  CO$_2$ ice is an important reservoir of carbon and oxygen in star and planet
forming regions. Together with water and CO, CO$_2$ sets the physical and
chemical characteristics of interstellar icy grain mantles, including
desorption and diffusion energies for other ice constituents. A detailed
understanding of CO$_2$ ice spectroscopy is a prerequisite to characterize
CO$_2$ interactions with other volatiles both in interstellar ices and in
laboratory experiments of interstellar ice analogs. We report laboratory
spectra of the CO$_2$ longitudinal optical (LO) phonon mode in pure CO$_2$ ice
and in CO$_2$ ice mixtures with H$_2$O, CO, O$_2$ components. We show that the
LO phonon mode position is sensitive to the mixing ratio of various ice
components of astronomical interest. In the era of JWST, this characteristic
could be used to constrain interstellar ice compositions and morphologies. More
immediately, LO phonon mode spectroscopy provides a sensitive probe of ice
mixing in the laboratory and should thus enable diffusion measurements with
higher precision than has been previously possible.
