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

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 be-tween 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 gamma-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 con-structed 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.

Automated summary

A series of Cu2-xMxCd compounds were prepared by high temperature solid state synthesis, and it was found that the valence electron concentration (VEC) correlated with phase stability. The incorporation of M (Zn, Ga, Ge, Sn) led to the formation of gamma-brass type and MgCu2 type Laves phases. The cubic Laves phases were stable within a narrow homogeneity range, whereas at higher M content, the stable pseudo-binary cubic phase transformed into binary compounds. First principles density functional theory calculations were used to explain the stability and bonding characteristics of the pseudo-binary Laves phases.