Investigation On Materials’ Electronic Properties Based On The “cluster-plus-glue-atom” Model

The “composition-structure-properties” correlations have always been longstanding topics with great importance in materials science. It is well-known that the physicochemical properties of condensed matters are dominated by their chemical composition and atomic structures. Since classical crystallographic method is limited to describe the structure of materials without long-range-order features, a series of cluster-based models have been proposed systematically upon the fact that atomic clusters can serve as the basic structural motifs of materials. Among various cluster-based models, the “cluster-plus-glue-atom” model can be used to describe the structure of almost all materials. Its general significance and application in the structural description and composition design of complex metallic alloys with multicomponents have been confirmed by numerous theoretical and experimental investigations. Denoted via a uniform cluster formula of [cluster](glue atoms)x, the cluster-plus-glue-atom model contains both the basic compostion information and structural information of materials, thus it lays the foundation to uncover the “composition-structure-properties” correlations of materials. The present work is based on this cluster-plus-glue-atom model. Firstly, we analyze the atomic cluster structures of typical alloy phases in Zr-/Ti-based systems by performing first-principles claculations. Then, a new method for determining the characteristic principal cluster and the corresponding cluster formula is proposed to describe the cluster-plus-glue-atom model. Finally, the concrete correlations between atomic cluster structure and electronic properties of materials are investigated in terms of the cluster-plus-glue-atom model. The following three aspects are mainly addressed in this thesis.(1) Based on the different atomic interaction in the cluster-plus-glue-atom model, a new method of central force field model is constructed to determine the characteristic principal cluster for the cluster-plus-glue-atom model. Since a relatively dense-packed atomic cluster can be defined around each atom in the structure of materials, then the characteristic principal cluster that best represents the local structural features of materials need to be selected from these various basic atomic clusters. Considering the interaction between atoms inside the cluster is stronger than that between the clusters corresponding to the glue atoms, while the atomic interaction can be reflected by interatomic force constants(IFCs), then the cluster-plus-glue-atom model can be described by the IFCs. By combination with the atomic dense-packing principle, the central force field method is developed to determine the characteristic principal cluster. The validity and reasonability of this method are checked for the alloy phases in Cu-Zr, Cu-Ti and Al-Ni-Zr systems, by comparision with the available experimetal and theoretical results. It shows that through method of the central force field model, the characteristic principal cluster of the cluster-plus-glue-atom model and the corresponding cluster formula can be obtained conveniently.(2) Electronic configuration plays a crucial role in dominating the structure and properties of materials. Upon the determination of principal cluster and cluster formula, an electron counting rule hidden in the cluster-plus-glue-atom model is analyzed by virtue of the atomic cluster structures and electron concentration related concept. Since the cluster formula of alloy phases is equivalent to the molecular formula of common ionic compounds and covalent compounds, by analogy with the octet rule and its extension followed by most of ionic compounds and covalent compounds, it is revealed that the alloy phases including some intermetallics, quasicrystals and amorphous alloys, also conform to similar electron counting rule. This potential electronic rule hidden in the cluster-plus-glue-atom model indicates that there are certain numbers of valence electrons in the unit cluster formula(Ne/u) of complex metallic alloys, which is described by the electron concentration(e/a) multiplied by the total number of atoms in the cluster formula(Z), i. e. Ne/u=e/a× Z. For convenience, it is designated as the certain electrons cluster formula. Accordingly, the cluster formula issued from the cluster-plus-glue-atom model can be perceived as not only the compostion unit and structural unit of these materials, but also their electronic unit and molecular formula. Finally, a possible interpretation for understanding this electron counting rule is proposed on the basis of complex metallic alloys’ microscopic atomic cluster features.(3) Electrochemical potential(ECP) is an important parameter to control and mediate the equilibrium of stable electronic system. Based on the free electron approximation model and the cluster-plus-glue-atom model, the correlation between atomic cluster structure and ECP is constructed in the present work. It is deduced that the ECP(?) for the system of interest is proportional to the reciprocal of atomic cluster radius(r) squared, i. e. ?=k/r2. The correlation reflected by this equation is systematically validated by the available experimental and theoretical results. Furthermore, some other physicochemical quantities associated with ECP, including Fermi energy, work function and chemical hardness, are also found to be correlated with the atomic cluster radius. Accrodingly, it is revealed that the atomic cluster radius corresponding to the cluster-plus-glue-atom model, can be regarded as an effective structural parameter to measure the ECP and its related physicochemical properties of materials.