Effect of ferroic domain pattern changes on the Raman spectra of some ferroic crystals

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 ν3 and ν4 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 (ΔTS ≅ 3–5 K) and in Raman spectra studies (ΔTS ≅ 12 K). No changes were observed in the pattern of ferroelastic domain structure at the temperature TII–III ≅ 265 K, at which C2/c → P2/n structural phase transition takes place. On the other hand, at TIII–IV ≅ 135 K additional domains with W′-type of domain wall orientation were found.