, and the pressure-induced direct-indirect band gap transition is studied in detail.

Mechanical and Electronic Properties of XC6 and XC12

A series of carbon-based superconductors XC6 with high Tc were reported recently. In this paper, based on the first-principles calculations, we studied the mechanical properties of these structures, and further explored the XC12 phases, where the X atoms are from elemental hydrogen to calcium, except noble gas atoms. The mechanically- and dynamically-stable structures include HC6, NC6, and SC6 in XC6 phases, and BC12, CC12, PC12, SC12, ClC12, and KC12 in XC12 phases. The doping leads to a weakening in mechanical properties and an increase in the elastic anisotropy. C6 has the lowest elastic anisotropy, and the anisotropy increases with the atomic number of doping atoms for both XC6 and XC12. Furthermore, the acoustic velocities, Debye temperatures, and the electronic properties are also studied.