Nonlinear effects in the symmetric phase of layered ferroelectric materials

Özet

The layered ferroelectric $TlInS_2$ possessed a space group of symmetry $C_{2h}^6$ at room temperature undergoes succession of phase transitions below $T_i$ = 216K with decreasing temperature [1]. In the present communication the results of nonlinear behaviour of pyroelectric effect and second harmonic generation observation in the symmetric phase of $TlInS_2$ were reported. Above of $T_i$ = 216K in the paraelectric phase, the unusual behaviour of pyrocurrent as found. The pyrocurrent undergo the maximum at T = 238K with abruptly changes its sign with decreasing temperature. The influence of beas field increases the maximum of pyrocurrent considerably. Chang in the sign of applyed field induces the swiching of pyrocurrent peak at T = 238K. Besides that, near the above mentioned temperature the appearance of second harmonic generation signal have been founded out. The temperature dependence of second harmonic generation in paraelectric phase exhibits a rather unusual behaviour: the second harmonic intensity undulation is observed with decreasing temperature in the range $T_i$ < T < 240K. Heat capacity studies [2] have been shown the existance a series of anomolies in the paraelectric phase at T < 250T. Nevertheless, there are no evidence about symmetry breaking at these temperatures 3. In order to explane of observed nonlinear effects in $TlInS_2$ the following model can be proposed. We assume that the distorsion of $InS_4$ tetrahedron deformes $In_4S_{10}$ polyhedron structural units. This can reduce the local symmetry of each layer and induced local polarization. These mentioned effects are due to the appearance of local polarization below ~ 240K. By taking into account a polytype structure of $TlInS_2$ crystals, it can be supposed that a quasiperiodic space distribution of local polarization along c-axis in the volume of the crystal takes place. This can bring to appearance of optical Fibonacci superlattice where enhancement of the second harmonic generation occurs. Due to the quasiperiodisity of local polarization the should be present. Coupling of second-and third harmonic generation are assure the undulation character of second harmonic generation in paraelectric phase of $TlInS_2$ .