Giant dielectric relaxation in TlGaTe<Subscript>2</Subscript> crystals

The conductivity and temperature-frequency dependences of the permittivity of TlGaTe₂ crystals have been studied. Strong dielectric relaxation has been revealed. It has been shown that the mechanism of dielectric relaxation is associated with hopping of Tl ions over vacancies in the thallium sublattice due to the transition of the system to the superionic state.     

The effects of thermal activation on the complex conductivity of Ag2HgI4 in the microwave region

The complex conductivity of polycrystalline Ag₂HgI₄, a superionic conductor, has been measured as a function of temperature at 10, 24 and 70 GHz. Both conductivity and permittivity exhibited sharp changes at the β?α phase transition. The microwave conductivity of the β-phase was found to be insensitive to temperature changes and that of the α-phase has thermal activation energies lower than that of dc. The observed monotonic increasing conductivity, decreasing permittivity, together with thermal activation effects are indicative of hopping ionic transport.     

Dielectric properties,ionic conductivity and phase transitions in La<Subscript>2</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics

The dspersion dielectric permeability and ion conductivity of La₂Mo₂O₉ ceramics was studied. It was established that the observed low-frequency dielectric dispersion is due to relaxation effects related to high ion conductivity. It is shown that the phase transition in La₂Mo₂O₉ has characteristic features of a superionic phase transition.     

Preparation, crystal structure, and electrical properties of thallium monosulfide in the vicinity of high-temperature phase transitions

Single crystals of thallium monosulfide TlS with excess sulfur (4 at. %) are grown. The existence of monoclinic (of the TlGaSe₂ type), tetragonal ordered (of the TlS type), and subtetragonal phases of thallium monosulfide is revealed using x-ray powder diffraction. The temperature dependences of the electrical conductivity and permittivity of thallium monosulfide single crystals indicate that, in the temperature range 401–411 K, these crystals undergo a phase transition to a state with superionic conduction.     

Investigation of ionic conduction in the mixed AgI-PbI2 system

Study of ionic conductivity in AgI-PbI₂ system has a function of composition and temperature shows that PbI₂ has a definite role in the process of superionic phase transition. It has been found that at 80 mole % of AgI, superionic phase transition temperature passes through a minimum value of as low as 105°C. The maximum conductivity (at room temperature) is also obtained in this region. The results are discussed qualitatively in terms of a lattice loosening model.     

Dielectric properties and phase transitions in crystals of TlInS x Se2 − x solid solutions

This paper reports on a study of the dielectric permittivity and electrical conductivity of single-crystal TlInS _x Se_{2 − x} solid solutions as functions of temperature and composition. It has been found that the dielectric permittivity ɛ and electrical conductivity σ decrease with increasing x and increase with growing temperature. The temperature dependences ɛ = f(T) and σ = f(T) for TlInS _x Se_{2 − x} crystals have been demonstrated to reveal anomalies in the form of maxima, which suggests that these crystals undergo phase transitions. The temperatures of the phase transitions increase with increasing x.     

Phase transitions in CuBiP2Se6 crystals

Investigation results of dielectric (20?Hz–1?MHz) properties of layered CuBiP₂Se₆ crystals are presented. The temperature dependence of the static dielectric permittivity reveals the first-order “displacive” antiferroelectric phase transition at T _c?=?136?K. In the paraelectric phase, at low frequencies, dielectric spectra are highly influenced by the high ionic conductivity with the activation energy of 2473?K (0.21?eV). In the antiferroelectric phase the electrical conductivity and its activation energy (531.1?K (0.045?eV)) are considerably smaller. At low temperatures, the temperature behaviour of the distribution of relaxation times reveals complex freezing phenomena. A part of long relaxation time distribution is strongly affected by external direct current (DC) electric field and it is obviously caused by antiferroelectric domain dynamics.     

