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.     

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

The dependences of the permittivity and electrical conductivity of TlInS₂ and TlGaS₂ single crystals on the temperature and electron beam irradiation dose have been studied. It has been established that, as the electron irradiation dose increases, the electrical conductivity σ significantly increases, whereas the permittivity ? decreases over the entire temperature range covered (80–320 K). It has been shown that anomalies in the form of maxima in the temperature dependences σ(T) and ?(T) are observed in the regions characteristic of phase transitions in TlInS₂. Irradiation of the TlInS₂ and TlGaS₂ crystals with electrons to doses of 10¹⁵ and 10¹⁶ cm^?2 does not affect their phase transition temperatures. The dispersion curves of the permittivity ? of the TlGaS₂ crystal have been constructed.     

Characteristics of traps in TlInS2 single crystals

Characteristics of charge traps in TlInS₂ single crystals are investigated by the use of thermally stimulated current (TSC) technique. The TSC spectra of the sample from 80 K to 300 K are recorded at a constant heating rate. The spectra reveal that there are several trapping levels associated with the complex structure of overlapping peaks. The experimental results indicate that the traps in TlInS₂ associated with the spectra in the measuring range of temperature obey the monomolecular (first order) kinetics. Thus, the spectra are resolved into first order shaped peaks by the use of computerized best fit procedure. The trapping parameters; such as the energy depth, temperature dependent frequency factor and capture cross section, together with concentrations of the corresponding six discrete levels are computed. These centers all having low capture cross sections with strong temperature dependence are found to be at the energies of 0.11 eV, 0.22 eV, 0.25 eV, 0.26 eV, 0.29 eV and 0.30 eV with high concentrations of 6.6 × 10¹⁶, 2.0 × 10¹⁷, 3.3 × 10¹⁷, 9.6 × 10¹⁶, 2.3 × 10¹⁷ and 4.0 × 10¹⁷ cm^?3, respectively.     

Optical and photoelectric properties of TlInS2 layered single crystals

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.     

Exciton absorption parameters in TlGaS2 crystals

It is shown that, because the shape of the exciton absorption curve in crystalline TlGaS₂ is described by the Fano antiresonance profile, the experimentally observed exciton peak corresponds to a modified state which is the result of the configuration interaction of a discrete state (exciton) with the quasi-continuum of conduction-band states. The oscillator strength for the transition to the discrete (“pure”) exciton state is calculated as F ₀=1.22×10^?2. The exciton transition selection rules are calculated for two assumed symmetry groups, D _2h and D _4h . An analysis of the selection rules for the dipole-allowed exciton transition permits one to conclude that the symmetry group for the TlGaS₂ crystal is D _2h .     

Optical properties of thallium indium disulfide (TlInS2) single crystals

Transmission, photoluminescence, and reflectance spectra of TlInS2 single crystals grown by the Bridgman–Stockbarger method were measured at 4.2 K near the fundamental absorption edge. Narrow lines at ~2.5535 and ~2.5694 eV were observed in the transmission spectrum and assigned to ground and excited free-exciton states, respectively. The free-exciton binding energy and band-gap energy Eg were found to be ~21.2 meV and ~2.5747 eV, respectively. A recombination mechanism was proposed for the TlInS2 near-band-edge and deep luminescence.     

Anisotropy of the hopping conductivity in TlGaSe2 single crystals

The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe₂ single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe₂ single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N _F=1.3×10¹⁹eV·cm³)^?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe₂ single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe₂ single crystals is investigated.     

Magnetodielectric effects in Co-implanted TlInS2 and TlGaSe2 crystals

The results of investigations of dielectric and magnetodielectric properties of ternary layered TlInS₂ and TlGaSe₂ ferroelectric crystals implanted with 40 keV Co⁺ ions at the fluency of 1.0 × 10¹⁷ ion/cm² are presented. The temperature dependences of the dielectric susceptibility of Co-implanted samples showed that the formation of metal nanoparticulate composite layer in the near-surface irradiated region as a result of high-fluency Co implantation causes considerable shifts of well-known successive structural phase transition points to high temperatures in heating regime. It has been revealed that the application of the magnetic field in the direction perpendicular to implanted surface results in shifting of the phase transition points to low temperature region. The observed peculiarities are considered as magnetocapacitance (magnetodielectric) effects, which appeared as a result of magnetoelectric lock-in interaction between domains of ferroelectric and ferromagnetic substances of the composite structure.     

