Raman Spectroscopy of Soft and Rigid Modes in Ferroelectric TlInS₂

Abstract

Low-frequency Raman spectra of a TlInS, single crystal in the vicinity of a ferroelectric phase transition are reported. In the ferroelectric phase a soft mode interacting with some rigid modes is observed. The temperature dependence of the soft mode frequency is found t o be proportional to ( T , -T)W. An unusual behaviour of the rigid mode (w = 20 cm-1) is observed. This fact is interpreted in terms of inherent iattjce defects which cause nonhomogeneous broadening. C~~H~T O~J I~I E T~L I Y~C I E O~O@a30~0ro nepexoaa. B c e r~e~o @ a a e oBHapymeHa MnrKaR kkCJIe~OBaHb1 HB3KOYaCTOTHbIe CIIeKTpbI HPc MOHOIEpMCTaJIna TlInS, B OKPeCTHOCTEl MOXa, B3aHMOQefiCTByJOlrl;aH C PHXOM WeCTHLIX PeIIIeTYaTbIX MOA. BbIaeneHa TeMIIepaTyp-HSlR 3aBBCMMOCTb YaCTOTbI MHrHOfi MOZbI, KOTOPaR OHa3aJIaCb IIpOIIOpL(H0HaJtbHOfi = 20 Cm-' IIpH TeMIIepaTypaX HHme 200 K. Pe3yJIbTaT HHTepIIpeTHpyeTCFI B lIpeEIIOJI0-HWHIIH 0 Cylrl;eCTBOBaHHH CO6CTBeHHbIX ne@eKTOB, IIpBBO~RU@lX IE HeOHHOPOAHOMY YIIIMPeHLIJO ZeCTIEOfi MOHbI.(T, -T)lfz. 06HapyxeHo Tmme aHoMajIbHoe noseneme @OPMM mecTKofi MOAM w =

Ferroelectric phases in the polytypes of TlInS2 ternary compound

“…Comparing the respective temperature dependences of ε -1 (Т) and с(T) for C-and 2C-polytypes of TlInS 2 crystals, it follows that that in 2C-polytype a PT to an incommensurate phase occurs at the temperature T i ≈ 219 K and a ferroelectric PT, which is not accompanied by quadrupling of the unit cell along an axis C, occurs at the temperature Т = Т с ≈ 202 K. The fact that Curie-Weiss law (Curie constant C ≈ 3x10 3 K) is satisfied with decreasing temperature from Т с ≈ 202 K and also that soft mode frequency of which in a wide area of temperatures 30 K ≤ Т ≤ 200 K is proportional to (Т с -Т) 1/2 [4], has been revealed in the Raman spectra of TlInS 2 crystals, enable to suppose that 2C-polytype is proper ferroelectrics with modulated structures which are considered as type II incommensurate systems. Then, taking into account the existence of intergrown different polytypes of TlInS 2 [7], we can understand the mechanism of occurrence and coexistence of two various incommensurate modulated structures in those crystals, one of which has a two-component order parameter, and in the other structure the order parameter is uniaxial spontaneous polarization [15].…”

“…In particular, it concerns the appearance of the polarized state in these crystals. There can be no doubt, that phase transition (PT) to an incommensurate phase is the preceded transition in a polar state [1][2][3][4][5][6]. According to results of different authors, the temperature T i of PT paraphase-incommensurate phase can slightly change from crystal to crystal (T i =213-219 K).…”

“…Identical ferroelectric soft mode was discovered in the Raman spectrums later [4]. According to the microscopic theory it means, that ferroelectric PT is the proper transition of "order-disorder" type.…”

Influence of Isomorphic Atom Substitution on Lattice Anisotropy of Thallium Dichalcogenide Layered Mixed Crystals

“…Now let us consider the variation of the lattice volume on the compositions of the mixed crystals studied. It is well known from the literature that TlGaSe 2 and TlInS 2 sequentially undergo low-temperature transitions to an incommensurate phase and a ferroelectric phase at T i = 115 K, T c = 108 K and T i = 213 K, T c = 189 K, respectively [8][9][10][11]. However, in TlGaS 2 crystals no evidence for a soft mode behavior has been found.…”

Temperature Dependence of Raman-Active Modes of TlIn(0.95Se0.05)2 Single Crystals

“…Several works were devoted to Raman spectroscopic studies of TlIn(S 1− Se ) 2 solid solutions (0 < < 1), where the compositional transformation of Raman spectra was investigated mostly at room temperature [15][16][17][18][19], as well as at lower temperatures [19]. With regard to the studies of vibrational spectra in a broad temperature range, including the phase transition range, such data were obtained for TlInS 2 crystals by several research teams [20][21][22][23][24][25][26][27][28][29]. A detailed analysis of the polarized Raman spectra of TlInS 2 was performed in a broad temperature range, including the temperature dependences of spectral positions and half-widths of low-frequency modes, and a conclusion was made on the existence of soft modes [24,25].…”