Electronic band structures of mixed valence compounds Cs₂Au₂X₆ (X = Cl, Br, I)

Electronic band-structure calculations are carried out for mixed valence compounds Cs₂Au₂X₆ (X = Cl, Br, I) by using the self-consistent augmented-plane-wave (APW) method. The calculations are performed for an ideal cubic perovskite-type structure as well as a tetragonal structure observed at ambient pressure. For the cubic structure, conduction bands consist of hybridized Au 5dγ and Cl 3p (or Br 4p, I 5p) states and have (dpσ) antibonding character. A part of the Fermi surface shows clearly strong nesting feature for a wavevector (Π/a₀, Π/a₀, Π/a₀), where a₀ denotes a lattice constant of the cubic structure. The nesting effect of the Fermi surface is responsible for the lattice distortion observed at ambient pressure. For the tetragonal structure, an energy gap of about 1 eV appears in the electronic band structure. Optical spectra and contact charge density at Au nuclei calculated based on the band structure show qualitative agreement with recent measurements.