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.     

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.     

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.     

Thermoelectric power of superionic conductor Ag7I4PO4

The thermoelectric power (θ) of Ag₇I₄PO₄ superionic conductor has been studied for the first time from 4 to 75°C; 80°C being its decomposition temperature. Relation between θ and temperature can be expressed by the equation

$\text{?θ = 0.20}\text{10}{}^3\text{T}\text{?0.255}$

for this solid. Analysis of the data yields 0.20 eV as heat of transport of Ag⁺ ion which is very close to its activation energy 0.21 eV. This supports the prediction of the theory of Rice and Roth based on “free-ion” model. This is also in consonance with earlier theories on heats of transport of ions in ionic solids. Indications have been found to justify the assumption that Ag₇I₄PO₄ has average structure.     

Photolysis of MAg4I5 superionic films

Photolysis studies on MAg₄I₅ films are reported. The electrical conductivities of MAg₄I₅ films have been found to change when exposed to mercury light. This is explained on the basis of a model similar to Mott-Gurney theory of photolysis for silver halides. The formation of silver specks has been confirmed by photomicrographic studies.     

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.     

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.     

Phase transitions in the superionic protonic conductor CsHSeO4 investigated by Brillouin spectroscopy

A Brillouin investigation in CsHSeO₄ has been performed over the temperature range 20–165 °C which includes two phase transitions, in particular the transition to the superionic phase near T_s = 129 °C. We observed strong discontinuities for elastic constants C₁₁, C₂₂ and C₃₃ at T_s and a broadening of the Brillouin lines above T_s. The results are discussed on the basis of a linear coupling between strains and mobile protons.     

The phonon spectra of the superionic conductor KBiF4 (a CaF2 Analogue)

Theoretical calculations of the phonon infrared and Raman response in simple superionic conductors such as AgI, CuI, and CaF₂ types are based on two fundamental assumptions. First, most of the response can be understood in terms of a breakdown of the selection rules due to disorder (lack of translational symmetry) and second, harmonic lattice dynamics can be used with a good degree of accuracy. This is tested here experimentally in the superionic conductor K_1?xBi_1+xF_4+2x which has the CaF₂ structure (x=0.0 is analogous to 2CaF₂). By varying x we increase the disorder (via F-ion interstitials and vacancies) and measure broad temperature independent reduced Raman and infrared responses. The broad response is dependent on x in a manner consistent with the first assumption and the lack of temperature dependence is consistent with the second assumption. In order to understand the transverse optic vibrational frequencies (ω_TO) we have found that plots of ω_TO²vs. μ^-1 (reduced mass) are very helpful. The linearity of such plots, for example, for tetrahedrally bonded AgI, CuI, CuBr, CuCl (formal charge Z=1) and the difference of such results for similar materials but with formal charges of 2, 3, and 4 is surprising and not as yet understood. Other conclusions are discussed at the end of the paper.     

Raman measurements of the superionic conductor AgI

Using the Raman technique, we have observed large abrupt reversible changes in the phonon spectrum at the transition temperature, T_c = 147°C, of the superionic conductor AgI. The defect nature of the high temperature phase completely breaks down the selection rules and allows the Raman measurements to give a measure of the frequency dependent conductivity, σ(ω), assuming a frequency independent matrix element. Similarities as well as differences in σ(ω) above and below T_c are shown.     

Ion dynamics and sublattice melting in the superionic conductor PbF2

The F¹⁹ relaxation rates in PbF₂ exhibit anamolous changes as the temperature is increased into the superionic phase. These changes confirm the concept of a continuous melting of the fluorine sublattice and show that the fluorine motion in the superionic phase is highly correlated.     

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.     

The sodium conductivity paths in the superionic conductors Na5RESi4O12

From an extended single crystal analysis on the Y variant of a recently discovered new class of extraordinary ionic conductors, the crystallographic sites available to the Na⁺-ions were located. Part of the Na⁺-ions ($\text{7}\text{3}$ of a Na⁺-ion per formula unit) appear to be mobile and are located in the space between stacks of Si₁₂O₃₆ rings. The size of the channels containing the partly occupied Na⁺-sites was changed by substitution of the RE-ion. The highest conductivities and smallest activation energies were found for RE-ions having radii comparable to that of a Na⁺-ion.     

