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.     

Photoinduced color centers creation in superionic crystals RbAg 4 I 5

Abstract

A new phenomenon of a reversible photoinduced coloration caused by light irradiation is discovered and investigated in superionic RbAg ₄ I ₅ crystals. The reversible photoinduced absorption is found to be a result of irradiation by light with wavelengths in the region from 420 nm to 450 nm. The proposed mechanism of the discovered effect is associated with ambipolar diffusion of screened by mobile ions optically excited electronic carriers. The processes of color centers creation in superionic crystals RbAg ₄ I ₅ due to additive coloring in iodine vapours, ionic implantation and γ-ray irradiation are considered.     

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)     

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.     

The effect of point defects on the ionic conductivity of RbAg4I5 solid electrolytes

Abstract

The effect has been studied of additive coloring on the magnitude of ion conductivity of RbAg₄I₅ crystals. It has been found that slight changes of silver stoichiometry of 10^?3 at.% can lead to considerable variations of the ionic conductivity Δ[sgrave]_i/[sgrave]_i ? 0.1. The dependence has been observed of the magnitude of ion conductivity on the ratio between the integral intensities of the main bands in the photoluminescence spectrum of the γ-phase of RbAg₄I₅ which associated with the luminescence centres containing vacancies and interstitials of silver cations.     

Phase transitions and dynamics of superionic conductors

A Hamiltonian is constructed for superionic conductors taking into account the mutual interactions and transports of the mobile ions, and their interactions with the phonons of the cage. Under special conditions we recover the phenomenological results of Rice et al. and Huberman in regard to phase transitions. Two transition points are shown to be possible in the presence of the mutual interactions of cations (as observed in RbAg₄I₅ and CuBr). Structural phase transitions involving the cage when exist are found to occur at the same temperature at which the conductivity becomes critical, in agreement with experiments. Dynamical aspects of our Hamiltonian are also discussed. The collective modes of the phonon-cation system are calculated and used to explain the abrupt disappearance of certain modes above the critical temperature as observed in the Raman spectra of RbAg₄I₅, KAg₄I₅ and AgI. Our theory does contain the possibility that there is no soft mode with nonzero frequency, in accordance with the existing experimental situation. Our Hamiltonian is compared to others. The similarities between superionic conductors and other systems (Hubbard model, ferroelectrics, Jahn-Teller systems, molecular crystals) are emphasized.     

Influence of samarium on optical absorption in thin films of the solid electrolyte RbAg4I5

The fundamental optical absorption of films of the solid electrolyte RbAg₄I₅ electrolyte films decreases (by approximately 25%) after vacuum evaporation of Sm films onto them, and a broad strong-absorption band with a maximum at 2.4 eV appears within the bad gap. The films bleach after 5–10 days in dry air. The observed phenomena are attributed to a high concentration (∼3×10²⁰ cm⁻³) of point defects, including F-centers, in nonstoichiometric RbAg₄I₅:Sm, and also to the oxidation of Sm. In colored films the ionic conductivity is σ⋍0.9σ ₀, and in bleached films it is close to the initial value σ ₀. Fiz. Tverd. Tela (St. Petersburg) 39, 1544–1547 (September 1997)     

Electronic properties of C60 single crystals doped with lithium by electrodiffusion

Diffusion of lithium cations in C₆₀ single crystals driven by electric field has been detected and studied. A novel technique for fullerene crystal doping based on injection of ions through a “superionic crystal/C₆₀ single crystal” heterojunction has been suggested. It has been found that lithium doping of C₆₀ single crystals brings about an ESR signal, and this signal as a function of time has been investigated. The electronic conductivity in Li_xC₆₀ crystals has a nonmetallic nature. Reflection spectra measured in the IR band have shown that the reflectivity due to free electrons gradually decreases with time, which correlates with the evolution of signals due to ESR and microwave conductivity. Lithium doping of crystals increases the oscillator strength of the T _1u (4) vibrational mode and shifts it to lower frequencies (from 1429 cm⁻¹ to 1413 cm⁻¹), which indicates that one electron is present at the C₆₀ molecule, and this fact may be treated as evidence that the LiC₆₀ phase is generated in a C₆₀ crystal. Zh. éksp. Teor. Fiz. 116, 1706–1722 (November 1999)     

Comparison of the Raman spectra of superionic conductors AgI and RbAg4I5

The Raman spectra of superionic conductors AgI and RbAg₄I₅ have been measured in both cation disordered and normal phases. Similarities in the observed spectra of these two materials are discussed. The frequency dependent conductivity is obtained from the spectra and compared to infrared results. Several possible interpretations of the observed spectra are considered.     

Influence of grain sizes on the ionic conductivity and the chemical diffusion coefficient in copper selenide

Ionic conductivity and chemical diffusion coefficient have been studied for superionic polycrystalline Cu_1.75Se copper selenide within the temperature interval 300–500 K. An increase in ionic conductivity with an grain size increase is observed. In our opinion, this fact is caused by lower activation energy for the bulk diffusion than that for the grain boundary diffusion.     

