TlInS2 Crystals Research Articles

Low-temperature X-ray studies of TlInS2, TlGaS2, and TlGaSe2 single crystals

The results of X-ray diffraction studies of the unit-cell parameters and thermal-expansion coefficients of TlInS2, TlGaS2, and TlGaSe2 crystals in the temperature range 100–300 K are described. It is shown that the unit-cell parameters of all the studied crystals gradually increase with increasing temperature. The temperature dependences of these parameters exhibit anomalies in the form of bends and kinks at temperatures corresponding to phase transitions in the crystals. The thermal-expansion coefficients along the [001] crystallographic direction of the crystals under study are determined. It is found that their values slightly change with increasing temperature.

Multiphonon absorption processes in layered structured TlGaS2, TlInS2 and TlGaSe2 single crystals

The infrared transmittance and Raman scattering spectra in TlGaS2, TlInS2 and TlGaSe2 layered single crystals grown by Bridgman method were studied in the frequency ranges of 400–1500 and 10–400cm−1, respectively. Three, three and five bands observed at room temperature in IR transmittance spectra of TlGaS2, TlInS2 and TlGaSe2, respectively, were interpreted in terms of multiphonon absorption processes.

Effect of ionizing radiation on the dielectric characteristics of TlInS2 and TlGaS2 single crystals

The temperature dependences of the electrical conductivity and the permittivity of TlInSe2 and TlGaTe2 crystals unirradiated and irradiated with 4-MeV electrons at a doze of 1016 cm−2 have been investigated. It has been established that electron irradiation leads to a decrease in the electrical conductivity σ and the permittivity ɛ over the entire temperature range under study (90–320 K). It has been revealed that the TlInSe2 and TlGaTe2 single crystals undergo a sequence of phase transitions characteristic of crystals of this type, which manifest themselves as anomalies in the temperature dependences σ = f(T) and ɛ = f(T). Electron irradiation at a doze of 1016 cm−2 does not affect the phase transition temperatures of the crystals under investigation.

Phase transitions and incommensurability in the layered semiconductor TlInS2—an NMR study

We report on the first NMR study of powder and single crystal samples of thallium indium sulfide, TlInS2. The crystal under study is a pure single-layer TlInS2 polytype. Our findings show that transformation from the high temperature paraelectric phase to the low temperature ferroelectric phase occurs via an incommensurate phase that exists in the temperature range from Tc = 192 K to Ti = 205 K. On approaching the phase transition at Ti from above, the crystal exhibits a soft mode behavior. A discrepancy in the literature data on the phase transitions in TlInS2 is discussed and ascribed to polytypism of the TlInS2 crystals.

Radiation-stimulated processes in irradiated hexagonal TlInS2 crystals

The anisotropy of electrical conductivity in hexagonal TlInS2 crystals irradiated by γ-quanta is investigated. It is established that irradiation by low doses (~50 krad) gives rise to accumulation of radiation defects both in the tetrahedral space and in the layer plane. As a result, there is an increase in electrical conductivities σ⊥C and σ∥C. As the dose is increased (above 200 krad), more complex defects are formed due to interaction of the radiation defects with the inhomogeneities, thus reducing the increase of electrical conductivity in both directions.

Magnetic properties of Co implanted TlInS2 and TlGaSe2 crystals

The results of investigations of magnetic properties of ternary layered TlInS2 and TlGaSe2 ferroelectric crystals implanted with 40 keV Co+ ions at the fluency of 1.0×1017 ion cm−2 are presented. It has been revealed that high-fluence implantation with Co ions results in metal nanoparticle formation in the near-surface irradiated region. The calculations of Co concentration profiles and SEM studies show that the metal nanoparticles are located under the surface at the depth of about 20 nm, and they originate the irregular-shaped bumps on the surface. The Co-implanted samples exhibited superparamagnetic behaviour at high temperatures and ferromagnetic state at temperatures lower than Tb, where Tb is a “blocking temperature” of superparamagnetic nanoparticles. It has been suggested that the observed phenomena can be discussed on the basis of strong magnetic dipolar interaction between Co nanoparticles inside the granular composite film formed as a result of implantation.