Specific Features of the Structure and the Dielectric Properties of Sodium–Bismuth Titanate-Based Ceramics

The phase formation, specific features, and the dielectric properties of the ceramics of compositions from the region of morphotropic interface in the (Na_0.5Bi_0.5)TiO₃–BaTiO₃ system modified by Bi(Mg_0.5Ti_0.5)O₃ and also low-melting additions KCl, NaCl–LiF, CuO, and MnO₂ that favor the control of the stoichiometry and the properties of the ceramics have been studied. The ceramics are characterized by ferroelectric phase transitions that are observed as jumps at temperatures near 400 K and maxima at T_m ~ 600 K in the temperature dependences of the dielectric permittivity. The phase transitions at ~400 K demonstrate the relaxor behavior indicating the existence of polar domains in the nonpolar matrix. An increase in the content of Bi(Mg_0.5Ti_0.5)O₃ favor a decrease in the electrical conductivity and dielectric losses of the samples, and the relative dielectric permittivity at room temperature ε_rt is retained quite high, achieving the highest values ε_rt = 1080–1350 in the ceramics modified with KCl.     

Dielectric properties for the ordered-perovskite cuprate Sr2Cu(Re0.69Ca0.31)O6

Dielectric properties are reported on polycrystalline cubic ordered-perovskite cuprate Sr₂Cu(Re_0.69Ca_0.31)O₆ in the frequency range 10 Hz-100 kHz at temperature from 300 to 500 K. Both the dielectric permittivity and dielectric loss factor are found to be frequency and temperature dependent. The enhanced value of the low frequency dielectric permittivity is associated to ionic polarization and interfacial phenomena. The material is found to possess significantly high dielectric permittivity. The calculated ac conductivity suggests semiconducting behaviour for the Sr₂Cu(Re_0.69Ca_0.31)O₆.     

Pecularities of change transfer mechanism and dielectric properties of glasses Ag-Sb-S-I

Temperature dependence of electronic and ionic components of electric conductivity and dielectric permittivity of amorphic compounds Ag_0.25Sb_0.25S_0.5, (AgSbS₂)_0.95· (SbSI)_0.05 and (AgSbS₂)_0.9·(SbSI)_0.1 in the temperature range of 290–380 K have been investigated. The experimental data show that glasses investigated are solid electrolytes. Ionic conductivity as well as dielectric permittivity of the glasses Ag-Sb-S may be changed by introduction of SbSI.     

Fluorine mobility in an aluminum-doped CeF<Subscript>3</Subscript> crystal: NMR and conductivity studies

Electrical conductivity and NMR measurements were carried out for an aluminum-doped CeF₃ crystal to study the influence of substitutional impurities on the superionic fluorine mobility. Activation enthalpy was found to remain constant from low temperatures to about 325 K and to increase as compared to that of a pure CeF₃ crystal. Above about 325 K, a trend towards gradual conductivity saturation was observed. This change was ascribed to a superionic phase transition not accompanied by structural transformations. NMR also revealed some alterations in the local fluorine dynamics as compared to that reported for a pure CeF₃ crystal. According to NMR measurements, the superionic phase transition near 325 K results in acceleration of the exchange between rigid or slow fluorine in the F₁, F₂, and F₃ sublattices, while highly mobile F₁ fluorine ions move independently at least until 400 K.     

Point defect parameters for β-PbF2 from a computer analysis of measurements of ionic conductivity

The conductivity of four crystals of β-PbF₂ has been measured. The results for three of these crystals, one doped with La³⁺ and two doped with Na ⁺ impurity, have been analysed with a non-linear least squares programme to yield the thermodynamic parameters of defect formation and migration below the superionic transition. These parameters are compared with previous values and are used to provide estimates of the defect concentrations in the superionic phase. This work suggests that the superionic phase is not massively disordered and that β-PbF₂ is not significantly different from other fluorites.     

Enhancement of dielectric and ferroelectric properties in ferroelectric superlattices

I studied theoretically the enhancement of remanent polarization and dielectric permittivity of interfacial-coupled ferroelectric superlattices based on the Landau–Ginzburg theory. Our model adopts the Landau–Khalatnikov equation to describe hysteresis behavior and takes the time-dependent space-charge-limited conductivity into account to investigate the ferroelectric and dielectric properties of ferroelectric superlattices. The results are in good agreement with recent experimental observations on the enhancement of remanent polarization and permittivity of BaTiO₃/SrTiO₃ superlattices and heterolayered Pb(Zr,Ti)O₃ thin films.     

