Electronic properties of ionic surfaces: A systematic theoretical investigation of alkali halides

Bulk and surface electronic band structure properties of alkali halides are investigated by using density-functional theory (DFT) in generalized gradient approximation, the GW many-body quasi-particle theory and a quantitative ionic model that combines the atomic ionization potentials and electron affinities with all other parameters determined from first-principles DFT calculations. It is confirmed that the GW approach can predict the band gaps of all alkali halides in good agreement with the experiment with a mean absolute error of about 0.3 eV compared to the available experimental data. The ionic model can well reproduce the trends in the experimental band gaps and ionization potentials, and in some cases can even lead to a quantitative agreement with experiment, but it has severe limitations for the systems containing small ions like Li⁺ for which a reliable estimation of the polarization contribution is difficult to attain. The latter finding can provide clues for the development of new theoretical approaches to ionic systems by combining the ionic model with more sophisticated consideration of the ionic polarization.