Abstract
Molecular rotation in cubic ammonium nitrate was studied by using a single crystal obtained by slow cooling from the melt. The observed X-ray scatterings of two different kinds were used: one is the Laue-Bragg scattering and another the diffuse scattering. The first and the second order diffuse scatterings of disk-like shape (sin(\varphi⁄2)⁄λ×10−8=0.1945, 0.3089) perpendicular to the direction a* and its equivalents in the reciprocal lattice were observed in addition to the diffuse scattering due to ordinary thermal vibration. These diffuse scatterings were considered to be due to the rotation of nitrate groups.
By the study on the Laue-Bragg scattering two models of the hindered rotation were selected and it was found by the study on the diffuse scattering that one of these two models is most probable. Further, the correlation among the individual rotating nitrate groups was considered for interpreting the characteristic intensity distribution of the first order diffuse scattering as well as the existence of the second order diffuse scattering. One type of the correlations which is specified by a parameter P denoting the degree of the randomness in the distribution of the rotation-axes was taken into consideration for the analysis of the diffuse scattering. The model thus obtained is closely connected with the model of the rotation in the tetragonal ammonium nitrate and it seems that there still exists correlation not only between the neighbouring nitrate groups but also between remoter ones.
