Electron-phonon interaction and structural instability of mixed valence compounds Cs 2Au 2X6 ( X = Cl, Br, I)

Abstract

The electron-phonon interaction of mixed valence compounds Cs 2Au 2 X 6 ( 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 Cs 2Au 2 X 6 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.