Calculation of Energy Bands in Alkali Halides

The electronic energy bands in LiF, NaCl, and KBr have been calculated using the augmented-plane-wave (APW) method. The usual APW method is improved upon by including nonspherical terms in the potential and by varying the Slater free-electron-exchange approximation in order to match experimental band-gap data. It was found that the nonspherical terms have about an 0.8-eV effect in LiF, a 0.4-eV effect in NaCl, and a 0.1-eV effect in KBr. It was found that if the exchange potential is increased by about 10%, experimental band-gap data can be matched. Transitions calculated from the energy-band calculations were compared with experimental optical-absorption and optical-reflectance data. Agreement is generally good. Using numerical differentiation, the effective mass at the bottom of the conduction band was calculated. The effective masses were found to be approximately 1.2mₑ for LiF, 0.6mₑ for NaCl, and 0.5mₑ for KBr. Experimental values exist only for KBr and are in good agreement.