Far-Infrared Properties of Lattice Resonant Modes. III. Temperature Effects

We have measured the temperature-dependent absorption spectrum associated with resonant modes in KBr: ${\mathrm{Li}}^{+}$, NaCl: ${\mathrm{Cu}}^{+}$, KI: ${\mathrm{Ag}}^{+}$, and Mn${\mathrm{F}}_2$: ${\mathrm{Eu}}^{2+}$. A comparison is given of the measured temperature dependence with that expected from a linear-coupling theory using the known static stress coupling coefficients. We find that the linear-coupling model is capable of explaining the measured temperature dependences provided the active resonant mode is coupled to another inactive resonant mode, but not if the coupling is to a Debye spectrum of lattice modes.