Ionic photoconductivity of RbAg4I5 superionic crystals

A new effect of illumination on ionic conductivity and activation energy of migration of mobile Ag⁺ cations in RbAg₄I₅ superionic crystals has been detected and studied. Reversible changes in the ionic conductivity due to illumination of superionic crystals are caused by reversible changes 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 has been 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 has been detected. This recovery of the ionic conductivity is due to excitation of centers in complexes generated by previous illumination of tested samples. Zh. éksp. Teor. Fiz. 112, 698–706 (August 1997)     

Specific heat and thermal conductivity of superionic conductors in the superionic phase

The temperature dependences of the specific heat and the thermal conductivity of crystalline superionic conductors LnF₃ (Ln = La, Ce, Pr), Li₂B₄O₇ and α-LiIO₃ in the superionic phase have been investigated experimentally. The specific heat C _p and the thermal conductivity K are observed to increase monotonically over a wide range of temperatures above the Debye temperature Θ_D. This increase is attributed to the relaxational interaction of high-frequency phonons with two-level systems. Fiz. Tverd. Tela (St. Petersburg) 39, 1548–1553 (September 1997)     

The influence of elastic stress fields on ionic transport through a 〈superionic crystal〉-〈electrode〉 heterojunction

The appearance of a current in an external circuit has been observed upon elastic deformation of a local region of the superionic crystal RbAg₄I₅. The dependence of the magnitude and sign of the deformation current on the region of application of the local load on the sample is examined, and the temporal characteristics of the processes are investigated. The influence of an elastic deformation on processes taking place at the 〈superionic crystal〉-〈electrode〉 heterojunction is investigated, and a mechanism of generation of the deformation current is proposed. The generation of photostimulated currents upon illumination of a local region of the superionic conductor by light corresponding to intracenter excitation of optically active centers is considered. It is shown that the elastic stress fields arising around photoexcited centers are responsible for the generation of photostimulated currents. Fiz. Tverd. Tela (St. Petersburg) 41, 1766–1771 (October 1999)     

Semiconductor-metal transition in FeSi in ultrahigh magnetic fields up to 450 T

The magnetic susceptibility and conductivity of single-crystal iron monosilicide are investigated in ultrahigh magnetic fields up to 450 T at a temperature of 77 K. It is found that the conductivity of iron monosilicide increases continuously by two orders of magnitude as the magnetic field increases. The results obtained can be interpreted as a semiconductor-metal transition induced by the magnetic field. The dependence of the conductivity on the magnetic field is described well on the basis of the spin-fluctuation theory. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 326–330 (25 August 1998)     

Effect of free electrons on growth of “cancers” in mixed electronic-ionic conductors

The reasons for the anomalously rapid (for solids) growth of “cancers” (filamentary crystals, hills, pores, and other formations) on the surface of samples of superionic conductors with mixed electronic-ionic conductivity (of the type Cu_2−x Se and Ag₂Te) are discussed. The effects are attributed to the acceleration of mobile ions and activated ions in the “core” of the superionic by free electrons in the joint “chemical” diffusion of ions and electrons in the samples. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 97–100 (25 July 1996)     

Lithium superionic conductors Li3InBr6 and LiInBr4 studied by 7Li, 115In NMR

Li₃InBr₆ undergoes a phase transition to a superionic phase at 314 K associated with a steep increase of the conductivity (σ = 4 × 10^− 3 Scm^− 1 at 330 K). This superionic phase is isomorphous with Li₃InCl₆ in which a positional disorder at the In³⁺ site is introduced. A pseudo cubic-close-packing of the bromide ions is formed in this phase. On the other hand, a new superionic phase of LiInBr₄ was found above ca 315 K and its structure was confirmed to be a defect spinel. The dynamic properties of the cations in these two superionic phases were investigated by ⁷Li and ¹¹⁵In NMR spectroscopy.     

Enhancement of ionic conductivity in the composite solid electrolyte system: TlI–Al2O3

This paper reports the heterogeneously doped alumina (Al₂O₃) on the ionic conductivity of thallium iodide. Composite materials of formula (1 − x) TlI–xAl₂O₃, x = 0–0.7 have been prepared and studied by X-ray diffraction, differential scanning calorimetry, and electrical conductivity. X-ray diffraction and differential scanning calorimetry proved the formation of composite in this binary system. The maximum enhancement observed is about three orders of magnitude with respect to the host material. The enhancement of electrical conductivity in comparison with pure thallium iodide can be interpreted with the space charge layer model. Moreover, the increased content of alumina in the system leads to the disappearance of phase transition β–α thallium iodide, which is usually observed in the pure compound. This behavior was explained by stabilizing effect of β-phase at high temperatures and suppression of α-phase at higher contents of alumina.     

