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

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 T_d¹ in which the NH₄⁺ ion symmetry is T_d. 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.