Raman results in the superionic conductor RbAg4I5

Raman measurements between 77°K and 296°K are reported for the superionic conductor RbAg₄I₅. Careful attention is paid to the temperature region of the two phase transitions at 121°K and 208°K. We can detect no shifts in any of the numerous phonon modes except one at 22.9 cm^-1, which abruptly and reversibly appears in the lowest temperature phase. Raman results for the isomorphic material KAg₄I₅ are the same with the same mode appearing in the low temperature phase. Thus, the results in these systems are markedly different from those in AgI, where there are very large changes at the superionic conducting transition temperature.     

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.     

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.     

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.     

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.     

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 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.     

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.     

Novel RCo5Ga7 intermetallic compound

The novel RCo₅Ga₇ (R=Y, Tb, Dy, Ho and Er) intermetallic compounds have been synthesized, and their crystallographic and magnetic properties have been studied using X-ray diffraction and magnetic measurement. RCo₅Ga₇ crystallizes in an orthorhombic structure with ScFe₆Ga₆ type. The space group is Immm, and Z=2. According to the structural refinement result, the 2a, 4e, 4f, 4g, 4h, and 8k crystal positions are occupied by 2R, 4Ga^I, 4(Ga^II, Co^I), 4Ga^III, 4(Ga^IV,Co^II), and 8(Co^III,Ga^V), respectively. The RCo₅Ga₇ intermetallic compound can be stabilized in the range of the radius ratio of R_Re/R_(Co,Ga)<1.36. The RCo₅Ga₇ compound exhibits a paramagnetic behavior. The magnetization at 5 K ranges from 28.93 to 40.62 emu/g.     

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.     

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.     

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.     

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 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.     

Raman scattering in K2Pt(CN)4Br0.3 · 3H2O

Raman scattering experiments on K₂Pt(CN)₄Br_0.3 · 3H₂O are reported between 5 and 300 K as a function of temperature. A line of A₁ symmetry detected at 44 cm^?1 shows interesting temperature dependent properties. It is concluded from a comparison of the frequency, symmetry, and scattering intensity of this line with theoretical predictions that the excitation concerned represents the amplitude mode of the charge density wave (the line observed in infrared absorption being the phase mode). No Peierls transition is observed, but the results are consistent with a Peierls distortion present at all temperatures. The findings are correlated with inelastic neutron scattering and infrared studies. Finally, the CN stretching modes at 2189 and 2173 cm^?1 and the water mode at 3490 cm^?1 are studied as a function of temperature.     

Symmetry and lattice-dynamic aspects of structural phase transitions in (CH3NH3)2MnCL4 and related compounds

A group theoretical analysis of the second-order structural phase transition in (CH₃NH₃)₂MnCL₄ at 394°K and of similar transitions in ethyl and propyl compounds (D¹⁷_4h→D¹⁸_2h) was performed. The soft mode transforms according to the τ^x₅-irreducible representation at the X-point of the Brillouin-zone boundary and its eigenvector is discussed. The transition is of the order-disorder type and is caused by a slowing down of the hindered rotation of NH₃-groups. Knowing the symmetry of the order parameter, a thermodynamic potential expansion was constructed and expected anomalies in material constants around the transition temperature are briefly discussed. The high temperature phase transitions in analogous copper compounds are explained as a sequence D¹⁷_4h→D¹⁸_2h→D¹⁵_2h. The second of these phase transitions is driven by a soft mode transforming as the τ^Y₇-representation at the D¹⁸_2h Brillouin zone boundary.     

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.     

Diode laser line strength measurements of the (ν4 + ν5)0 band of 12C2H2

A diode laser spectrometer that was operated in sweep integration mode was used to measure individual line strengths for 17 R-branch transitions of the (ν₄ + ν₅)⁰ combination band of ¹²C₂H₂ at 7.4 μm. Analysis of these results gives a band strength S_v = 64.4 ± 2.0 cm^?2 atm^?1 at 296 K. Line-broadening parameters for several of these transitions were determined by using both N₂ and He as broadening gases.     

Spectroscopic and photoluminescence properties of Ho doped Ba0.65Sr0.35TiO3 nanocrystals

Ho³⁺ doped Ba_0.65Sr_0.35TiO₃ (BST) nanocrystals was prepared by sol-gel method. The structural and morphological properties of the nanocrystals were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The absorption spectrum, photoluminescence spectrum and fluorescence time decay curve were measured at room temperature. Based on the Judd-Ofelt (J-O) theory, the J-O intensity parameters Ω_t (t=2, 4, 6) of Ho³⁺ doped BST nanocrystals were calculated to be 0.67×10⁻²⁰ cm², 1.11×10⁻²⁰ cm² and 1.09×10⁻²⁰ cm², respectively. The emission probabilities, radiative lifetimes and branching ratios of the different Ho³⁺ transitions were also determined. The emission cross sections of the important intermanifold transitions ⁵F₄,⁵S₂→⁵I₈, ⁵F₅→⁵I₈ and ⁵F₄→⁵I₇ have been calculated from the luminescence spectrum. The room temperature fluorescence lifetime of the ⁵S₂→⁵I₈ transition for Ho³⁺ in BST nanocrystals was measured and the radiative quantum efficiency was estimated to be 61.9%.