Pretransitional effect below the ferroelectric-paraelectric phase transition in beta-LiNH4SO4

Detailed birefringence measurements of hydrogenated and deuterated beta-LiN H4SO4 in the close vicinity of the high-temperature phase transition (T-1 = 462 K for hydrogenated, T1 = 465 K for deuterated) are reported. The measu rements of the sample were accompanied by simultaneous observation in the p olarizing microscope. The temperature dependence of the birefringence data displays an additional anomaly at T-1(') approximate to 459 K for hydrogena ted and T-1(') approximate to 462 K for deuterated crystals. In the tempera ture interval between T-1(') and T-1 a regular striped structure was observ ed in the polarizing microscope. Among several possible interpretations of this phenomenon, the most appealing is the assumption of lamellar coexisten ce of the paraelectric and ferroelectric phase over a finite temperature in terval.