TRENDS IN THE BAND-STRUCTURE OF DEFECT TETRAHEDRAL COMPOUND SEMICONDUCTORS - OXIDES AND OTHER SYSTEMS WITH 4-2 LOCAL COORDINATION

A tight binding Hamiltonian is used to investigate the electronic structure and the average local character of eigenstates in a large class of compounds with 4-2 local coordination. General theorems are established for AX//2 and AB X//4 compounds relating the existence of gaps to the local environment. General expressions are derived concerning the ″s″ or ″p″ character of the eigenstates on any atom. The highest valence band is shown to be in all cases a pure p non bonding or lone pair band on the X atom. One finds quite generally that the valence band of AX//2 systems splits into three subbands for which only two distinct possibilities can exist. Additional gaps are obtained for AB X//4 systems. A detailed numerical investigation is made especially in the important case of oxides.