Stabilization of the low-temperature phase near the layered crystal surface

Abstract

The temperature dependence of the integrated band intensities in the ε''(ω) and (-1/ε(ω)) spectra of a TlGaSe2 single crystal was investigated. The above spectra were calculated from IR-reflection ones. A two-stage increase of integrated intensities τω of bands, appearing at the phase transition (PT) in the ferroelectric phase, was observed. The beginning of the first stage corresponds to the temperature Tc1, which is 10-20 K higher than Tc determined in other works. The beginning of the second stage corresponds to the temperature near Tc. Since IR-reflection spectra are formed in the surface layer of thickness λ/n(λ) (λ is the wavelength, n(λ) is the refractive index), the observed dependence τω (T) might be explained by successive passing of two PT's; the first one takes place at T=Tc1 in a surface layer of thickness d < λ/n (λ). The second one occurs in bulk at T=Tc. In a thin monocrystalline film (d ≃ 1 μ) a monotonous increase of Iω (T) was observed, i.e.there, as it was expected, only one PT at T=Tc1 takes place. The fact that the surface has an influence on the temperature of the surface layer PT was confirmed by investigations of reflection spectra from the surfaces oriented differently with respect to the axis [image omitted].