F. T. Salmanov

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.

Superionic conductivity in TlGaTe2 crystals

Temperature dependences of electrical conductivity σ(T) and permittivity ɛ(T) of one-dimensional (1D) TlGaTe2 single crystals are investigated. At temperatures higher than 305 K, superionic conductivity of the TlGaTe2 is observed and is related to diffusion of Tl+ ions via vacancies in the thallium sublattice between (Ga3+Te22−− nanochains. A relaxation character of dielectric anomalies is established, which suggests the existence of electric charges weakly bound to the crystal lattice. Upon the transition to the superionic state, relaxors in the TlGaTe2 crystals are Tl+ dipoles ((Ga3+Te22−)− chains) that arise due to melting of the thallium sublattice and hops of Tl+ ions from one localized state to another. The effect of a field-induced transition of the TlGaTe2 crystal to the superionic state is detected.

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.

Ionic conductivity and dielectric relaxation in γ-irradiated TlGaTe2 crystals

The switching effect, field and temperature dependences of the permittivity and conductivity of TlGaTe2 crystals subjected to various γ-irradiation doses are studied. Under rather low electric fields, the phenomenon of threshold switching with an S-shaped current-voltage characteristic containing a portion with negative differential resistance is observed in the crystals. In the region of critical voltages, current and voltage oscillations and imposed modulation are observed. Possible mechanisms of switching, ionic conductivity, disorder, and electrical instability in TlGaTe2 crystals are discussed.

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.

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.