Proton Spin—Lattice Relaxation Temperature Dependence in Ammonium Bromide

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Abstract
The lambda transition at 234.5°K in ammonium bromide has been detected by proton spin—lattice relaxation temperature dependence measurements. There is a small but detectable change in T1 at the lambda point. In order to obtain the maximum detail in this temperature region, a special probe was designed to minimize temperature gradients in the sample. Use of this probe allows evaluation of the change in T1 at the transition and determination of the apparent activation energy immediately below the transition. The previously reported x‐ray, dilatometric, extensometric, and acoustical data are best interpreted in terms of a conceptual picture of domains defined by the orientation of tetragonal axes in the region immediately below the lambda point and the indistinguishability of the domains above this temperature. The τc data are consistent with this concept. The degree of order seems to be affected by the orientations of the tetragonal axes in the domains. Comparison of the acoustical data and the τc data suggests that the elastic constants are much more dependent on the electrostatic changes due to the volume changes than is the temperature dependence of τc. The latter seems to depend mainly on the degree of order in the crystalline lattice.