Polarized reflection spectra between 4.000 and 20 cm -1 and absorption spectra (at temperatures of liquid helium) between 2.000 and 650 cm -1 have been measured of the isomorphous crystals NaNO3, CaCOa, MgCO 3 (D~a) and of CaMg (CO3) 2 (C32i). The wavenumbers of the fundamental infrared transitions and of the twophonon-combinations have been determined. The combinationbands show a structure, which allows some insight into the wavevector dispersion of the phonon branches of these crystals. Selection rules have been worked out for several points in the interior and on the border of the Brillouin zone. By comparison with the spectra they give evidence for the presence of critical points of the phonon-branches for wavevectors parallel to the trigonal axis of the crystal.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.