Abstract
The dynamical critical phenomena associated with the structural phase transition of N Br ( °K) have been studied by neutron-scattering techniques. The measurements of the low-energy part of neutron spectra of NBr single crystals have been carried out along the principal symmetry directions of cubic reciprocal space. The spectra observed along the [110] direction show a "triple-peak" structure. The central (quasielastic) component is clearly due to the coupled relaxational motion of displacement of ions and the flipping of N ions. The width (full width at half-maximum) of the central component is less than the instrumental resolution ( meV). In addition to the central component, there exists a well-defined T phonon mode at the [110] zone boundary whose eigenvector exactly corresponds to the spontaneous displacement of ions in the low-temperature phase. However, this phonon mode does not show any appreciable softening nor broadening throughout the temperature region of °K. Rather the intensity of the central component at the point critically increases. These experimental results are analyzed by looking upon the system as a pseudospin-phonon coupled system. It is shown that the observed neutron spectra are satisfactorily explained with such a formalism by taking the relaxation time of the flipping of an independent ammonium ion longer than 1.3 × sec. The distribution of intensity of the central peak shows two maxima in the reciprocal space, one at the point (zone center) and another at the point ([110] zone boundary). This gives direct evidence for the existence of the two competitive interactions: direct interaction between ammonium ions and indirect interaction via phonon modes. This competition is postulated to explain the two successive phase transitions which occur in NBr.
- Received 30 October 1973
DOI:https://doi.org/10.1103/PhysRevB.9.4429
©1974 American Physical Society