F.A. Mikailov

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 properties and metastable states in ferroelectric TlInS2 crystals

Abstract A phase of antiferroelectric type of hysteresis preceding the transition to the ferroelectric state has been observed in Thallium Indium sulfide crystals. A possible coexistence of polar regions and domains of the ferroelectric and the T C1 - T C2 phases over a wide range of temperature is suggested from analysis of dielectric studies. A metastable behaviour of permittivity observed at thermocycling is explained by existence of a chaotic state below the ferroelectric phase transition temperature T C2 = 201 K. A model of two polar sublattices in the region of phase coexistence in TlInS2 is offered.

Succession of the low temperature phase transitions in TlInS2 crystals

Abstract Two additional incommensurate phase transitions (PTs) at T i = 206 K and T tr = 79 K were observed in TlInS 2 crystals. Peculiarities of the temperature behaviour of the dielectric susceptibility and the dielectric loops allowed to consider the PTs at T c1 = 204 K and T c2 = 201 K as incomplete lock-in transitions and the PT at T≈195 K as a temperature range of the final lock-in transition. The PT at T tr = 79 K was considered as an isomorphous transition. The experimental results of the second harmonic generation signal at low temperature are also in favour of such a conclusion. Peculiarities of the temperature behaviour of the dielectric properties at low temperature are qualitatively explained by means of the theory of two non-equivalent sublattices.

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.

Memory effect in layered semiconductor TlInS2 with incommensurate phase

Abstract The temperature dependence of pyroelectric current was studied for the ferroelectric-semiconductor crystal TlInS2 in the temperature range of commensurate and incommensurate phases. A remarkable shift of the lock-in transition point to lower temperatures has been observed on heating runs after turning from a heating to a cooling from different temperatures within the incommensurate phase. The observed effect was interpreted by the pinning of modulation due to mobile defects in the crystal having layered structure and was attributed to a kind of memory effect.

Phase transitions and metastable states in TlGaSe2

The results of measurements of the dielectric constant of TlGaSe2 in temperature range of successive phase transitions are presented. An anomaly in the temperature dependence of the real part of dielectric constant in TlGaSe2 has been observed at about 242 K in addition to anomalies at 115, 108, and also near 65 K as reported in previous publications. The presence of temperature hysteresis effects in temperature interval between 115 and 242 K allowed making a conclusion about possible existence of an incommensurate phase in the mentioned temperature range. A model of succession of the structural phase transitions in TlGaSe2 has been suggested.

Improper and proper ferroelectric phase transitions in TlInS2layered crystal with incommensurate structure

The dielectric constant of TlInS2 crystal was measured in the temperature range including incommensurate (IC1 and IC2) and commensurate (C1 and C2) phase transitions under external bias electric field. It has been revealed that maxima in the temperature dependence of dielectric constant of TlInS2 at the C1 and C2 phase transition points shift to higher temperatures. The anomaly at IC2 transition point shifts to lower temperatures on applying the external electric fields. A new complete theoretical model including the presence of two order parameters and two polar sublattices in TlInS2 has been suggested.

Electron Paramagnetic Resonance (EPR) Investigations of Iron-Doped Ferroelectric Ternary Thallium Chalcogenides

Abstract In this review article, we summarize the results of the first investigations of EPR spectra of iron-doped ternary thallium chalcogenides TlInS2, TlGaSe2, and TlGaS2 in a wide temperature interval including previously reported structural phase transitions. In these compounds, Fe ions substituted Ga and/or In sites in crystal structure and served as local probes to study ligand crystal field and its local symmetry around these paramagnetic sites. An overview of our recent EPR studies carried out at room temperature and in the temperature interval including structural phase transitions is provided. The results revealed the fine structure of EPR spectra of the paramagnetic Fe3 + ions and the local symmetry of crystal field (CF) at Fe3 + site and CF parameters. As a result of summarizing the low-temperature EPR investigations a discussion about active atomic groups, possible atomic displacements, and the local symmetry changes during the low-temperature phase transformations is presented.

Dielectric constant and EPR spectra of Fe doped TlInS2 crystal near the structural phase transitions

Dielectric constant and electron paramagnetic resonance (EPR) spectra of Fe-doped TlInS2 crystal has been investigated in the temperature range of 5–300 K. The influence of Fe impurities on dielectric properties of TlInS2 crystal has been considered in comparison with earlier observed results from pure TlInS2 compounds. Considerable decrease of the dielectric constant, as well as the change of the shape of its temperature dependence in a result of influence of doped iron atoms, has been observed. It has been established that EPR lines exhibit remarkable splitting and appearance of additional resonance lines at the temperatures lower than 200 K, which is attributed earlier as the temperature of the ferroelectric phase transition. The simulation for low-temperature EPR spectra has been performed taking into account the appearance of four additional centers in addition to four equivalent centers observed from EPR spectra at room temperature. Such transformations are considered as result of nonequivalent displacements of different groups of Tl atoms during the structural phase transitions.

Thermal hysteresis and memory effects in TlGaSe2 crystal with incommensurate phase

Temperature dependencies of dielectric permittivity of TlGaSe2 have been measured under various thermal cycles. Peculiarities of anomalies in temperature dependencies of dielectric permittivity corresponding to structural phase transitions at 108 and 115 K are discussed. The coexistence of two different incommensurate structures in TlGaSe2 was proposed. The phase transitions at 108 and 115 K are considered as commensurate lock-in transitions. As a result a new model of the structural phase transitions in TlGaSe2 has been suggested.