[en] Detailed birefringence measurements of hydrogenated and deuterated β-LiNH₄SO₄ in the close vicinity of the high-temperature phase transition (T₁ = 462 K for hydrogenated, T₁ = 465 K for deuterated) are reported. The measurements of the sample were accompanied by simultaneous observation in the polarizing microscope. The temperature dependence of the birefringence data displays an additional anomaly at T₁' approx. 459 K for hydrogenated and T₁' approx. 462 K for deuterated crystals. In the temperature interval between T'₁ and T₁ a regular striped structure was observed in the polarizing microscope. Among several possible interpretations of this phenomenon, the most appealing is the assumption of lamellar coexistence of the paraelectric and ferroelectric phase over a finite temperature interval. (author)