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)     

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)     

Dufour effect in superionic copper selenide

The Dufour effect has been observed in the superionic conductor copper selenide, Cu_2−x Se. This effect is the opposite of the Soret thermal diffusion effect which was previously only observed in gaseous and liquid systems. Fiz. Tverd. Tela (St. Petersburg) 40, 242–244 (February 1998)     

Ion-motion disorder in a tysonite superionic conductor from 19F NMR data

The disorder in ion motion in a LaF₃ superionic conductor with tysonite structure is determined from NMR spectra. The method is sensitive to dynamic effects with correlation times of 5×10⁻³<τ<10⁻⁶ s. Within the 260–400-K interval, ion movement is observed only on one of the sublattices (F₁), and it exhibits a correlation time distribution. The shape of the distribution is well fitted by a log-Gaussian. The distribution in correlation times reflects noticeable spatial fluctuations of local potentials in the LaF₃ superionic conductor. Fiz. Tverd. Tela (St. Petersburg) 41, 1616–1620 (September 1999)     

Experimental studies of the thermal properties of thin films by a probe technique with periodic heating

The thermal properties of SrTiO₃ thin films are investigated experimentally by recording the amplitude and phase of the temperature oscillations of a flat probe with the heat flux perpendicular to the plane of the film. Results are given from measurements of the specific heat and thermal conductivity of a leucosapphire substrate and the thermal conductivity of a SrTiO₃ film of thickness 2 μm. Fiz. Tverd. Tela (St. Petersburg) 39, 1299–1302 (July 1997)     

Sodium superionic conductor sputter-deposited coatings

Thin sodium superionic conductor (Nasicon) coatings are deposited on rotating substrates by co-sputtering in the reactive mode of a Zr-Si and a Na₃PO₄ target. The influence of the discharge current and of the target-to-substrate distance is investigated owing to the targeted Na₃Zr₂Si₂PO₁₂ stoichiometry. A thermo-structural analysis shows that the amorphous as-deposited coating of convenient composition crystallises around 700 °C in the monoclinic structure. Electrical measurements performed at room temperature after various annealing treatments indicate a ionic conductivity of about 2·10⁻³ S·cm⁻¹, consistent with that of bulk Nasicon. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.     

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.     

Ionic conductivity of isostructural crystals of the superionic conductors Li3Fe2(PO4)4 and Li3Sc2(PO4)3

The results of measurements of the ionic conductivity σ in Li₃M₂(PO₄)₃ (M=Fe, Sc) single crystals along various crystallographic directions are analyzed. Possible causes of the different behavior of σ in the isostructural crystals are discussed: a jump of the conductivity in the transition to the superionic phase in Li₃Sc₂(PO₄)₃ and its absence in Li₃Fe₂(PO₄)₃; the existence of a conductivity maximum in different crystallographic directions (along the c axis in Li₃Sc₂(PO₄)₃ and along the a axis in Li₃Fe₂(PO₄)₃). Fiz. Tverd. Tela (St. Petersburg) 39, 83–86 (January 1997)     

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.     

High-temperature behaviour of average structure and vibrational density of states in the ternary superionic compound AgCuSe

Results of simultaneous thermal analysis (STA), synchrotron powder diffraction (in the range 300–973 K) and inelastic neutron scattering (at 285 and 505 K) on non-superionic β- and superionic α-AgCuSe are reported. The sample is stable in argon on heating. The volume change at the superionic phase transition is about 5%. A model for the average structure of α-AgCuSe is proposed. No anomalies in the temperature dependence of the parameters of the average structure were revealed. Ionic conductivity in α-AgCuSe can originate from cation jumps in “skewed” 〈100 〉 directions between nearest-neighbour tetrahedral sites via the peripheries of the octahedral cavities. A correlation between the temperature dependence of the cation redistribution in α-AgCuSe and the temperature dependence of the ionic conductivity is supposed. Various contributions (anharmonic effects, time-average static disorder and phonon-phonon scattering) to the widths of individual phonons upon temperature increase lead to pronounced changes in the neutron-weighted densities of states of β- and α-AgCuSe and accompany the superionic phase transition as well.     

Dielectric properties of superionic lead fluoride

Recent experimental studies show that the dielectric constant of lead fluoride increases rapidly as the superionic transition temperatureT _c is approached. In this work the static dielectric constant (ε) is calculated theoretically for low and high temperature limits. Assuming conduction to take place through a percolation mechanism,ε is found to diverge at the threshold temperatureT _th where ionic conduction starts.     

Heat transport over nonmagnetic lithium chains in LiCuVO<Subscript>4</Subscript>, a new one-dimensional superionic conductor

The thermal conductivity of three single-crystal samples of the quasi-one-dimensional spin system of LiCuVO₄ with different concentrations of defects (primarily, vacancies on the lithium sublattice) was measured along the crystallographic a axis (along the nonmagnetic lithium chains) in the temperature interval 5–300 K. An increase in thermal conductivity from that of the crystal lattice was revealed for T>150–200 K. This increase can be accounted for only by assuming LiCuVO₄ to be a superionic conductor. This assumption was confirmed by measuring its electrical conductivity in the temperature interval 300–500 K. Li⁺ ions move over vacancies on the lithium sublattice (conducting channels) and act as charge carriers in LiCuVO₄. It is shown that LiCuVO₄ is a fairly good superionic conductor with application potential.     

