Effects of Ga content on the stability of Fe-X (X = 6.25, 12.5, 18.75, 25 at.% Ga) alloys from first-principles calculations

Abstract

First-principles calculations based on density functional theory (DFT) were employed to investigate the influence of Ga concentration on the stability of binary Fe–Ga alloy. The contents of Ga element are 6.25, 12.5, 18.75 and 25 at.% for Fe[Formula: see text]Ga, Fe[Formula: see text]Ga, Fe[Formula: see text]Ga[Formula: see text] and Fe[Formula: see text]Ga alloys, respectively. Cohesive energy, elastic properties, electronic structures and phonon properties of optimized Fe–Ga models were calculated. Our results of calculated elastic constants and elastic moduli are in good accordance with the determined available literature. Our study shows that with increasing Ga content, the absolute value of cohesive energy of Fe–Ga alloy is decreasing. From the calculations of elastic constants, we found that the value of [Formula: see text] decreases with the increasing Ga content, which indicates that the stability of Fe–Ga along the [001] direction is decreasing with the increasing Ga content. The calculated bulk moduli of Fe–Ga alloys increase with increasing Ga content because of the increase of the average bonding number evaluated by our calculations of densities of states. Conversely, the shear moduli decrease with increasing Ga content, which indicates that the stability of Fe–Ga is decreasing with the increasing Ga content. Tetragonal shear elastic constant [Formula: see text] almost decreases linearly with increasing Ga content, which follows the same decreasing trend as the cohesive energy and the shear modulus. It is worth noting that with the increasing Ga content, the pseudogap between two major peaks in the minority-spin state became softer. The softness brings the Fermi level into a high state, which lowers the stability of the Fe–Ga alloy. According to the phonon calculations, the acoustic mode of Fe-25 at.% Ga appeared to be softening along the [001] direction, which is in good accordance with the direction predicted by the calculations of elastic constant. But this softening trend cannot be observed in the phonon dispersion spectrum of Fe-6.25 at.% Ga, which proved that Fe-6.25 at.% Ga is more stable than the Fe-25 at.% Ga alloy.