Influence of Valence Electron Concentration on Laves Phases: Structures and Phase Stability of Pseudo-Binary MgZn2- xPdx.

A series of pseudo-binary compounds MgZn2- xPdx (0.15 ≤ x ≤ 1.0) were synthesized and structurally characterized to understand the role of valence electron concentration ( vec) on the prototype Laves phase MgZn2 with Pd-substitution. Three distinctive phase regions were observed with respect to Pd content, all exhibiting fundamental Laves phase structures: 0.1 ≤ x ≤ 0.3 (MgNi2-type, hP24; MgZn1.80Pd0.20(2)), 0.4 ≤ x ≤ 0.6 (MgCu2-type, cF24; MgZn1.59Pd0.41(2)), and 0.62 ≤ x ≤ 0.8 (MgZn2-type, hP12: MgZn1.37Pd0.63(2)). Refinements from single-crystal X-ray diffraction indicated nearly statistical distributions of Pd and Zn atoms among the majority atom sites in these structures. Interestingly, the MgZn2-type structure re-emerges in MgZn2- xPd x at x ≈ 0.7 with the refined composition MgZn1.37(2)Pd0.63 and a c/ a ratio of 1.59 compared to 1.64 for binary MgZn2. Electronic structure calculations on a model ''MgZn1.25Pd0.75'' yielded a density of states (DOS) curve showing enhancement of a pseudogap at the Fermi level as a result of electronic stabilization due to the Pd addition. Moreover, integrated crystal orbital Hamilton population (ICOHP) values show significant increases of orbital interactions for (Zn,Pd)-(Zn,Pd) atom pairs within the majority atom substructure, i.e., within the Kagomé nets as well as between a Kagomé net and an apical site, from binary MgZn2 to the ternary ''MgZn1.25Pd0.75''. Multi-centered bonding is evident from electron localization function (ELF) plots for ''MgZn1.25Pd0.75'', an outcome which is in accordance with analysis of other Laves phases. [ABSTRACT FROM AUTHOR]