Evidence for Order Parameter Spatial Fluctuations in TlGa(Se_1−xS_x)₂ Crystals by Raman Spectroscopoy

Raman spectra of the ferroelectric layered crystals TIGa(Se_1−xS_x)₂ are studied for concentrations x = 0, 0.05, 0.15 over a wide temperature range. The contribution of the order parameter spatial fluctuations to the spectral line parameters is defined by comparing the concentration dependence of the low-frequency integral intensity to that of the splitting of a doubly degenerated hard mode at ≈ 55 cm⁻¹. Large spatial fluctuations of the order parameter are assumed to be caused by the presence of an order–disorder component in the ferroelectric instability of the layered crystal. The order—disorder component is thought to be related to a narrow one-dimensional soft phonon band.