Studies of structure, thermal, and electrical properties for Cs 5 H 3 (SO 4 ) 4 crystal

Electrical conductivity, differential scanning calorimetry, and X-ray diffraction measurements were performed on a pentacesium trihydrogen tetrasulfate, Cs₅H₃(SO₄)₄, crystal. The transition entropy at a superionic phase transition and the activation energy of proton migrations in the superionic phase were determined to be 58.2 J K⁻¹ mol⁻¹ and 0.48 eV, respectively. The crystal structure of Cs₅H₃(SO₄)₄ at room temperature was refined. The electrical conduction in Cs₅H₃(SO₄)₄ was discussed with the refined structure.     

High-temperature behaviour of average structure and vibrational density of states in the ternary superionic compound AgCuSe

Results of simultaneous thermal analysis (STA), synchrotron powder diffraction (in the range 300–973 K) and inelastic neutron scattering (at 285 and 505 K) on non-superionic β- and superionic α-AgCuSe are reported. The sample is stable in argon on heating. The volume change at the superionic phase transition is about 5%. A model for the average structure of α-AgCuSe is proposed. No anomalies in the temperature dependence of the parameters of the average structure were revealed. Ionic conductivity in α-AgCuSe can originate from cation jumps in “skewed” 〈100 〉 directions between nearest-neighbour tetrahedral sites via the peripheries of the octahedral cavities. A correlation between the temperature dependence of the cation redistribution in α-AgCuSe and the temperature dependence of the ionic conductivity is supposed. Various contributions (anharmonic effects, time-average static disorder and phonon-phonon scattering) to the widths of individual phonons upon temperature increase lead to pronounced changes in the neutron-weighted densities of states of β- and α-AgCuSe and accompany the superionic phase transition as well.     

Propagation of nonequilibrium phonons in ferroelectric ceramics and single crystals

Phonon transport in ferroelectric ceramics and single crystals has been experimentally investigated. Our measurements indicate that, in the temperature range 1.7–3.8 K studied, the effective phonon diffusion coefficient behaves as D _eff∞T ⁻⁵ in ferroelectrics with a broadened phase transition. This experimental dependence is in accord with the presence of a plateau in the thermal conductivity of such materials. The scattering by domain walls in BaTiO₃ single crystals has been identified, and our results are in quantitative agreement with calculations. Zh. éksp. Teor. Fiz. 115, 624–631 (February 1999)     

Ion-motion disorder in a tysonite superionic conductor from 19F NMR data

The disorder in ion motion in a LaF₃ superionic conductor with tysonite structure is determined from NMR spectra. The method is sensitive to dynamic effects with correlation times of 5×10⁻³<τ<10⁻⁶ s. Within the 260–400-K interval, ion movement is observed only on one of the sublattices (F₁), and it exhibits a correlation time distribution. The shape of the distribution is well fitted by a log-Gaussian. The distribution in correlation times reflects noticeable spatial fluctuations of local potentials in the LaF₃ superionic conductor. Fiz. Tverd. Tela (St. Petersburg) 41, 1616–1620 (September 1999)     

Features of the optical absorption of crystals of the fullerene C60 in the region of the orientational phase transition

The absorption coefficient of perfect single crystals of the fullerene C₆₀ is measured in the energy range 1.6–2.1 eV at temperatures from 4.2 to 300 K. An absorption fine structure is discovered in the and is assigned to electronic and vibronic transitions with the production of free excitons and excitons localized on structural defects. It is shown that in the region of the structural phase transition from a face-centered cubic structure to a simple cubic structure the absorption coefficient undergoes a jump, which is associated with an energy shift of the free exciton line toward lower energies. It is discovered that spatial inhomogeneity, which is associated with the growth of the new phase from a finite number of nuclei, appears in the crystal at the time of this transition. Zh. éksp. Teor. Fiz. 114, 2211–2224 (December 1998)     

Structures responsible for quasisynchronism in second-harmonic generation in BaTiO3:Fe

Second harmonic generation (SHG) of light in iron-doped BaTiO₃ crystals occurs primarily at an angle of about 7° to incident λ=1.06 μm radiation. It is shown that the quasisynchronism is accounted for by a 90° domain structure with a spatial wave vector q∥[011] and a spatial period of about 3 μm. This result may serve as a basis for interpretation of the anomalous SHG enhancement near the phase transition. Fiz. Tverd. Tela (St. Petersburg) 41, 1076–1079 (June 1999)     

