Abstract
A detailed observation of X-ray diffuse scattering in CsCl-type NH 4 Br has been carried out in order to investigate the property of the microscopic interaction to cause the order-disorder phase transitions in this substance. The experimental results are summarized as follows. (i) Diffuse scattering peaked at the super lattice points has been observed up to 30 K above the critical temperature of the cubic (disordered) to tetragonal phase transition. (ii) The temperature dependence of the diffuse scattering shows a typical feature of critical scattering observed in the process of ordering of various physical quantities. In the present case, the critical scattering is attributed to the correlated fluctuation of displacements of Br - ions. (iii) The distribution of the diffuse scattering intensity in the reciprocal space is anisotropic, the intensity contours showing an elongation along [110] direction of the cubic unit cell. These experimental results are consistent with the theory 1) in which the system is treated as an Ising spin-phonon coupled system.
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