The Vibrational Spectra of Molecules and Complex Ions in Crystals III. Ammonium Chloride and Deutero-Ammonium Chloride

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Abstract
The infra‐red spectra of thin non‐scattering films of NH4Cl and ND4Cl were obtained at 28°, −78° and −190°C. A convenient low temperature transmission type cell usable for such films is described. No indication of fine structure due to free rotation of the NH4+ ions was found. Instead, evidence is presented for the existence, both above and below the λ‐point, of a torsional lattice mode involving the NH4+ ions. The limiting frequencies of the torsional oscillations were observed at about 390 and 280 cm−1 for NH4Cl and ND4Cl, respectively. These values agree quite well with the frequencies calculated on the basis of a purely electrostatic potential function. The spectra of the low temperature modifications indicate strongly that the structures belong to the space group Td1 in which the NH4+ ion symmetry is Td. Of the eight observed bands, two are assigned to the triply degenerate fundamentals ν3 and ν4, one to the overtone 2ν4, one to the combination ν2+ν4 which resonates strongly with ν3, one to the combination of the totally symmetric mode, ν1, with the limiting lattice frequency, ν5, and two to the combinations involving the lattice torsional mode, ν6, i.e., ν4+ν6 and ν2+ν6. The spectra of the room temperature modifications are consistent with a structure in which the NH4+ ion tetrahedra are randomly distributed between the two possible equilibrium orientations in each unit cell. The λ‐point transformations are probably simple order‐disorder transitions between the two modifications.

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