Electronic Structure of Alkali Metal Oxides and Sulfides

The energy band structure, density of electronic states, imaginary part of the complex permittivity, and valence charge distribution over different crystal planes are calculated for eight M₂A (M : Li, Na, K, or Rb; A : O or S) compounds within the formalism of the local electron density functional with the use of the nonlocal pseudopotentials. The series dependences of the band structure parameters are established. It is demonstrated that the band structures of these crystals differ significantly for the anion and cation. A conclusion on the different characters of hole transport in (Li, Na)₂A and (K, Rb)₂A is drawn based on the specific features of their upper valence band structure. The general regularities and distinctive features of the spectral dependence ₂(E) for the examined M₂A series are discussed. The valence density maxima are localized on anions and interstices, so that chains are formed from the second anionic neighbors covalently bonded by the charge between the anions. Wells with the minimum density formed between the anion chains in oxides may form channels for cation transport and are responsible for the ionic conductivity.