The elastic stiffness constants of β-NH 4 LiSO 4 around its phase transition at 460 K

摘要: The complete tensor of the elastic stiffness constantsC ij (i,j=1 to 6) of β-NH4LiSO4 has been measured in the temperature range 290 K to 540 K including the ferroelectric phase transition at 460 K, by the ultrasonic pulse echo overlap method. Some ultrasonic attenuation coefficients were determined. The elastic stiffness constants were calculated using Landau Theory. The elastic stiffness constants are all well described within this theory with the exception ofC 66, which can not be reproduced with coupling terms allowed by group theoretical arguments. This together with double peaks observed in the specific heat and in sound attenuation in some directions leads one to suspect an intermediate phase between the paraelectric and the ferroelectric phases.