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 T_b, where T_b 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.     

dc ionic conductivity in single crystals of lead nitrate

dc ionic conductivity measurements on solution grown single crystals of lead nitrate were made in the temperature range of 100 to 410°C. Activation energies corresponding to both extrinsic and intrinsic regions have been calculated.     

Proton conduction in LaSrScO3 single crystals

Transparent single crystals of La_1−xSr_xScO₃ (x = 0.01, 0.03) were grown by the floating zone method. The optical and the electrical properties were studied by infrared absorption spectra and AC-impedance measurements, respectively. Our results showed that proton conduction becomes dominant in La_0.97Sr_0.03ScO₃ single crystals below 700 °C. The electrical conductivity increases as the amount of Sr increases. At 600 °C, La_0.97Sr_0.03ScO₃ had a proton conductivity of 4.89 × 10^− 3 S/cm. This value is almost the same as that in the literature for La_0.9Sr_0.1ScO₃ ceramic samples. It was revealed from infrared absorption spectra that several different sites are occupied by protons in this material. Furthermore, it was found that the intensity of high-frequency O–H bands can be related to the dopant-concentration dependence of the electrical conductivity.     

Study of dc ionic conductivity in single crystals of RbNO3

dc ionic conductivity has been studied on single crystals of RbNO₃ grown by slow evaporation method. The range of temperature covered is from room temperature to its melting point atmospheric pressure. An attempt is made to correlate the known phase transitions with the variation of ionic conductivity with temperature.     

3D hopping conduction in SnO2 nanobelts

The temperature dependence of the electrical transport of an individual tin oxide nanobelt was measured in darkness from 400 to 5 K. We found four intrinsic electrical transport mechanisms through the nanobelt. It starts with thermally activated conduction between 400 K and 314 K, followed by nearest‐neighbor hopping conduction between 268 K and 115 K and variable range hopping conduction below 58 K, with a crossover from the 3D Mott to the 3D Efros–Shklovskii regime at 16 K. We claim that this sequence reveals the three‐dimensional nature of the electrical transport in the SnO₂ nanobelts, even though they are expected to behave as one‐dimensional systems. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of temperature and uniform compression on local-center luminescence in TlGaS2 layered crystals

Changes in the local-center luminescence spectrum initiated by structural defects in the TlGaS₂ crystal are studied in the temperature range from 2 to 77 K and under a hydrostatic pressure up to 35 kbar. The effect of hydrostatic compression can be explained by the relative slipping of the two elementary layers comprising the layer stack, which results in a lowering of the symmetry of the cavities occupied by the thallium atoms.     

Raman spectra and conduction properties of V1−xTixO2 single crystals

Raman scattering and electrical conductivity measurements have been performed on mixed single crystals of V_1?xTi_xO₂ system. The results show that the formation of homopolar bonds between vanadium ion pairs in the orthorhombic phase produces little alteration of the conduction and vibrational properties compared with the insulator-metal transition, which occurs at a slightly higher temperature.     

Polaron hopping conduction in La0.85Sr0.15MnO3 single crystal

Temperature dependences of electrical resistance and thermopower of a La_0.85Sr_0.15MnO₃ single crystal were measured in the paramagnetic and ferromagnetic dielectric phases. It was shown that charge transfer in both phases is due to the variable-range polaron hopping over the localized states. The activation energies in both phases linearly depend on T ^3/4 but differ from each other by a constant which is approximately equal to the exchange energy kT _c. The results obtained are in compliance with the concept of percolation character of metal-insulator transition in manganites.     

Thermal effect on the dielectric function and small polaron hopping conduction in organic molecular crystals

A Green function formalism is applied to study the dielectric function spectra and the small polaron hopping conduction in organic molecular crystals. In the calculations, the electron-phonon interaction is considered within the Hartree-Fock approximation, and the temperature effect is taken into account. Our theoretical approach is based on the polar electron-phonon interaction (Fröhlich type) to characterize the non-degenerate polaron gas, with the assumption of the electronic hopping between the first-neighbor.