Dielectric Anomaly and Conductivity at Ferroelectric Phase Transition in TlInS2 Doped with Different Impurities

The low-frequency dielectric function and conductivity at ferroelectric phase transition have been analyzed in monoclinic TlInS2 crystals doped with different impurities. Complementary, room-temperature optical absorption and photoconductivity have been examined to identify shallow and deep energy levels caused by the introduced impurities. It is found that only shallow impurities broaden and shift dielectric anomaly towards lower temperatures. Contrary to our expectation, the dark conductivity at the temperature of the dielectric anomaly is maximum rather than minimum. The dielectric breakdown of the shallow impurities is proposed to be responsible for the observed behavior of the dark conductivity in monoclinic TlInS2.

Activated impurity states in the incommensurate phase of ferroelectric semiconductor TlInS2

The effect of annealing within the incommensurate phase on the dielectric function ε 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.

Anisotropy of electric conductivity in irradiated TlInS2 crystals

The anisotropy of electric conductivity in hexagonal TlInS2 crystals irradiated with γ-quanta are studied. It is found that, upon light exposure (∼50 krad), radiation defects accumulate in the interlayer space and in the layer plane. As a result, the electric conductivity σ⊥ and σ‖ decreases. As the exposure dose increases (above 200 krad), due to the interaction between the radiation defects and the initial inhomogeneities, complex defects appear with the result that the electric conductivity in either direction increases exponentially.

Influence of photonic excitations on the electrical parameters of TlInS2 crystals

AbstractThe photo‐excitation effect on the current transport mechanism in TlInS2 crystals has been studied by means of dark and illuminated conductivity measurements. The temperature‐dependent electrical conductivity analysis in the temperature region of 110‐340 K revealed the domination of the thermionic emission and the thermally assisted variable range hopping (VRH) of charge carriers above and below 160 K, respectively. Above 160 K, the conductivity activation energies in the dark are found to be 0.28 and 0.15 eV in the temperature regions of 340‐240 K and 230‐160 K, respectively. In the temperature region of 110‐150 K, the dark variable range hopping analysis revealed a density of localized states of 1.99×1022 cm–3eV–1, an average hopping distance of 0.53 nm and an average hopping energy of 79.65 meV. When the sample was photo‐excited, the values of the conductivity activation energies, the density of localized states near the Fermi level and the average hopping energy were observed to decrease sharply with increasing illumination intensity. On the other hand, the average hopping distance increased with rising illumination intensity. Such behaviours were attributed to the Fermi level shift and/or trap density reduction by electron‐hole recombination. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ferroelectric phases in the polytypes of TlInS2 ternary compound

AbstractThe measurements of the real part of the dielectric constant and X‐ray diffraction studies of different polytypes of TlInS2 crystals are presented in the temperature range T = 160–250 K. Substantial influence polytypism on the occurrence of a polarized state in TlInS2 crystals has been revealed. It is shown that phase transition in 2C‐polytype, unlike C‐polytype, is not accompanied by a quadrupling of elementary cell along C axes, it is characterized by the other order parameter and occurs at higher temperatures.(© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polytypes in ferroelectric TlInS2 and its dielectric and optic properties

AbstractPolytypes (Ps) of monoclinic TlInS2 with lattice parameters c (c ≈ 15 Å, c ≈ 30 Å, c ≈ 60 Å, c ≈ 120 Å and c ≈ 240 Å) have been obtained by long time annealing in vacuum or in sulfur vapour (P c ≈ 240 Å) of TlInS2 crystal at different temperatures‐T. From the temperature dependence of dielectric constant ϵ (T) of pure Ps it is established, that well known ferroelectric phase transition (FPT) in TlInS2 takes place with single peaks of dielectric anomaly (DA) at slightly different temperatures for different Ps. Samples with pure P are unstable and are returning to mixed Ps state again after some time. The most unstable is the P with c ≈ 15 Å. It is shown that FPT and optic measurements can be used as Ps analysis methods in TlInS2 samples. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Time‐dependent dielectric behaviour in incommensurate TlInS2 crystal