Thermal conductivity of crystalline fullerite C60 in the simple cubic phase

The behavior of the thermal conductivity k(T) of bulk faceted fullerite C₆₀ crystals is investigated at temperatures T=8–220 K. The samples are prepared by the gas-transport method from pure C₆₀, containing less than 0.01% impurities. It is found that as the temperature decreases, the thermal conductivity of the crystal increases, reaches a maximum at T=15–20 K, and drops by a factor of ∼2, proportional to the change in the specific heat, on cooling to 8 K. The effective phonon mean free path λ _p, estimated from the thermal conductivity and known from the published values of the specific heat of fullerite, is comparable to the lattice constant of the crystal λ _p∼d=1.4 nm at temperatures T>200 K and reaches values λ_p∼50d at T<15 K, i.e., the maximum phonon ranges are limited by scattering on defects in the volume of the sample in the simple cubic phase. In the range T=25−75 K the observed temperature dependence k(T) can be described by the expression k(T)∼exp(Θ/bT), characteristic for the behavior of the thermal conductivity of perfect nonconducting crystals at temperatures below the Debye temperature Θ (Θ=80 K in fullerite), where umklapp phonon-phonon scattering processes predominate in the volume of the sample. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 651–656 (25 April 1997)     

The ionic Seebeck effect and heat of cation transfer in Cu2−δSe superionic conductors

The ionic Seebeck coefficients of Cu_2?δSe superionic conductors are measured in the temperature range 340–380°C. The data obtained are used to determine the heat of transfer Q _i of copper ions as a function of the degree of nonstoichiometry and the temperature. The heat of transfer of copper cations increases from 0.19 to 0.22 eV as the degree of nonstoichiometry δ increases from 0.015 to 0.050. It is noted that the heat of transfer Q _i tends to increase with an increase in the temperature. Assumptions regarding the specific features of the cation diffusion in the Cu_2?δSe superionic conductors are made from the observed closeness of the heat of transfer and the activation energy for ionic conduction.     

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)     

Temperature-dependent nepheloxetic shift of the 5 D 0→7 F 0 luminescence line of a Eu3+ ion in a superionic Na5Y(:Eu)Si4O12 single crystal

A decrease in the wavelength of the ⁵ D ₀→⁷ F ₀ luminescence line of a Eu³⁺ ion in a single crystal of the superionic conductor Na₅Y(:Eu)Si₄O₁₂ is observed as a consequence of raising the temperature. The effect is interpreted as the result of a change in the distribution of mobile Na⁺ cations in the second coordination sphere of the luminescing rare-earth ion. Fiz. Tverd. Tela (St. Petersburg) 39, 1997–2000 (November 1997)     

Molecular dynamics simulation of the superionic conductor BaF2

Molecular dynamics simulations of the BaF₂ fluoride crystal were carried out over a wide range of temperatures in order to study structural and transport characteristics in the low-temperature, the high-temperature superionic, and the molten state. The experimental temperature dependence of the lattice constant was taken into account. A sharp change in total energy of the system in the vicinity of T=1200 K indicates a phase transition to the high-temperature state with a transition energy U=(18.8±0.2) kJ/mol which is close to the value of the latent heat Q=18.36 kJ/mol obtained experimentally at 1275 K. Calculation of the radial distribution functions g(r) shows that in the high-temperature superionic state the F^– sublattice is already disordered while the Ba²⁺ sublattice stays regularly ordered, which keeps the crystal in the solid state. In the low-temperature state both sublattices are regularly ordered, and in the molten state both sublattices are disordered. The calculated diffusion constants of F^– in the superionic state is about 10^–9m²/s which is a typical value for superionic conductors. The temperature dependence of the diffusion constant obeys the Arrhenius equation. Higher statistical moments of the trajectories are used to characterise the type of ion movement.     

Particle correlation in superionic conductors

A molecular-dynamics method is applied to study the particle correlation in superionic conductor α-AgI. It is found that there is a close connection between the particle correlation λ_i,j (i,j=I or Ag) and the radial distribution function. The values of λ_II and λ_IAg at the nearest neighbor site are below 0.3. The values of λ_AgAg at the nearest neighbor site and λ_II at the next nearest neighbor site are about 1/2 ≈ 1/3 of it of λ_II (and λ_IAg) at the nearest neighbor site.     

Self-organization of particles in the superionic phase of lanthanum trifluoride

When a laser beam (λ = 396.3 nm) propagates through a nonuniformly heated superionic LaF₃ crystal in the direction of lattice constant c, alternating bright and dark fringes arise on the screen. The fringes run from the warmer face to the colder one. The number and width of the fringes are found to depend on the temperature gradient in the crystal: the smaller (greater) the temperature difference between the faces, the smaller (greater) the number of moving fringes. It is the author’s opinion that this effect is due to the wave transfer of applied heat and reflects the self-organization and collective displacements of the ions of the “quasi-liquid” sublattice in a nonuniform temperature field. A qualitative model of heat wave transfer in the LaF₃ superionic phase is suggested.     

Lithium superionic conductors Li3InBr6 and LiInBr4 studied by 7Li, 115In NMR

Li₃InBr₆ undergoes a phase transition to a superionic phase at 314 K associated with a steep increase of the conductivity (σ = 4 × 10^− 3 Scm^− 1 at 330 K). This superionic phase is isomorphous with Li₃InCl₆ in which a positional disorder at the In³⁺ site is introduced. A pseudo cubic-close-packing of the bromide ions is formed in this phase. On the other hand, a new superionic phase of LiInBr₄ was found above ca 315 K and its structure was confirmed to be a defect spinel. The dynamic properties of the cations in these two superionic phases were investigated by ⁷Li and ¹¹⁵In NMR spectroscopy.