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)     

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.     

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)     

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.     

Photoconductivity and light stimulated contact potentials in β-RbAg4I5

Photoconductivity and photoelectric effects have been established in thin polycrystalline films of β - RbAg₄I₅. The spectral and the temperature dependence of the phenomena have been studied. The results are interpreted using a model in which ionic processes dominate in the photoconductivity when the exciting photon energy less than 3.2 eV.     

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)     

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.     

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.     

RDF studies of the superionic conductor RbAg4I5 using EXAFS

Measurements of the extended X-ray absorption fine structure (EXAFS) on the Ag K-shell absorption in RbAg₄I₅ show that in all three crystalline phases the equilibrium position of the Ag ions is near the center of tetrahedra formed by the iodine atoms. In addition, Ag-Ag correlations, which persist into the superionic α-phase, are observed.     

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.     

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.     

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.     

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.     

Thermal expansivity in superionic RbAg4I5

We have measured the length of a RbAg₄I₅ crystal in the temperature range between 130 K and 300 K using a fused silica LVDT dilatometer system. The expansivity is a smooth, nearly linear function of temperature. There is no anomaly such as a kink, discontinuity, or critical behavior at T_c = 209 K where the superionic β to superionic α phase transition occurs.     

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)     

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).     

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.     

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.     

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.     

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.