X-Ray Diffuse Scattering of Paraelectric Ammonium Sulfate

Ammonium sulfate was studied by X-ray diffraction. The integrated intensities for some of the Bragg reflections decreased discontinuously at the transition temperature -50°C as the temperature was decreased. The fact cannot be explained by the typical order-disorder model. X-ray diffuse scattering was studied at room temperature and -47°C. It was observed that the intensities of diffuse streaks along the [100] direction in the reciprocal space were markedly enhanced as the specimen was cooled. The diffuse scattering was analysed by the soft mode model and the observed intensity distribution was explained qualitatively the assumed softening of the libration of sulfate ions which satisfy the B 1u symmetry. The possibility of the analysis of the diffuse scattering by the order-disorder model was also discussed.