Optical-Phonon Relaxation by Electronic Excitations in CoF2

We propose a new phonon relaxation mechanism, in which the phonon couples to two electronic excitations, nearly resonant in energy. As a striking illustration of this effect we explain the anomalous temperature dependence of the linewidth of the $\overrightarrow{\mathrm{k}}$, $E_g$ phonon in Co${\mathrm{F}}_2$ by its coupling to physically renormalized exciton-magnon states which detune with increasing temperature.