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.     

Dielectric relaxation in complex perovskite oxide BaCo1/2W1/2O3

Polycrystalline BaCo_1/2W_1/2O₃ (BCW) is prepared by the solid-state reaction technique. The X-ray diffraction study of the compound at room temperature reveals the monoclinic phase. The field dependence of the dielectric constant and the conductivity are measured in the frequency range from 50 Hz to1 MHz and in the temperature range from 300 to 413 K. An analysis of the real and imaginary parts of the dielectric permittivity with frequency is performed. The frequency-dependent maxima in the imaginary impedance are found to obey an Arrhenius law with an activation energy=0.86 eV. The frequency-dependent electrical data are also analysed in the framework of the conductivity and modulus formalisms.     

Superionic behavior in the xAgI–(1−x)CsAg2I3 polycrystalline system

A superionic phase behavior (with DC ionic conductivities higher than 0.01 S/cm) has been observed in xAgI–(1−x)CsAg₂I₃ (x≈0.67) polycrystalline system grown by slow evaporation using AgI and CsI powders (molar ratio Cs/Ag=0.25) as starting salts and an aqueous solution of HI as solvent. The transition from the normal-to- the superionic state is first-order with a hysteretic behavior in temperature centered at about 116 °C as reflected by thermal (DSC) and electrical conductivity measurements. This mixture is composed of CsAg₂I₃ and AgI crystalline phases and an additional amorphous AgI phase that explains the glassy-type behavior observed in the superionic phase transition.     

Light induced phenomena in superionic crystals

The phenomena of the reversible transformation of concentration and a structure of luminescence centers and of the concentration of mobile silver ions in local irradiated region of RbAg₄I₅ superionic crystals are studied. A new effect of illumination on ionic conductivity and activation energy of migration of mobile Ag⁺ cations in RbAg₄I₅ superionic crystals are observed. Reversible changes in the ionic conductivity due to illumination of superionic crystals are caused by reversible change in the structure of electronic centers caused by elastic strain around these centers. The effect of elastic deformation on the process of ionic transport and activation energy for diffusion of mobile silver cations are studied. Photostimulated recovery of the ionic conductivity after its change due to preliminary illumination of a RbAg₄I₅ superionic crystal with light of wavelength λ=430 nm are detected. This recovery of the ionic conductivity is due to excitation of centers in complexes generated by previous illumination of tested samples.     

Effect of conductivity on the dielectric characteristics of cyano-ethyl ester of poly(vinyl alcohol)

Frequency dependences of the real and imaginary parts of the dielectric permittivity of films prepared from cyano-ethyl ester of poly(vinyl alcohol) have been studied within a frequency range of 25 Hz to 1 MHz at temperatures from 266 to 323 K. The frequency dependence of the total conductivity of the polymer dielectric has been calculated. At T < T _g , it has been established to follow a power law course with an exponent close to unity, which evidences hopping pattern of the conductivity. The anomalous increase of the dielectric losses and dielectric permittivity in the low-frequency range at T ≥ T _g is attributed to formation of double electric layers at the electrode-polymer boundary.     

Electrodynamic properties of lead Zirconate-Titanate thin films in the terahertz frequency range

The transmission/reflection spectra of bilayer structures consisting of thin amorphous and polycrystalline Pb(Zr_0.52Ti_0.48)O₃ ferroelectric films deposited on dielectric substrates of magnesium oxide MgO and sapphire α-Al₂O₃ were measured in the frequency range of 5–4000 cm^?1. Based on these spectra and using the dispersion analysis method, the spectra of complex dielectric permittivity ?*(ν) and dynamic conductivity σ′(ν) of the films were simulated, the electrodynamic parameters of the films were determined, and the dielectric dispersion responsible for the formation of static permittivity was found.     

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

DSC and complex impedance studies of the protonic conductor (NH₄)₄H₂(SeO₄)₃, which undergoes a superionic phase transition of first order at T_s = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔC_p of the crystal are proportional: d(lg σ)/dT = XΔC_p/RT_s + const, as found for MAg₄I₅ crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at T_s, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH₄)₄H₂(SeO₄)₃ crystal above T_s.