Understanding Electron Compounds With Cluster-Plus-Glue-Atom Model

Electron compounds are a special kind of intermetallic with specific electron concentration. Even though they are made of different elements with different structures, they can be classified into three different groups with three different electron per atom, e/a, values, which are1.5(21/14),1.615(21/13),1.75(21/12).In comparison with size factor and negativity factor, the electron factor has prominent influence on the stability of those structures. With the development of theories and experiments, the concept of electron compounds expands from crystalline phases to quasi-crystalline phases, quasi-crystalline approximants and amorphous structures; the stabilization mechanisms are also developed from Fermi Surface-Brillouin Zone effect in reciprocal space for crystalline and quasi-crystalline structures to Friedel oscillation in real space for bulk metallic glasses.In our research group, cluster-plus-glue-atom model is used to divide a structure into cluster part and glue atom part, which has been used to design and optimize bulk metallic glasses. Then cluster-plus-glue-atom-model is combined with Friedel oscillation into cluster-resonance model, in which the relationship between electron per atom with the atom number in cluster of ideal metallic glasses are proved to be e/a=24/Z, and the electron per unit cluster for ideal metallic glasses is24.Based on conclusions from cluster-resonance, cluster-plus-glue-atom model is applied to crystalline electron compounds to obtain principal clusters, cluster formulas, electron per unit cluster and possible24-electron clusters.First, the rule of e/u values in crystalline electron compounds is discussed. After considering overlapping between clusters in crystalline electron compounds, it is discovered that electron compounds can be classified according to the e/u values. For phases with e/a=21/14, e/u is3for AgCd, AgMg, AgZn, AuCd, AuMg, AuZn, BeCu, CaT1, CuZn, MgT1, SrT1; e/u is6for Ag3In phase; e/u is9for Au5Sn phase. For phases with e/a=21/13, e/u is21for Ag9In4, Al4Cu9, Au9In4, Cu9Ga4, Cu9In4; e/u is10.5for Ag5Cd8, Ag5Zn8, Au5Cd8, Cu5Zn8. For phases with e/a=21/12, e/u is21for AuCd3phase; e/u is7for Cu3Ge phase; e/u is4.7for CuZn3phase. Cluster-plus-glue-atom model is advantageous in presenting short-range order, so it is an effective way to distinguish phases with identical e/a values whose component elements and structures are quite different and a good compensation of e/a. Second, possible24-electron clusters are presented and the relation e/a=24/Z in electron compounds is discussed. And it is shown that the relation can only be partly satisfied in electron compounds.