R. M. Sardarly

Superionic conductivity and switching effect with memory in TlInSe2 and TlInTe2 crystals

The temperature dependences of the conductivity σ(T) and the switching and memory effects in one-dimensional TlInSe2 and TlInTe2 single crystals have been studied. A specific feature is found in the dependence σ(T) above 333 K, which is related to the transition of crystals to the state with superionic conductivity. It is suggested that the ion conductivity is caused by the diffusion of Tl+ ions over vacancies in the thallium sublattice between (In3+Te22−)− and (In3+Se22−)− nanochains (nanorods). S-type switching and memory effects are revealed in TlInSe2 and TlInTe2 crystals, as well as voltage oscillations in the range of negative differential resistance. It is suggested that the switching effect and voltage oscillations are related to the transition of crystals to the superionic state, which is accompanied by “melting” of the Tl sublattice. The effect of electric-field-induced transition of TlInSe2 and TlInTe2 crystals to the superionic state is found.

Ferroelectricity and polytypism in TlGaSe2 crystals

Abstract The present report deals with a complex of investigations of the temperature dependences of the static dielectric constant, the spontaneous polarization and the pyroelectric coefficient in different TlGaSe2 crystal modifications. It has been shown that in the temperature region below Tc dielectric hysteresis loops are observed and in this case spontaneous polarization grows according to a linear law and reacts the value of 1.3 × 10−7C cm−2.

The influence of cation impurities on phase transitions in the TlInS2 compound

The temperature dependences of the permittivity of TlInS2 crystals doped with 0.1 at. % Cr, Mn, Yb, Sm, Bi, or La are investigated, and the dependence of the phase transition temperature on the ion radius of the dopant impurity is determined. It is revealed that manganese and chromium substitute for indium in the TlInS2 crystal lattice, whereas ytterbium, samarium, bismuth, and lanthanum ions occupy octahedral holes in an In4S10 tetrahedral complex and, thus, produce an internal pressure responsible for the shift in the temperature of phase transitions toward the high-temperature range.

Effect of nonstoichiometry on the dielectric permittivity of TlInS2 near phase transitions

Temperature-dependent dielectric permittivity measurements and differential thermal analysis demonstrate that even slight deviations from stoichiometry have a significant effect on the dielectric properties of ferroelectric TlInS2 crystals.

Superionic Conductivity in One-Dimensional Nanofibrous TlGaTe2 Crystals

We study the temperature dependence of the electrical conductivity σ(T) of TlGaTe2 crystals under an electric field (E = 181 V/cm). A strong sharp increase (×1500) in σ(T) is observed at T = 242 K (along the c-axis) and T = 267 K (perpendicular to it). This increase is attributed to a transition towards a superionic conductivity behavior. Over the superionic transition threshold, disordering of the Tl+ sublattice in TlGaTe2 occurs, and nanoscale topological disordering arises owing to aperiodicity in placing Ga3+Te22- nanofibers. Terahertz time-domain spectroscopy reveals absorption lines at approximately 0.2 THz that may be attributed to the libration oscillations of the nanofibers.

On the polarization caused by bulk charges and the ionic conductivity in TlInSe2 crystals

TlInSe2 crystals are investigated in dc and ac electric fields in the temperature range of 100–400 K. A decrease in the electrical conductivity σ with time in a dc field is revealed. The complex-impedance spectra Z*(f) are measured in the frequency range of 10–106 Hz. Diagrams in the (Z″-Z′) complex plane are analyzed using the method of equivalent circuits. It is shown that the electrical properties of TlInSe2 crystals in the investigated ranges of temperatures and frequencies are determined by hopping conductivity and the accumulation of charge carriers near blocking platinum electrodes.

Characterization of Selenide, Sulfide and Telluride Materials by Terahertz Time-Domain Spectroscopy

We characterize, by terahertz time-domain spectroscopy, the far infrared properties of different selenide, sulfide and telluride compounds and crystals. We propose a new method to extract, from the experimental data, the refractive index and absorption of such materials that exhibit strong absorption lines. We illustrate this method by measuring the terahertz response of doped TlInS2 crystals. We report the observation of a strong absorption peak at 1.242 THz in InSe, which could be attributed to the excitation of TO phonon (half layer shear mode).

Relaxor properties and the mechanism of conduction in TlInS2 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.

Relaxor properties of Fe-doped TlInS2

The relaxor properties of Fe-doped TlInS2 crystals are investigated. It is shown that Fe-doped TlInS2 compounds exhibit all features inherent in relaxor ferroelectric materials. The temperature range of existence of the microdomain (relaxor) state and the temperature of the transition from this state to a macro-domain state are determined.

Conductivity over localized states of the system of (TlInSe2)1–x (TlGaTe2) x solid solutions

The temperature dependences of electrical conductivity over localized states in the solubility region of mixed crystals of compositions (TlInSe2)1–x(TlGaTe2)x at x = 0, 0.1 0.2, 0.8, 0.9, and 1 are studied. It is established that hopping conductivity with a variable hop length over localized states near the Fermi level occurs in this system. The conductivity activation energy is determined; and the density of states in the vicinity of the Fermi level, their dispersion, localization radius, and average carrier hop distance for all compositions are estimated; and the concentration dependences of the calculated parameters are plotted.