Energy Band Structure and Fundamental Optical Absorption of Alkali Cuprous Halide and Alkali Silver Halide Crystals

The fundamental optical absorption bands have been found at about 4.5 eV in the alkali cuprous halide (ACH) crystals of K₂CuCl₃, K₂CuBr₃, Rb₂CuCl₃ and Rb₂CuBr₃, and from 3.9 to 5.2 eV in Rb₂CuI₃. From the measured ultraviolet photoelectron spectra of the ACH's and the alkali silver halide (ASH) crystals, it has been concluded that the uppermost valence band of the ACH and the ASH are the d-band of the cuprous ions and the ρ-band of the halogen ions, respectively. The energy band structure of K₂CuCl₃ and Rb₂AgI₃ has been calculated by an augmented-planewave method and it is shown that the direct band gap exists at the Γ point in each crystal. The features of the calculated valence bands are consistent with the optical absorption and the photoelectron spectra. The calculated effective mass of an electron at the bottom of the conduction band shows small anisotropy.