AbstractThe results of measurements of temperature and time dependencies of the dielectric constant of TlInS2 in the temperature interval of successive incommensurate and commensurate phase transitions have been presented. Considerable decrease of the dielectric constant in a temperature interval including successive phase transitions was observed after annealing the sample at a fixed temperature in the ferroelectric state. Additionally, it has been revealed that the time dependencies of the real part of dielectric susceptibility and dielectric losses angle tangent of TlInS2 crystal in the incommensurate phase exhibits an unusual behaviour, which is accompanied by exponential decreasing of the dielectric constant and increasing of dielectric losses angle tangent by time. The observed effects are qualitatively explained using defect‐density‐wave model and phenomenological theory of incommensurate systems containing discommensurations undergoing nucleation and evolution processes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Optical and photoelectric properties of TlInS2 layered single crystals

Abstract Single crystals of the layered compound TlInS2 were grown by direct synthesis of their constituents. The spectral and optical parameters have been determined using spectrophotometric measurements of transmittance and reflectance in the wavelength range 200–2500 nm. Absorption spectra of thin layers of TlInS2 crystals are used to study the energy gap and the interband transitions of the compound in the energy region 2–2.4 eV. The dispersion curve of the refractive index shows an anomalous dispersion in the absorption region and a normal one in the transmitted region. The direct and indirect band gaps were determined to be 2.34 and 2.258 eV, respectively. Photoconductivity measurements at room temperature resolve the structure that can be identified with the optical transition.

XPS of incommensurate and electronic structure of thallium chalcogenides

The paper presents the X-ray photoelectron spectra (XPS) of the valence band (VB) and of the principal core levels (CL) for the TlInS2 and TlInSe2 single crystals. The XPS were measured with monochromatized Al Kα radiation in the energy range of 0 -- 1400 eV at different temperatures. The resolved splitting of XPS in the incommensurate phase of the ferroelectric semiconductor TlInS2 single crystal was obtained. The VB is located from 0.6 to 10 eV below the Fermi level. Experimental energies of the VB and CL are compared with the results of quantum mechanical ab initio calculations of the molecular model of the TlInSe2 crystal. The electronic structure of the VB and CL is described theoretically by quantum-mechanical Hartree-Fock calculations. The surface and bulk atoms influence the shape of the VB and CL, which is crystallographic plane dependent. The chemical shifts in the both crystals for the Tl, In, S and Se states are obtained.

Dielectric relaxation in ferroelectric TlInS2 layered crystals within metastable chaotic state

AbstractThe results of investigations low frequency dielectric relaxation in layered ferroelectric TlInS2 crystals are presented. The measurements were performed in the temperature range of 180‐230 K and in the frequency range of 5 kHz–1 MHz. Two different relaxation processes were observed in mentioned temperature interval. The crystal has “slow” and “fast” relaxation mechanisms in low and high frequency region, respectively. The presence of two different relaxation mechanisms in TlInS2 is discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room-Temperature Mid-, and Far-Infrared Absorption and Electrical Properties of Intercalated GaSe and TlInS2 Crystals

We report room-temperature measurements of the mid- and far-IR absorption throughout the 400 – 4000 cm−1 and 10 – 700 cm−1 spectral ranges and the resistivity of layered p-GaSe and p-TlInS2 intercalated with Li+. Intercalation was performed by immersing Bridgman grown crystals in 0.5 M solutions of LiCl in distilled water at ambient conditions. The crystal structure and the stoichiometry of the grown crystals were determined by X-ray diffraction and XRF methods. It is shown that intercalation does not change the frequency of the IR-, and Raman active low-frequency “rigid layer” mode (GaSe), the space symmetry group or the lattice parameters of the crystals. It was found that for both crystals, the resistivity versus time dependencies are nearly the same. Three ranges in the resistivity-intercalation time dependencies were explained qualitatively. The resistivity increase due to intercalation was explained by assuming that the intercalated lithium ions act as ionized donors and compensate the host p-type crystal. The highest degree of compensation for GaSe and TlInS2 crystals was achieved after intercalation during 12 and 10 days, respectively.

Relaxor Properties and the Mechanism of Conduction in TlInS[sub 2] Crystals Exposed to Gamma Irradiation

The density of states at the energy levels associated with radiation-induced defects, the localization length of a defect center, and the hopping distance of charge carriers are determined in a TlInS2 crystal. It is demonstrated that, by varying the dose of gamma irradiation, it is possible to control the dielectric properties of ferroelectrics and to attain a stable relaxor state. In the temperature range of existence of this state, charge carriers execute tunneling from electron levels in the band gap through potential barriers created by an incommensurate superstructure of the TlInS2 crystal.