Photostimulated growth of whiskers in AgI-type superionic crystals

The growth of whiskers inside and on the surface of superionic crystals (AgI, CuI, RbAg₄I₅) is considered. The crystals are exposed to radiations with different spectral compositions at temperatures above and below the temperature of the superionic phase transition. The chemical composition, structure, and properties of whiskers are studied with optical microscopy, scanning electron microscopy, transmission electron microscopy, and photoluminescence. The mechanisms of photostimulated growth of whiskers in semiconductors with a high ionic conductivity are discussed, and the role of the “molten” cation sublattice during nucleation and growth of whiskers is considered.     

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.     

Superionic transitions in NASICON-type solid electrolytes

Four different methods, temperature dependent X-ray diffraction, impedance measurements, calorimetry and dilatometry, were employed to investigate the superionic transitons of NASICON type materials. Two singularities are found at 160 – 170 °C and about 230 °C. The first one is related to a second order phase transition close to the temperature of structural phase transition; the second one coincides with the inflection in the temperature dependence of conductivity. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–15, 1995     

Thermal stability and impedance spectroscopy studies of (Cu, Ni) substituted Bi4V2O11 ceramics

Polycrystalline samples of the oxygen ion conductor Bi₂V_0.9Cu_0.1−xNi_xO_5.35 with various contents of nickel (0 ≤ × ≤ 0.1) were investigated. The X-ray powder diffraction revealed the tetragonal structure of all compositions. DTA curves exhibit effects due to phase transition, one endothermic effect during heating and one exothermic one during cooling. The impedance of the ceramics with Pt electrodes was measured in the frequency range 10⁻¹–10⁷ Hz at constant temperatures between 350 and 920 K. The conductivity was determined by nonlinear least-squares analysis of the impedance spectra. Separation of the total resistance into grain interior and grain boundary components was feasible at temperatures below 580 K. The transition temperatures observed in DTA coincide with those observed in conductivity measurements. A phase transition, involving a reordering process of the oxygen ions is considered to be responsible for this phenomenon. The frequency dependent part of the intragrain conductivity was modeled by a constant phase admittance. The effective hopping rate was estimated by comparing the frequency dependent part and the dc limit of the intragrain conductivity. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997.     

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.     

Semiconductor-metal transition in FeSi in an ultrahigh magnetic field

We study the conductivity and magnetic susceptibility of single-crystal iron monosilicide in ultrahigh magnetic fields (up to 500 T) at low temperatures. The experimental methods used in measuring the conductivity and magnetic susceptibility are discussed. At 77K we detect a gradual increase in the conductivity of iron monosilicide by more than a factor of 100 as the magnetic field gets stronger. At 4.2K we detect a first-order phase transition in a field of 355 T accompanied by a sudden change in the value of the magnetic moment by 0.95 μ _B per iron atom and a transition to a phase with high conductivity. The results are discussed within the scope of the spin-fluctuation theory. Zh. éksp. Teor. Fiz. 116, 1770–1780 (November 1999)     

Electrical properties of A′Ca2Nb3O10 (A′=K, Rb, Cs) layered perovskite ceramics

The electrical conductivity properties of Dion-Jacobson type layered perovskites A′Ca₂Nb₃O₁₀ (A′=K, Rb, Cs) was investigated under different gas atmospheres. An increase in the electrical conductivity by about 2–5 orders in magnitude in both ammonia and hydrogen atmospheres is observed compared to air. Among the members of the series, the compound with the smallest size of the alkali ion, i.e. KCa₂Nb₃O₁₀, exhibits the highest conductivity. In air and hydrogen, a single activation energy value in the range 0.25 – 0.80 eV is observed, while in ammonia a sharp increase in the electrical conductivity is found at about 500 °C. The activation energy at low-temperatures (300–500 °C) is attributed to ionic motion and at higher temperatures (500–700 °C) to both defect formation and ionic motion. The unusual electrical conductivity behavior in ammonia is explained on the basis of the model developed for alkali halides. EMF measurements reveal that the layered perovskites are ionic (proton) conductors. The electrical conductivity changes as a function of the ammonia gas concentration; accordingly, layered perovskites appear to be useful solid electrolytes in galvanic cells for practical applications, e.g. for gas sensors. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.