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)     

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.     

The absence of a measurable hall effect in the superionic conductor RbAg4I5

Thin film samples of RbAg₄I₅ have been prepared by evaporation and several of their electrical properties have been determined. The behavior of the conductivity of these films is identical to that found in bulk RbAg₄I₅. An attempt to reproduce the Hall effect results of Kaneda and Mizuki was unsuccessful, and we conclude that the Hall mobility of silver ions in this material is less than 2 × 10^-3cm²/V·sec.     

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.     

Proprietes electrogalvaniques et electroniques du sulfure d'argent β: Domaine d'existence

The study of electrogalvanic and electronic properties of the form β of silver sulfide has been performed on polycrystalline samples whose non stoichiometry is controled by coulometric titration with the cell Ag/RbAg₄I₅/Ag₂₊?S/Pt.The data obtained lead to the adoption of an electronic model according to which the Frenkel defects of the silver sublattice are completly ionized. The variation with temperature of the electronic properties near the stoichiometric composition between 23 and 176°C, the temperature of transition between the two forms β and α of Ag₂S, enables the forbidden gap E_i = (1,35?1,5 × 10^?3T) eV to be computed. The existence range has been drawn between 69°C and the α ? β transition temperature.The study of the ionic conductivity has made it possible to detect two ranges of preferential conduction and to determine the mechanism of migrations of the intersticial silver ions.     

The electrical response of RbAg4I5/graphite inhomogeneous interfaces

The ac impedance of the cell graphite/graphite + RbAg₄I₅/graphite + RbAg₄I₅/graphite was investigated in the frequency range from 10⁻⁴ to 3 × 10⁵ Hz. A model assuming the graphite + RbAg₄I₅ mixture to represent a porous interface could explain the observed dependences in nearly the whole frequency range with the exception of the lowest frequencies. Attempts to improve the agreement presuming adsorption, discharge of charge carriers or diffusion of neutral species failed. An excellent fit could be achieved assuming a frequency dependent ac conductivity of the interface, approximately proportional to the frequency, as in the bulk of many solids with low conductivity, such as amorphous semiconductors or dielectrics.     

Proprietes electroniques et electrogalvaniques du seleniure d'argent β: Domaine d'existence

A study of the electronic and electrogalvanic properties of β silver selenide has been performed on polycrystalline samples whose non-stoichiometry is controlled by eoulometric titration with the cell Ag/RbAg₄I₅/Ag_2+? Se/Pt. The adoption of an electronic model according to which the Frenkel defects of the silver sublattice are completely ionised allows us to compute independently the non-stoichiometry with the two formulaes δ = [Ag^o_i] ? [V'_Ag] and δ = n ? p. The homogeneity range has been drawn at positive temperatures and the formation enthalpy of the Frenkel defects is H'_F = 0,40 eV.     

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)     

UV absorption of RbAg4I5-RE (Sm, Yb) thin-film systems

UV spectra of samples prepared by vacuum deposition of Sm and Yb thin films on 100–200-nm thick films of the RbAg₄I₅ solid electrolyte (SE) at 300–350 K contain strong absorption bands peaking at about 4.3 and 5.0 eV. After deposition of ～5 nm of Sm, the ionic conductivity σ of the samples decreases from σ ₀ to ≈0.9 σ ₀, and the SE lattice parameter, from 11.24 to ≈11.15 Å, with the x-ray reflection halfwidth increasing from 0.5 to 0.8°. Further growth of Sm concentration in the samples changes the x-ray diffraction pattern, the absorption at 4.3 and 5.0 eV increases, a new absorption edge forms at 3.8 eV, and σ decreases down to ～10^?2 σ ₀. It is conjectured that the strong UV absorption bands in heavily defected silver halides of the RbAg₄I₅-Sm(Yb) system is genetically related to the 4d ¹⁰→4d ⁹5s electronic transitions in free Ag⁺ ions.     

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.     

High-voltage conductivity of α-RbAg4I5, α-KAg4I5, and α-KCu4I5 superionic conductors

It is found that the conductivity of RbAg₄I₅, KAg₄I₅, and KCu₄I₅ solid electrolytes in the ??-phase rises with the electric field strength and tends to a limit value in fields on the order of 1 MV/m. The high-voltage limit of conductivity exceeds the low-voltage one by 100% or more. The application of an intense electric pulse raises the conductivity of the electrolytes by 35%, and the activated state exhibits an anomalously long relaxation time.     

Ion dynamics in solid electrolytes described by the concept of mismatch and relaxation

Abstract

Below the far-infrared frequency regime, conductivity spectra of crystalline ion conductors like RbAg₄I₅ and others display remarkably uniform characteristics. Well-known approximations include the universal dynamic response (UDR) and the nearly constant loss (NCL) behaviour. We now present a new non-power-law, non-KWW master curve. Its shape is shown to be equivalent to a proportionality between two particular functions of time. These functions are interpreted as rates of mismatch relaxation via the so-called single-particle and many-particle routes. The proportionality of relaxation rates is the central statement of the concept of mismatch and relaxation (CMR).     

