The electronic structure of disordered systems.

A theoretical study has been carried out on the electronic structure of concentrated disordered alloys and liquid semiconductors made up of components which are metallic in the pure liquid state (e.g. Mg-Bi, Li-Bi). For the latter there is strong evidence to suggest that chemical complexes form at the critical concentration (Mg3 Bi2, Li2Bi). The nature of the bonding is discussed in depth to reveal that it is unlikely that the bonding is ionic in the same sense as sodium chloride. Two separate calculations on the single hydrogen molecule in an electron gas show that the binding energy decreases with increasing electron number density. The assumption of the covalently bonded Mg3 Bi2 complex, and the associated change in binding energy with varying electron environment away from the critical composition, does not provide good agreement with experimental thermodynamic and electron transport data. Charge transfer within disordered systems is an effect associated with interatomic bonding. Its relationship with electronegativity difference and atomic cell size is considered in detail. No simple correlation is found to exist between charge transfer and electronegativity difference. A phenomenological study carried out on the liquid semiconducting Mg-Bi system has demonstrated that it is possible for an energy gap to occur at the critical composition with the filling of the s-p bands on the bismuth atoms. No presumptions of molecular formation are made. It does appear that this interpretation can provide a scheme, in terms of the positions of the elements within the periodic table, to explain the occurrence of many liquid semiconductors.