Investigations on Cmc2$_1$-Si$_2$P$_2$X structures and physical properties by first-principles calculations

TL;DR

This study predicts new Cmc2$_1$-Si$_2$P$_2$X structures and explores their mechanical, electronic, and optical properties using first-principles calculations, revealing their stability, semiconducting nature, and optical characteristics.

Contribution

First-principles predictions of novel Cmc2$_1$-Si$_2$P$_2$X structures and comprehensive analysis of their physical properties.

Findings

All four compounds are mechanically and dynamically stable.

They are indirect bandgap semiconductors with decreasing band gaps from S to Po.

Cmc2$_1$-Si$_2$P$_2$X exhibits the highest hardness and covalent character.

Abstract

The new structures, Cmc2$_1$-Si$_2$P$_2$X (X=S, Se, Te, and Po), are predicted, and their mechanical, electronic and optical properties are investigated with the density functional theory, by first principles calculations. The elastic constants of the four compounds are calculated by the stress-strain method. The calculations of the elastic stability criteria and phonon dispersion spectra imply that they are mechanically and dynamically stable at zero pressure. The mechanical parameters, such as shear moduli $G$, bulk moduli $B$, Young's moduli $E$ and Poisson's ratios $v$ are evaluated by the Voigt-Reuss-Hill approach. The Cmc2$_1$-Si$_2$P$_2$X has the largest hardness due to the largest Young's modulus in the four compounds, and it is a covalent crystal. The anisotropies of their mechanical properties are also analyzed. The band structures and densities of states, which are calculated by using HSE06, show that Cmc2$_1$-Si$_2$P$_2$X compounds are indirect bandgap semiconductors, and the values of the band gaps decrease with increasing atomic number from S, Se, Te, to Po. In addition, the longitudinal sound velocity and transverse sound velocity for Cmc2$_1$-Si$_2$P$_2$X have been investigated. The dielectric constant, electron energy loss, refractive index, reflectivity, absorption and conductivity are analyzed to gain the optical properties of Si$_2$P$_2$X.