Anisotropic conductivity in a channel-structured superionic conductor: Li2Ti3O7

From measurements on single crystals of Li₂Ti₃O₇, the conductivity is determined to be predominately ionic with an anisotropy of $\text{σ}{}_{\mathrm{b}}\text{σ}{}_{\mathrm{a}}\text{≈4}$ and $\text{σ}{}_{\mathrm{c}}\text{σ}{}_{\mathrm{b}}\text{≈7}$. This anisotropy is significant but is not sufficient to classify this channel-structured ramsdellite material as a one-dimensional conductor. A conduction path through the ramsdellite crystal structure consistent with the determined anisotropy is presented.     

Local ionic motion in the superionic conductor Li3N

Anisotropic dielectric behavior of the superionic conductor lithium nitride (Li₃N) is reported for low temperature where the dc ionic conductivity can be neglected (T<17oK). For E ⊥ c the static dielectric constant followed a Langevin-Debye (ε ∝ 1/T) type law from 170 to 80K which changed to a temperature independent value below 40K. Debye relaxations were measured between 10 and 50K in the frequency range 10Hz to 1MHz. The observed properties are discussed in terms of a locally restricted ionic motion in shallow potentials. Different types of potentials are considered and an overdamped oscillator model is proposed.     

Ambient-pressure superconductivity at 8 K in the organic conductor β-(BEDT-TTF)2I3

We report that the superconductivity with the onset temperature as high as 8 K in the organic conductor of β-(BEDT-TTF)₂I₃, which was achieved by an application of a soft pressure, i.e. 1.3 kb, remains stable with even higher transition temperature after a subsequent release of pressure to the ambient pressure. Correspondingly, higher values of the upper critical field, H_c2, are observed. (H_c2 > 20 k0e at 2 K along the c¹-axis.)     

Raman study of charge-density-wave phase transition in quasi-one-dimensional conductor (TaSe4)2I

Polarized Raman spectra were obtained in the quasi-one-dimensional conductor (TaSe₄)₂I above and below the charge-density-wave (CDW) transition temperature (T_c=263 K). The Raman intensities of many peaks become intenser and two of the phonon peaks shift to higher frequency with decreasing temperature. Moreover a new broad peak at about 90 cm^?1 and a new peak around 166 cm^?1 appear in the low-temperature phase. The polarization characteristic shows that the former is assigned to totally symmetric mode. The damping constant of the phonon at 90 cm^?1 increases markedly with increasing temperature. The frequency shifts to higher frequency as the temperature increases and the coupling coefficient is approximately proportional to (T_c?T)^{$\text{1}\text{2}$}. This peak becomes Raman active owing to the CDW phase transition. The temperature dependence of the damping constant and the frequency shift may have a relation to the dynamical properties of the CDW phase transition.     

The Raman spectra of the superionic conductor CuI in its three phases

We report Raman measurements of the superionic conductor CuI in all three phases. We show that the major features of the spectra, particularly the highest temperature α'-phase, can be accounted for by simple lattice dynamics calculations if the defect nature of the phase is taken into account. Thus, while some of the more specialized ideas that have been proposed to explain these types of spectra are undoubtedly appropriate, the major features actually can be explained rather straightforwardly.     

Temperature variation of electrical conductivity and absorption edge in Cu7GeSe5I advanced superionic conductor

Single crystals of Cu₇GeSe₅I superionic conductor were grown by chemical transport. Their electrical conductivity in the frequency range 1.0×10⁶–1.2×10⁹ Hz and in the temperature range 196–295 K was measured. Cu₇GeSe₅I crystal is shown to exhibit a rather high electrical conductivity (σ₂₉₅=64.0 S/m at 295 K) and a low activation energy (ΔE_a=0.125 eV). Optical absorption edge of Cu₇GeSe₅I crystals in the temperature range 77–300 K was studied, the temperature dependences of the optical pseudogap and Urbach energy being obtained. The effect of different types of disordering on the Urbach absorption edge and electron–phonon interaction parameters was investigated.     

Thermoelectric power of (Me4N)2Ag13I15 and (Et4N)2Ag13I15

Thermoelectric power using reversible silver electrodes and electrical conductivity on the compressed pellets of (Me₄N)₂Ag₁₃I₁₅, and (Et₄N)₂Ag₁₃I₁₅ have been measured between room temperature and below 160°C. The results of θ can be expressed by the equations:?θ = 0.115 (10³/T)+0.2905VK^?1 and ?θ = 0.150 (10³/T) + 0.305mV K^?1; and those of conductivity by the equations; σ = 28.7 exp (?0.17eV/kT) ohm^?1cm^?1 and $\text{σ = 216.6 exp (?0.24e}\text{V}\text{kT}\text{)}$ ohm^?1cm^?1; respectively for Me- and Et-electrolytes. The results are discussed and compared with those of previous authors.