Self‐Consistent Phonon Width Calculations in Alkali Halides

The cubic anharmonic phonon decay process is reinvestigated. The δ‐functions which describe energy conservation in the harmonic approximation are replaced by Lorentzians in the self‐consistent theory, whose widths are given by the sum of the line widths of the interacting phonons. For KI at room temperature, the critical points in the damping function are “washed out” almost completely by this line broadening effect and the shift function is reduced considerably, as compared to conventional calculations with constant width parameters.