Electronic states associated with the surfaces of one-dimensional ordered mixed crystals: I. Direct MO-LCAO method

The electronic states of one-dimensional mixed crystals, composed of two kinds of atoms arranged in an ordered sequence, have been examined by means of the MO-LCAO approach and the tight-binding approximation. The energy levels of the volume states are found to lie in two bands, which are separated by a forbidden energy gap, whose width is characteristic of the mixed crystal. Depending on their existence conditions, the surface and chemisorption states can have energies which lie either outside the two main bands ( outer states ) or in the gap between them ( inner states ). Comparisons are made with previous work on pure crystals.