M. H. Yu. Seyidov

Dielectric susceptibility behaviour in the incommensurate phase of TlInS2

Abstract Peculiarities of the temperature behaviour of the dielectric susceptibility of TlInS2 in the temperature interval of incommensurate (IC) phase have been theoretically investigated on the basis of recently suggested two-sublattice model of the phase transitions in TlInS2, which is based on the hypothesis about the coexistence of improper and proper ferroelectricity and also the presence of type I and type II incommensurations in the same compound. The temperature dependence of the dielectric constant of TlInS2 in the IC phase was calculated. An agreement between theoretical and experimental results has been obtained.

Effect of external fields on the memory effect of the incommensurate phase in the ferroelectric-semiconductor TlGaSe2

The effect of external fields (dc electric field, light illumination) on the memory effect of the incommensurate phase in the ferroelectric-semiconductor TlGaSe2 is studied using the measured dielectric constant. The results obtained are discussed. It is shown for the first time that the effect of external fields on the anomaly related to the memory effect in TlGaSe2 can be reduced to the following universal empirical rule: when a sample is held for many hours at a constant temperature T0 in the temperature range of the incommensurate phase in a dc electric field, the deflection amplitude in the low-temperature part of the anomaly in the temperature dependence of the relative change in the dielectric constant Δɛ/ɛ increases (the deflection in the high-temperature part of the Δɛ/ɛ anomaly disappears) as compared to this segment in the dependence obtained during isothermal annealing of this sample at the same temperature without an electric field. The crystal remembers its thermal history at a temperature that is several kelvins higher than T0. Light illumination increases the deflection amplitude in the high-temperature part of the Δɛ/ɛ(T) anomaly and shifts the temperature at which the crystal remembers its thermal history toward lower temperatures with respect to T0.

Photodielectric effect in TlInS2 activated by the La impurity

The influence of light on the low-frequency dielectric properties of the TlInS2 layered crystal is investigated for the first time. It is established that the illumination of the crystal during measurements leads to a substantial change in the behavior of the temperature dependence of the permittivity ɛ of the TlInS2 compound doped with the lanthanum impurity in the range of the existence of the incommensurate phase. The temperature dependences of the permittivity ε of the TlInS2 compound doped with the lanthanum impurity after preliminary cooling of the crystal in constant electric fields with different strengths are studied for the first time. The inference is made that the experimentally observed photodielectric effect is associated with the localization of charge carriers at defect levels in the band gap of the crystal with the formation of local polarized states.

Anomalies in the electrophysical, thermal, and elastic properties of layered ferroelectric semiconductor TlGaSe2: Instability in the electronic subsystem

The temperature dependences of the permittivity, dark current, and current-voltage characteristics of the layered ferroelectric semiconductor TlGaSe2 were measured over the range 77–300 K. The ε(T) curve measured perpendicular to the layers at low frequencies is discovered for the first time to exhibit an anomalous dip with clearly defined boundaries at ∼150 and ∼200 K. Electrical instability in the form of low-frequency oscillations of current in current-voltage characteristics of a sample in the same temperature range are also observed experimentally for the first time. It is shown that this instability occurs only if an electric field is applied to a sample using potential-controlling contactless electrodes in the form of thin mica spacer layers. The nature of the instability and its influence on various physical properties of the layered TlGaSe2 crystal are discussed.

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.

Photoelectric activity of structural defects of a single crystal of the ferroelectric-semiconductor TlInS2: La

Photoinduced current transient spectroscopy has been used in studying deep-level defects of the ferroelectric-semiconductor TlInS2. The specific features of defect detection in the lanthanum-doped and undoped single crystals have been compared. The explanation of the decrease in the photoelectric activity of defects in the doped sample has been proposed.

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.

Pyroelectric properties and structural defects of a layered TlInS2 crystal doped with lanthanum

The results of investigations of the pyroelectric current in a layered TlInS2 crystal doped with lanthanum are presented. A comparative analysis of the obtained results and data of photo-induced current transient spectroscopy has been performed. The revealed features of the contribution from electrically active defects to pyroelectric properties of the crystal have been discussed.

Photovoltaic currents and activity of structural defects in a ferroelectric–semiconductor TlInS2: La single crystal

This paper reports on the results of the investigation of electrically active defects in the crystal structure in a layered ferroelectric–semiconductor TlInS2: La crystal by photoinduced current transient spectroscopy (PICTS). It has been found that there are states of the crystal that differ in the magnitude of the photoresponse varying within four orders of magnitude, which is interpreted in terms of the differences in states of the domain structure of the crystal. The specific features of the recording of thermal emission from defects in the presence of an additional contribution from the photovoltaic component of the response of the crystal to excitation with light have been discussed.

Parametric resonance and photogalvanic currents in layered TlGaSe2 crystals

The results of the phenomenological study of the abnormal photoresponse kinetics in layered TlGaSe2 ferroelectric semiconductor have been discussed over the temperature range T of ~170–280 K corresponding to the paraelectric phase of crystal. Taking into account the alterations in the photoresponse kinetics temperature, the main mechanisms of anomalies caused by the spatial inhomogeneity of localized and nonlocalized charges in the bulk of the crystal have been assumed. The mechanism of parametric resonance is suggested to be favored by the photogalvanic currents in the crystal.