Tunability of Mg₂Si Bandgap by Formation of Mg₂(Si, C) with an Anti-Fluorite Structure Examined by First-Principles Calculations

Abstract

We used first-principles calculations to investigate the effects of replacing Si atoms in Mg₂Si with C atoms to tune its bandgap and enhance its thermoelectric performance. First-principles calculations suggest that the substitution of Si by C atoms in the Mg₂Si lattice forms Mg₈Si_4−zC_z (z = 1, 2, and 3) with a sustained anti-fluorite structure, which results in an unexpected bandgap contraction. However, the bandgap of Mg₈Si_4(1−x)C_4x composed of a supercell structure of a rhombohedral Mg₂Si primitive cell and Si-substitution with C has a wide bandgap when the C/Si ratio is sufficiently high. The formation enthalpies from Mg, Si, and C (diamond) are negative under pressures greater than ca. 15 GPa.

Fig. 6 Bandgap dependences of Mg₈Si₄ (□), Mg₈C₄ (○), Mg₈Si₃C₁ (▲), Mg₄Si₁C₁ (●), and Mg₈Si₁C₃ (×) on their unit-cell volume. Numerical values attached to these symbols indicate the assumed applied pressures in the unit of GPa during the geometrical optimization procedures.