Exciton—Optical-Phonon Coupling in CoF2

We present a theory of the temperature dependence of the frequency, linewidth, and infrared absorption strength of the $E_g$ optical phonon in Co${\mathrm{F}}_2$, assuming that the coupling between the phonon and the $\mathrm{AD}$ exciton is responsible for these effects. We find that a model based on the orbit-lattice interaction can account for these phenomena. We also predict that the $E_g$ phonon should be split by application of a magnetic field parallel to the $c$ axis. We estimate the "$g$ factor" associated with this splitting and find that one should be able to observe the splitting in readily attainable magnetic fields. The temperature dependence of the above phenomena in the paramagnetic and antiferromagnetic state is discussed. A point-charge estimate of the optical-phonon—exciton coupling constant produces a value much smaller than that required to fit the data.