A Molecular Orbital Model of Melting and Glass Formation
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A MOLECULAR ORBITAL MODEL OF MELTING AND GhASS FORMATION
M. E. I£BERHART, K. H. JOItNSON AND R. C. O'HANI)LEY Massachusetts Institute of Technology, 77 Massachusetts Avenue Cambridge, Mass., USA
ABSTRACT
SCF-Xo-SW calculations have been performed on the glass forming crystal ot' Ni•B. The resulting molecular orbital topology near the fermi level is similar to that previously obtained by SCF-Xa-SW calculation on the glass fomling crystals of Pd-Si and Cu-Zr. These similarties, it is conjectured, indicate melting is an electronically driven phase transition and that liquid and glass structures are more the result of electronic than kinetic factors. It is shown that the molecular orbital model of melting in transition mctals will generate structures similar to those resulting from the dense random packing of hard spheres method, and the molecualr orbital mechanism admits, in a qualitative way, the explanation of a broad range of liquid and glass properties.
INTRODUCTION
Recently metallic alloys have been produced in the amorphous or glassy state. These materials are characterized by the absence of long range order, and it is this property which makes these materials interesting technologically yet complicates the development of theories designed to explain their behavior. The understanding of crystalline properties has been inexorably tied to their periodic nature and therefore can provide only limited insight into the amorphous state. Calculations, which attempt to explain metal glass properties as stemming from local interactions, have met with some success and providc a promising direction for future efforts.
STRUCTURAL THEORIES
Attempts to understand the properties of metal glasses have resulted in the creation of a number of structural models. The two most successful of these models are the dense random packing of hard spheres model and the polyhedral or network model. The first model likens the structure of metal glasses to the structure obtained by the dense random packing of hard spheres (DRPHS). This model was originally proposed by Bernal [1] to explain the structure of monatomic liquids. It was later suggested by Cohen and Turnbull [2] that this model might also serve as a good representation for monatomic glasses. These ideas have been extended by others [3,4] who liken the structure of metal-metalloid binary glasses to the DRPHS in which the metal and metalloid are represented by different sized spheres. All investigators have generated essentially equivalent structures which may be descr,•bed as an assembly of polyhedral holes with atoms at the vertices. Five types of holes were characterized by Bernal; these are coordinated by 4, 6, 8, 9 and 10 atoms. The frequency of occurrence is 86, 6, 3.8, 3.7 and 0.5%, respectively.
104 The vast majority of the structure is, therefore, composed of distorted tetrahedra with the occasional occurrence of a trigonal prismatic or antiprismatic hole. The second model for glass structure derives from the fact that icosahedral symmetry is favored over cubocta
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