[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)

[en] The influence of changes in the pattern of ferroic domain structure on the Raman spectra of β-LiNH₄SO₄ and (NH₄)₃H(SO₄)₂ single crystals were studied. It was shown that the Raman spectra of β-LiNH₄SO₄ passed from the ferroelastic phase differ from those of 'as-grown' crystal and those of the crystal, which was in the paraelectric phase. Significant changes could be observed in the Raman bands related to triply degenerated ν ₃ and ν ₄ vibrations of the SO₄ tetrahedron. Detailed temperature studies of the Raman spectra of β-LiNH₄SO₄ close to the paraelectric-ferroelectric phase transition, exhibit anomaly of some internal vibrations of SO₄ in the temperature range where a regular large-scale structure is observed. Different types of evolution of the ferroelastic domain structure and temperature behaviour of the donor and acceptor vibrations were shown while heating and cooling the (NH₄)₃H(SO₄)₂ crystal. Different values of temperature hysteresis were found in temperature studies of the ferroelastic domain structure (ΔT _S ≅ 3-5 K) and in Raman spectra studies (ΔT _S ≅ 12 K). No changes were observed in the pattern of ferroelastic domain structure at the temperature T _II-III ≅ 265 K, at which C2/c → P2/n structural phase transition takes place. On the other hand, at T _III-IV ≅ 135 K additional domains with W'-type of domain wall orientation were found