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)     

The splitting of the infrared vibrational spectra of RbAg4I5 in the low-temperature γ-phase

Spectra of the dynamic conductivity σ(v) of the superionic RbAg₄I₅ single crystal in the γ-phase have been measured at the frequencies 2–33 cm^?1 by means of monochromatic submillimeter spectroscopy with resolution 0.001 cm^?1. Several tens of well-resolved narrow and intensive lines were observed at liquid helium temperature. The obtained results are qualitatively discussed.     

A correction method for thermally lowered fission track ages

Abstract

Onset of electric current has been detected on illuminating the near–contact regions of RbAg₄I₅ crystals. The spectral and temperature characteristics of this current have been studied. A method is proposed for studying the spectrum of electronic states in the α-phase of the RbAg₄I₅ crystal. The dependence of the magnitude of the current and its spectral characteristics on the particular region of the RbAg₄I₅ crystal which is exposed to the light was used for the analysis of the variations in the distribution of defects along the length of the sample and the processes which occur near the electrode-(solid electrolyte) junctions.     

Conductivity of the solid electrolyte RbAg4I5 in the microwave region

The high frequency ionic conductivity of RbAg₄I₅ single crystals was measured in the range from 0.1 MHz to 8 GHz using a microwave reflection method. In the whole temperature region studied (30°C to 135°C) the bulk conductivity was found to be frequency independent and to coincide with the latest published values for the static conductivity. This result is in contradiction with values reported formerly in literature but agrees very well with recent measurements on the structurally similar solid electrolyte AgI.     

Study of the phase composition of AgI microcrystals by exciton spectroscopy and differential scanning calorimetry

Differential absorption spectra of RbAg₄I₅ have been measured in the exciton absorption region of AgI within the temperature range 27–250 °C. In the same temperature range, the temperature behavior of the heat capacity of RbAg₄I₅, Rb₂AgI₃, and KAg₄I₅ have been obtained by differential scanning calorimetry. An analysis of the results suggests that, in AgI microcrystals less than r _cr in size, the upper boundary for stability of the low-temperature β modification is higher by several tens of degrees. Fiz. Tverd. Tela (St. Petersburg) 40, 852–854 (May 1998)     

Investigation of ion transport in Li<Subscript>8</Subscript>ZrO<Subscript>6</Subscript> and Li<Subscript>6</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> solid electrolytes

Lithium ionic conductivity and spin-lattice relaxation rates were measured in Li₈ZrO₆ and Li₆Zr₂O₇ solid electrolytes. It was found that the Li₈ZrO₆ solid electrolyte undergoes a transition to the superionic state in the temperature range 673–703 K. It was shown that Li⁺ ions are mobile in particular lattice positions of the Li₆Zr₂O₇ phase, and that ionic conductivity is monotonic at an activation energy of 79.4 kJ/mol.     

On the characteristic changes in the domain structure and conductivity of CsDSO4 crystals near the superionic phase transition

The appearance of a new domain structure against the background of the old domain structure is observed in CsDSO₄ crystals at 3 °C away from the superionic phase transition. It is established that the appearance of the new domain structure is accompanied by a gradual increase in the conductivity by 1.5–2 orders of magnitude, and then the conductivity increases abruptly by another two orders of magnitude at the temperature of the superionic phase transition. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 11, 871–875 (10 June 1996)     

Raman measurements of the superionic conductor KAg4I5

Raman measurements of the superionic conductor KAg₄I₅ are reported between 77K and 296K and compared to similar data of RbAg₄I₅. The mode frequencies for the two materials are very similar and a mode at 22.7 cm^?1 abruptly and reversibly disappears at the transition temperature, T_c=137 K, as is observed in RbAg₄I₅, at 121 K.     

Mössbauer studies of microscopic disorder in solid electrolytes

We implement for the first time Mössbauer Spectroscopy (MS) to investigate short-range properties of disorder in solid electrolytes. MS in¹²⁹I and¹¹⁹Sn was carried out in RbAg₄I₅ and as impurity in Ag₂Se, respectively. Measurements were performed both in the superionic and the normal phases. It is shown that localized cation hopping is an inherent feature of the α-AgI-type solid electrolytes. In RbAg₄I₅, at temperatures far belowT _c, a small fraction of Ag is still locally mobile and atT>T _c, its concentration increases exponentially. A strong linear temperature dependence of the point-charge electric field gradient is observed and explained in terms of local hopping. With¹¹⁹Sn in Ag₂Se we observe the onset of “local melting” of the Ag surrounding the SnSe₄ cluster at 50 K below the bulk superionic phase transition. The characteristic features of MS related to microscopic studies of solid electrolytes are fully described.     

Ionic conduction in KBr-KI mixed crystals

Ionic conductivity of KBr_1?xI_x (0 ? x ? 0.5) mixed crystals has been studied as a function of temperature in the range of 2 ionic conductivity and lowers the activation energy appreciably which is consistent with our earlier results on AgBr-AgI mixed crystals, and proposal that the purely elastic displacements (lattice strain) caused by the very “wrong size” of the substituent ions is primarily responsible for the behavior. The strain thus produced is expected to cause “lattice loosening” and hence lowering of the melting points (T_m) or the enthalpy of formation of the Schottky defects (H_s). A semiquantitative calculation shows this to be the case.