Superconducting MAX phases Nb2AC (A = S, Sn): An ab-initio study

TL;DR

This study uses ab-initio methods to analyze the structural, electronic, thermodynamic, and optical properties of Nb2AC MAX phases, revealing their conductive nature, superconducting potential, and optical characteristics for coating applications.

Contribution

It provides the first detailed analysis of thermodynamic and optical properties of Nb2AC MAX phases, including effects of atom substitution and superconducting behavior.

Findings

Nb2SC and Nb2SnC are moderately coupled superconductors.

Nb2SC is a better dielectric material.

Both phases show promise as coating materials in the 10-16.5 eV energy range.

Abstract

An ab-initio investigation of structural parameters, elastic, electronic, thermodynamic and optical properties of MAX phases Nb2AC (A = S, Sn) has been carried out by the plane wave psedudopotential method based on density functional theory (DFT). The effect on results of substitution of heavier Sn atoms for the lighter S atoms in the nanolaminate network has been made. The analysis of the electronic band structure shows that these compounds are electrical conductors, with contribution predominantly from the Nb 4d states. The temperature and pressure dependence of bulk modulus, Debye temperature, specific heats, thermal expansion coefficient of the nanolaminates are calculated for the first time using the quasi-harmonic Debye model with phononic effects. The estimated values of electron-phonon coupling constants (lambda ~ 0.49, ~0.59) imply that Nb2SC and Nb2SnC are moderately coupled superconductors. Further first time detailed analysis of all optical functions reveals that Nb2SC is a better dielectric material, and also both the phases, having similar reflectivity spectra, show promise as good coating materials in the energy regions 10-16.5 eV. Keywords: Nb2SC; Nb2SnC; First-principles; Mechanical properties; Quasi-harmonic Debye model; Band structure; Thermodynamic properties; Optical properties