Crystal structure, electronic structure and phase stability of the Cu2-xMxCd (M=Zn, Ga, Ge, Sn) pseudo-binary Laves phases: effect of valence electron concentration

Abstract

Starting from Cu2Cd, a series of compounds with the approximate formula Cu2-xMxCd (x ​∼ ​0.1–1) (M ​= ​Zn, Ga, Ge, Sn) were prepared by high temperature solid state synthesis. X-ray diffraction and energy dispersive X-ray spectroscopy were used for structure determination, phase analysis and compositional study. A correlation between the valence electron concentration (VEC) and phase stability was noticed in Cu2–xMxCd (M ​= ​Zn, Ga, Ge, Sn). A slight incorporation of M (M ​= ​Zn, Ga, Ge, Sn) for Cu into Cu2Cd (C14, MgZn2 type) leads to the formation of a solid solution of cubic γ-brass type Cu5Cd8 as a major phase. Further incorporation of M leads to the appearance of MgCu2 (C15) type cubic Laves phase as the major phase. The cubic Laves phases are stable within a narrow homogeneity range. At higher atomic percentage of M, the stable pseudo-binary cubic phase ceases to form and binary compounds start to appear. One ordered configuration from each of the four systems was constructed to calculate the density of states (DOS) and crystal orbital Hamilton population (COHP) in order to explain the stability and bonding characteristics of the pseudo-binary Laves phases applying first principles density functional theory. These phases are electronically stable and poor metallic in nature; where partially covalent hetero-atomic bonds have significant contribution towards the overall stability and bonding.