Nature of the λ Transition and the 56 cm−1 Anomalous Raman Mode in Ammonium Bromide

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Abstract
The temperature behavior of the Raman spectrum of NH4Br in the 56 cm−1 region is closely associated with the mechanism underlying the order‐disorder phase transition. This paper reports the detailed measurement and theoretical analysis of the spectral band shape, polarization, and integrated intensity of this low frequency mode. Above the phase transition temperature (Tλ), the scattering at 56 cm−1 is anomalous due to the fact that it is associated with a zone boundary T A phonon. Below Tλ, the tetragonal distortion of the crystal halves the size of the Brillouin zone and relocates the zone boundary T A phonon to the zone center. One of the new zone center phonons is Raman active (A1g symmetry) and can be observed in the a (cc) b scattering configuration. The intensity and spectral band shape of the A1g mode display unusual temperature behavior. In the vicinity of Tλ, a spectral structure exists in the αcc Raman spectrum (∼3 cm−1 at 230°K). Upon cooling of the crystal below Tλ, the structure disappears rapidly, accompanied by a large growth of the integrated scattering intensity. In addition to the intensity increasing Raman active mode, an intensity decreasing mode appears in the b (aa) c scattering configuration. This mode is not predicted by group theory and is attributed to the scattering from the order parameter for the reason that it is associated with the short‐range order that exists above Tλ. A microscopic theory is formulated, relating the intensity behavior associated with the a (cc) b and b (aa) c spectra to the long‐range order parameter. It is shown that it is the evolution of the long‐range order parameter which governs the increase of the a (cc) b as well as the decrease of the b (aa) c Raman spectral intensity.