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.     

Proprietes electroniques et electrogalvaniques dutellurure d'argent β domaine d'existence

A study of the electronic and electrogalvanic properties of β silver telluride has been performed on samples whose non-stoichiometry has been determined by coulometric titration with the cell Ag/RbAg₄I₅/Ag₂Te/Pt. The data lead to the adoption of a model of Frenkel defects on the silver sublattice that are fully ionised. Their formation enthalpy is H_F′ = 0,70 eV. On the tellurium side this model is completed by the association V_AgV_Ag^x. Th homogeneity range is reported from ?10°C to 132°C: it extends essentially on the tellurium side of the stoichiometric composition.     

UV absorption of thin-film RbAg4I5-RE (Sm, Yb) systems

UV-spectra of samples obtained by vacuum deposition of RE (Sm, Yb) thin films onto films of the solid electrolyte RbAg₄I₅ (300 – 350 K, thickness 100 – 200 nm) exhibit bands of strong absorption with maxima at 4.3 and 5.0 eV. After the deposition of Sm (≈ 5 nm), the ionic conductivity σ of the samples decreases to ≈ 0.9 σ₀ and the lattice parameter from 11.24 to 11.15 Å. The halfwidth of X-ray reflections increases from 0.5 to 0.8⁰. Further increase of the concentration of Sm in the samples changes the X-ray diffraction pattern. Under such conditions, the absorption in the regions of 4.3 and 5.0 eV further increases; a new single edge of absorption at 3.8 eV appears and σ declines to ≈10^?2 σ₀. It is supposed that a genetic relation exists between the UV-bands of strong absorption in the highly defective silver-halides of the RbAg₄I₅-RE system and the electronic transitions 4d¹⁰ ?4d⁹ 5s in free Ag⁺ ions.     

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)     

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.     

Transport properties and battery discharge characteristics of the Ag+ ion conducting composite electrolyte system (1−x)[0.75AgI: 0.25AgCl]: xFe2O3

Ion transport and battery discharge characteristic studies are reported for a new Ag⁺ ion conducting two-phase composite electrolyte system (1−x)[0.75AgI: 0.25AgCl]: xFe₂O₃, where 0 ≤ x ≤ 0.5 in molar weight fraction. An alternative single-phase host-matrix ‘annealed [0.75AgI: 0.25AgCl] mixed system/ solid solution’ has been used in place of the traditional host, AgI. Submicron size particles (<1 μm) of Fe₂O₃ has been used as second phase dispersoid. The composition 0.8[0.75AgI: 0.25AgCl]: 0.2Fe₂O₃, exhibiting the highest room temperature conductivity has been referred to as the optimum conducting composition (OCC). The reason for an enhancement of an order of magnitude in the conductivity from that of the pure host has been identified through direct determination of ionic mobility (μ) and mobile ion concentration (n) using transient ionic current (TIC) technique. The XRD study confirmed the coexistence of the constituent phases. The ionic transference number is found to be very close to unity. This reveals the fact that the silver ions are the sole charge carriers in the system. The results are discussed in the light of space-charge models proposed for the two-phase composite electrolyte systems. Solid state batteries, fabricated using OCC as electrolyte, Ag-metal as anode and mixtures of iodine & graphite, viz. (C+I₂), (C+KI₃), (C+(CH₃)₄NI₃), (C+(C₂H₅)₄NI₃), etc. as cathodes, were discharged under different load conditions. The battery with (C+I₂) cathode performed satisfactorily specially under low current drain states. Paper presented at the 2nd International Conference on Ionic Devices, Anna University, Chennai, India, Nov. 28–30, 2003.     

Solid state reactions and transport properties in the quasibinary system AgI/RbI

The reactions between AgI and Rb₂AgI₃, RbI and RbAg₄I₅ and AgI and RbI have been investigated by measurements of reaction layer thickness and by experiments with inert markers. A theoretical derivation of the growth kinetics for reactions with double layers is given. The Rb⁺-diffusion coefficients, which are needed to apply these relations to the reaction between AgI and RbI, have been obtained from an investigation of the other two reactions which form only a single product layer. The diffusion coefficient of Ag⁺in Rb₂AgI₃ was also determined from conductivity measurements on Rb₂AgI₃.     

Evidence for an order-disorder transformation in the solid electrolyte RbAg4I5

Specific heat and optical birefringence measurements made on RbAg₄I₅ are found to yield Ising-like critical exponents at the 208 K phase transition. A model involving indirect coupling of Ag⁺ sites through distortions of the I^--lattice is presented which is in agreement with experiment.