ELECTRONIC CONTRIBUTION TO LATTICE DYNAMICS IN INSULATING CRYSTALS

The electron contribution to the phonon spectrum is formulated in terms of the microscopic response function. The long-wavelength behavior of the lattice vibration is then investigated, with due attention to the long-range Coulomb interaction. An important quantity which emerges is the effective charge of the vibrating nuclei, which is shown to obey a sum rule. From this follows the existence of long-wavelength acoustic phonos. The infrared optical constants are also expressed in terms of the microscopic response function. © 1969 The American Physical Society.