Second harmonic generation in layered TlInS2 in the vicinity of the low-temperature phase transitions

Second-harmonic generation in the ternary layered semiconductor T1InS(2) excited with the wavelength lambda = 1.06 mum of YAG:Nd3+ laser is investigated in the temperature range corresponding to the low-temperature phase transitions. It is shown that the intensity of the second-harmonic signal corresponding to the non-linear coefficients d(eff), d(21), d(22) reveals peculiarities close to the commensurate-incommensurate phase transitions. The temperature hysteresis of the second-harmonic signal in the low-temperature region (below and close to T-i2=206K) is explained assuming that an incomplete lock-in transition in T1InS2 takes place at T-c1 = 204 K within the temperature range between a ferroelectric (T-c2 = 201 K) and an incommensurate (T-i2 = 206 K) phases. II is shown that the monoclinic point group symmetry Ca is preserved also at temperatures lower than the phase transition temperature T-c4 = 79K to a weak ferroelectric state.