Bond Orbital Model for IV-VI Compounds

A new bond orbital model is proposed for IV-VI compounds and applied for the evalu­ ation of polarity, static effective charge, cohesive energy and transverse effective charge of five IV-VI compounds PbS, PbSe, PbTe, SnTe and GeTe. The model contains five parameters which are determined so as to reproduce the results of OPW band calculation at a few points of high symmetry in the Brillouin zone. Numerical results of the model calculations are compared with corresponding experimental data and short discussion is given on the structural phase transition in SnTe and GeTe. J Many interesting physical properties of (V) compounds have been explored almost exhaustively. One of the most remarkable features among them is the structural phase transition: GeTe and SnTe undergo a phase transition from the rock-salt structure to the A-7 type structure, while PbTe, PbSe and PbS have no such transition. These differences among the (V) compounds with respect to the structural phase transition certainly reflect some differences in the electronic states which affect their cohesion and stability of the lattice structures. Thus it seems worth while clarifying the differences in these properties from their energy band structures, attributing the origin to the differences in the electronic states of the constituent atoms of these compounds. For this purpose, we propose in this paper a simple model which enables us to describe the main parts of the electronic energy band structures of the (V) compounds in a simple and global way. We apply this model to calculate the polarity, static effective charge, cohesive energy and transverse effective charge of each compound. The polarity defined in this paper will in some sense correspond to the ionicity first introduced by Pauling5J in his pioneering work on partially covalent materials, and it turns out that this quantity is useful for coordinating the properties of the (V) compounds. For the (IV) compounds, Phillips and Van Vechten succeeded in developing a new scheme of electronic theory to correlate various properties of the (IV) compounds in a unified manner.6J On the other hand, Harrison proposed a bond orbital model (BOM) to relate a wide range of properties of the (IV) compounds