Structural, mechanical, and thermodynamic properties of newly-designed superhard carbon materials in different crystal structures: A first-principles calculation

[Display omitted] •These carbon allotropes are mechanically stable due to the calculated elastic constants of single crystal satisfy the criterions of mechanical stability.•A larger bulk modulus not entirely correspond to a higher hardness, while there is a relationship between shear modulus and hardness that a larger shear modulus indicates a higher hardness.•All these carbon phases can be excellent candidate for superhard materials due to the hardness is more than 70 GPa.•The order of the elastic anisotropy for these carbon allotropes is C2/m-28 > Imma > C2/m-16 > P21/m > C2/m-20 > Cm-32 > Cm-40 > Amm2 > P2221 > I-4.•The thermal stability of these newly-designed carbon allotropes is better than that of diamond and the order is Cm-40 > C2/m-28 > I-4 > Imma > Amm2 > Cm-32 = C2/m-20 > P2221 > C2/m-16 > P21/m > Diamond, Cm-40 carbon has the strongest thermal stability. At present work, the ab-initio method was applied to systematically and deeply investigate the structural and mechanical properties, such as elastic, elastic anisotropy, thermal conductivity and thermodynamic properties of newly-designed superhard carbon in different crystal structures. The results of cohesive energies and formation enthalpies indicate that all carbon are energetically and structurally stable and P2221 carbon has the most stable structure among these carbon allotropes. On the basis of the single-crystal elastic constants, the elastic properties, such as bulk modulus B, shear modulus G and Young's modulus E, Poisson's ratio v and hardness HV were obtained, the values of G, E and HV is largest for P2221 carbon. The order of elastic anisotropy is C2/m-28 > Imma > C2/m-16 > P21/m > C2/m-20 > Cm-32 > Cm-40 > Amm2 > P2221 > I-4. Clarke's and Cahill's models are employed to calculate the minimum thermal conductivities of these carbon, and the order is P2221 > P21/m > C2/m-20 > C2/m-16 > Cm-40 > Imma > Amm2 > Cm-32 > I-4 > C2/m-28. Furthermore, the minimum thermal conductivities of these carbon allotropes are anisotropic in different direction. The analysis of thermodynamic properties can be obtained that the thermal stability of these newly-designed carbon allotropes is better than that of diamond and the order is Cm-40 > C2/m-28 > I-4 > Imma > Amm2 > Cm-32 = C2/m-20 > P2221 > C2/m-16 > P21/m > Diamond, Cm-40 carbon has the strongest thermal stability. It can be concluded from phonon density that the C2/m-28 carbon is dynamically metastable structure. The study in this work give a co