Ferid Salehli

The effect of impurities on the phase transitions in the ferroelectric semiconductors TlInS2 and TlGaSe2

The temperature dependences of the dielectric constants of the ferroelectric semiconductors TlInS2 and TlGaSe2 have been studied following their annealing within the incommensurate phase. Unusual memory effects accompanied by both a remarkable inflection of the temperature dependence curves in the incommensurate phase and various shifts of the incommensurate (Ti) and commensurate (Tc) phase transition temperatures have been revealed in both crystals. The observed effects are explained on the basis of a defect density wave model taking into account the interaction of modulation waves with charge carriers localized at impurity states. The thermally activated population of these states during the heating or cooling processes is responsible for the changes of the phase transition temperatures.

Dielectric spectroscopy and nonequilibrium phase transitions in TlGaSe2 layered crystals

The real and imaginary parts of the dielectric function of TlGaSe2 single crystals are investigated in the frequency range 30 Hz–13 MHz and at temperatures 80 K–300 K. A low frequency dispersion due to the interfacial polarization within the crystals is observed. The temperature dependence of relaxation time is obtained in a wide temperature region. A well- distinguished anomalous rise of relaxation time has been established in the 145 K–190 K temperature range for the first time far from the traditional phase transitions known in TlGaSe2 crystals. It is shown that the observed behaviour of relaxation time is due to instabilities in the electronic subsystem revealing themselves as a non-equilibrium phase transition. A close relationship between the observed anomaly and anomalous behaviour of the various physical parameters of the investigated crystals is proposed.

Unusual memory effects in an incommensurate phase of the TlInS2 ferroelectric semiconductor

The experimental data on memory effects in an incommensurate phase are analyzed both in undoped layered TlInS2 crystals selected from different technological batches and in TlInS2: La. Various types of unusual memory effect are detected. It is shown that the observed memory effects are due to pinning of the soliton superstructure by a defect density wave in the internal field of the electret state.

Charge disproportionation in TlGaSe2 crystals detected by dielectric spectroscopy

The electrical properties of TlGaSe2 ferroelectric-semiconductor with layer crystalline structure in the direction parallel to the layers plane is studied by impedance spectroscopy in the frequency range 30 Hz–15 MHz. The impedance spectra were analyzed together with earlier results for perpendicular direction. Low frequency dispersion due to the interfacial polarization from Maxwell–Wagner type space charge effect has been observed. It was established that anomalous rise of relaxation time in the 145–190 K temperature range far from the known phase transitions temperatures is characteristic only for the polarization perpendicular to the layers plane. The model of a specific phase transition taking place in 145–190 K temperature region assuming carriers accumulation was proposed. Charge disproportination is the main characteristic feature of this phase transition which leads to the charge density modulation in one crystallographic direction perpendicular to the layers plane of the crystal.

Effect of the “negative chemical” pressure on the temperatures of phase transitions in the TlInS2 layered crystal

The temperature dependences of the permittivity of TlInS2(1 −x)Se2x solid solutions have been experimentally investigated in the temperature range including the points of structural phase transitions in the solid solutions. It has been established that the isovalent substitution of selenium for sulfur in the anion sublattice of the TlInS2(1 − x)Se2x solid solutions shifts the phase transition temperatures Ti and Tc toward the low-temperature range with a simultaneous decrease in the temperature interval of the existence of the incommensurate phase. The T-x phase diagram is constructed for the solid solutions under study and the coordinates are determined for a critical point (of the Lifshitz type) at which the lines Ti(x) and Tc(x) converge in the phase diagram. The pattern of the T-x phase diagram for the TlInS2(1 − x)Se2x solid solutions has been analyzed within the phenomenological model of a virtual crystal.

Origin of structural instability in TlInS2(1−x)Se2x solid solutions

The influence of the isovalent substitution of selenium for sulfur in the anion sublattice of mixed TlInS2(1−x)Se2x crystals has been studied by means of dielectric measurements. It is shown that transformations in the In4(SxSe1−x)10 structural unit must be taken into account considering the nature of possible phase transitions in TlInS2(1−x)Se2 solid solutions. Possible mechanisms of the phase transition including instabilities in the electronic subsystem of pure TlInSe2 crystals are discussed.

Activated impurity states in the incommensurate phase of ferroelectric semiconductor TlInS2

The effect of annealing within the incommensurate phase on the dielectric function e of the TlInS2 single crystals has been investigated. It is shown that the effect of annealing is very close to the effect of doping by electrically active impurity La. The inference is made that the correlation between observed effects in annealed and doped crystals is conditioned by the internal electric fields induced by the activation (polarization) of native defects during the annealing procedure. The investigations of the second harmonic generation in undoped TlInS2 crystal and the pyrocurrent in TlInS2:La confirms the proposed model.

Temperature oscillating optical second-harmonic generation in TlGaSe2 and TlInS2 single crystals

Second-harmonic (SH) generation effects have been investigated in TlGaSe2 and TlInS2 layered crystals. Pronounced periodical changes of the SH signal with temperature are discovered. The observed effect is explained within the phase synchronism condition, which changes with temperature in the course of thermal expansion. A major role of the high value of dispersion for the refractive indexes of the investigated crystals in the observed phenomenon is proposed.