Electron-phonon interaction and structural instability of mixed valence compounds Cs₂Au₂X₆ (X = Cl, Br, I)

The electron-phonon interaction of mixed valence compounds Cs₂Au₂X₆ (X = Cl, Br, I) is studied microscopically on the basis of the realistic electronic band structure in their ideal cubic phase. The electron-phonon interaction between the conduction-band states near the Fermi level is especially strong for the stretching-type displacements of halogen atoms, which correspond to the E_g phonon mode at q = (π/a,π/a,π/a). The tetragonal distortion observed in Cs₂Au₂X₆ is originated from the characteristic dependences of the electron-phonon interaction on wavevectors and vibrational modes as well as the nesting features of the Fermi surface.