Phonon optics, thermal expansion tensor, thermodynamic and chemical bonding properties of Al4SiC4 and Al4Si2C5: a first-principles study

Abstract

The phonon spectra, chemical bonding, thermal and thermodynamic properties of Al₄SiC₄ and Al₄Si₂C₅ are calculated by first-principles using density functional theory. Raman and infrared (IR) active phonon modes and their eigenvectors are analyzed. Phonon mode-Grünseisen parameter and macroscopic Grünseisen constants are evaluated from phonon spectra. Employing quasiharmonic approximation (QHA), the thermal expansion tensor is obtained. The calculated volumetric thermal expansion coefficient (TEC) of Al₄SiC₄ is high than that of Al₄Si₂C₅; and the linear TEC in the [001] direction is slightly higher than that of the [100] direction for both compounds. The computed average linear TECs for Al₄SiC₄ and Al₄Si₂C₅ are 9.96 × 10⁻⁶ K⁻¹ and 8.9 × 10⁻⁶ K⁻¹ in a range from room temperature to 1500 K, respectively. Other thermal properties such as specific heats (C_V, and C_P), entropy (S), isothermal and isobaric bulk moduli (K_T and K_S) are also discussed. Using Slack's model, it is found that thermal conductivities are 59.9 W m⁻¹ K⁻¹ and 78.3 W m⁻¹ K⁻¹ at room temperature for Al₄SiC₄ and Al₄Si₂C₅, respectively. The response of chemical bonds to hydrostatic pressure is discussed using Milliken population analysis. We also plot the charge density and its reduced density gradient to reveal the covalent bonds in the structure.