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Brillouin scattering spectra of NH 4 LiSO 4 were measured in a temperature range of 20 ∼250°C including the ferroelectric phase transition temperature, T c =186.5°C. The principal indices of refraction n a and n c were also measured in the same temperature range in order to calculate the elastic constants from the Brillouin shifts. The elastic constants c 11 , c 22 , and c 33 all show quite similar temperature dependence; they are nearly constant above T c and increase monotonically with decreasing temperature below T c . It is shown, on the basis of the phenomenological theory, that the observed elastic anomalies are ascribed to the thermal expansion and/or the fourth-order coupling between the order parameter and the strains.
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Hirotsu et al. (1981) studied this question.
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