Motions of Molecules in Condensed Systems. VII. The Infra-Red Spectra for Single Crystals of Ammonium Nitrate and Thallous Nitrate in Polarized Radiation

Abstract

Infra-red spectra in plane polarized light for properly oriented, thin sections from single crystals of NH4NO3 (IV), NH4NO3 (III), and TlNO3 (γ) are described and interpreted. The selection of proper orientations for such work is indicated for crystals in general and potential difficulties with some types are noted. Spectra for the first two crystals show (1) that absorptions by molecular modes of nitrate ion, and their combinations, are strongly polarized in directions predicted for these structures by the general theory of crystalline spectra; (2) that envelope structure resulting from combinations between molecular modes and lattice modes is present but not uniquely polarized, again in agreement with the general theory; (3) that absorptions by molecular modes of ammonium ion appear to be unpolarized, indicating that these ions are disordered from rotation or other causes. Spectra for the third substance are used to complete the determination of its crystal structure. One band in this substance has a remarkable envelope consisting of a strongly polarized central branch and, at room temperature, three pairs of symmetrically distributed, virtually unpolarized satellites. The satellites become more numerous at lower temperature (see accompanying article). The spectra for the several substances suggest that optical branches of their lattice mode distributions are located between about 35 and perhaps 200 cm−1.