First-principles Study of Superconducting MAX Phases

Abstract

First-principles investigation of the structural, elastic, electronic, thermodynamic and optical properties along with theoretical hardness of the four superconducting MAX compounds Mo2GaC, Nb2AsC, Nb2InC and Ti2GeC have been carried out by the plane-wave pseudopotential method based on the density functional theory (DFT) implemented in the CASTEP code. The hardness has been studied by means of Mulliken bond population analysis and electronic energy density of states. The thermodynamic properties are derived from the quasi-harmonic Debye model with phononic effect. The calculated structural properties are in excellent agreement with experiments. The pressure effect on the structural properties of these MAX compounds has been investigated. The results show that both lattice constants and unit cell volume decrease almost linearly with the increase of pressure, while the hexagonal ratio increases gradually with increasing pressure. This implies that the lattice constant a decreases at a faster rate than c. Thus, the compressibility along c-axis is lower than that along a-axis. Our results on the elastic parameters indicate the elastic anisotropy and brittleness of the compounds. Nb2InC and Ti2GeC possess small elastic anisotropy but for other two phases it is comparatively large since they exhibit the maximal deviation from unity. The calculated elastic constants at different pressures exhibit the monotonous increase of the five independent elastic constants Cij with the pressure up to 50 GPa and satisfy the Born criteria for the mechanical stability with the prediction of no phase transition of the studied four MAX phases. The electronic structures calculations reveal that the chemical bonding of the MAX nanolaminates may be a combination of covalent, ionic and metallic in nature. The phase Nb2AsC is relatively soft and easily machinable compared to the other three metallic-ceramics due to its lowest hardness value. The investigated Fermi surfaces are formed mainly by the low-dispersive bands, which should be responsible for the presence of superconductivity in the four MAX materials. All optical functions are determined and analyzed for two different polarization directions. The theoretical findings are compared with available experimental data. The reflectivity spectra imply that the four MAX phases are the potential candidate materials for coating to reduce the solar heating. Finally, the thermodynamic properties such as bulk modulus, Debye temperature, volume thermal expansion coefficient and specific heats have been investigated successfully and analyzed in detail.