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.     

Infrared absorption studies on the superionic conductor ZrO2−Y2O3 crystal

Infrared absorption spectra of (1−x)ZrO₂−xYO_1.5, which is one of the superionic conductors, have been studied using thin films evaporated on Si plates. From absorption spectra obtained at normal incidence and at oblique incidence it is shown that ZrO₂−Y₂O₃ crystals have well-defined TO and LO infrared active phonons, though they have many defects. From the analysis of the spectra, x-dependences of the force constant and the effective charge are obtained, and it is found that the concept of the virtual ion crystal model is very useful to understand the infrared properties of the superionic conductor ZrO₂−Y₂O₃ crystal.     

Ion conductivity,structural features,and multifractal properties of grain boundaries in CuCr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>V<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>2</Subscript>

The ion conductivity, crystal structure, and multifractal parameters of the sections of grain boundaries in CuCr_1?x V _x S₂ superionic conductors with 0 ≤ x ≤ 0.3 have been investigated. It is established that an increase in the surface area of grain boundaries and complication of their shape in such compounds facilitate ion transport. The effect of crystal structure peculiarities on the grain structure of these compounds has been revealed.     

Nanoionics of advanced superionic conductors

New scientific direction — nanoionics of advanced superionic conductors (ASICs) was proposed. Nanosystems of solid state ionics were divided onto two classes differing by an opposite influence of crystal structure defects on the ionic conductivity σ_i (energy activationE): I) nanosystems on the base compounds with initial small σ_i (large values ofE); and II) nanosystems of ASICs (nano-ASICs) withE ≈0.1 eV. The fundamental challenge of nanoionics as the conservation of fast ion transport (FIT) in nano-ASICs on the level of bulk crystal was first recognized and for the providing of FIT in nano-ASICs the conception of structure-ordered (coherent) ASIC//indifferent electrode (IE) heteroboundaries was proposed. Nano-ASIC characteristic parameterP=d/λ _Q (d is the thickness of ASIC layer with the defect crystal structure at the heteroboundary, and λ _Q is the screening length of charge for mobile ions of the bulk of ASIC) was introduced. The criterion for a conservation of FIT in nano-ASIC isP≈1. It was shown that at the equilibrium conditions the contact potentialsV at the ASIC//IE coherent heterojunctions in nano-ASICs areV«k _BT/e. Interface engineering approach “from advanced materials to advanced devices” was proposed as fundamentals for the development of applied nanoionics. The possibility for creation on the base of ASIC//IE coherent heterojunctions of the efficient energy and power devices (sensors and supercapacitors with specific capacity ≈10^?4 F/cm² and maximal frequencies 10⁹–10⁰ Hz,) suited for micro(nano)electronics, microsystem technology and 5 Gbit DRAM was pointed out.     

Polaron transport in quasi-1d organic conductors and in some narrow band systems

A detailed review of large and small polaron transport theory in quasi-1d organic conductors and conducting polymers, and in some narrow band systems including superlattices, superionic conductors and superconductors with heavy fermions is presented. The strong-coupling large polaron mobility is evaluated in 1d and 3d crystals taking account of optical phonon dispersion. The current-voltage characteristics (CVC) for a a 1d acoustic polaron with saturation of the drift velocity V₀(E) in strong fields E→∞ near the sound velocity S is found. This effect has been recently observed by Donovan and Wilson in polydiacetylene PDA TS [8]. The small polaron (SP) spectrum in narrow band conductors is investigated and its strong anisotropy in quasi-1d organic conductors is proved. The CVC for SP is calculated and the characteristic maximum with the negative differential conductivity is found. An exact theory of the Gunn effect in these 1d systems is developed and the explicit analytic expressions for a domain structure are obtained. The domain stability is studied and the possibility of their experimental observing is discussed.     

Specific heat and thermal conductivity of superionic conductors in the superionic phase

The temperature dependences of the specific heat and the thermal conductivity of crystalline superionic conductors LnF₃ (Ln = La, Ce, Pr), Li₂B₄O₇ and α-LiIO₃ in the superionic phase have been investigated experimentally. The specific heat C _p and the thermal conductivity K are observed to increase monotonically over a wide range of temperatures above the Debye temperature Θ_D. This increase is attributed to the relaxational interaction of high-frequency phonons with two-level systems. Fiz. Tverd. Tela (St. Petersburg) 39, 1548–1553 (September 1997)     

Investigation of superionic phase transition in the CuCr1−x VxS2 system by x-ray diffraction and magnetic methods

The unit cell parameters and the distribution of copper atoms over different crystallographic sites in the CuCr_1?x V_xS₂ superionic conductors have been determined by x-ray diffraction. It is demonstrated that a jumpwise change in the copper occupancy of different crystallographic sites for the compositions with x=0, 0.05, 0.1, and 0.15 is associated with the transition to the superionic phase. The two-dimensional character of the ion transfer in the studied compounds is confirmed. No considerable anomalies in the temperature dependences of the paramagnetic susceptibility are revealed at temperatures of the superionic transition.     

Breakdown of absolute rate theory and prefactor anomalies in superionic conductors

We suggest that the anomalously low prefactors observed for the ionic hopping rate in defect-structure superionic conductors are due to a breakdown of absolute rate theory. A treatment that takes dissipative processes into account is introduced and a new expression for the prefactor is obtained. We present NMR data on the superionic conductor Li₂Ti₃O₇ that supports these ideas and extract the value of the elemental energy transfer per collision.     

The role of disorder in superionic conductors

This paper illustrates from a phenomenological point of view why the study of superionic conductors is essentially a study of disorders. Crystals that are good ionic conductors lack a long-range order in their mobile-ion sublattice. Moreover, low-temperature anomalies typical of amorphous materials appear to be rather common in superionic crystals. In several solid electrolytes the coupling between “disorder modes” typical of glasses and translational degrees of freedom of the ions can be shown to enhance ionic diffusion. The observed, or expected, properties of these superionic conductors are briefly discussed. The hypothesis that disorder may often play a dynamic role in ion transport in solids suggests ways to synthesize materials of technological interest.     

On the Urbach rule in sulphur-implanted Cu6PS5I superionic conductors

Cu₆PS₅I superionic crystals, grown using chemical vapour transport, were implanted by sulphur ions. The ion implantation effect on the phase transitions is studied by temperature isoabsorption investigation of the optical absorption edge. For the implanted crystals the optical absorption edge shape is studied in the temperature range 77-320 K, the parameters of exciton-phonon interaction, resulting in the Urbach behaviour of the optical absorption edge, are determined, the temperature dependences of the optical pseudogap and Urbach energy are obtained. The implantation effect on the ordering-disordering processes in Cu₆PS₅I superionic conductors is studied.     

Activation energy of ion motion in the nanodimensional lattice of LaF<Subscript>3</Subscript> superionic conductor

Quantum chemistry calculations of the intracrystalline potential relief in the nanolattice of LaF₃ superionic crystal that contains 1200 ions and measures 3.5 × 2.0 × 2.2 nm along the x, y, and z axis, respectively, have been performed. Using the MOPAC 2012 program package, the potential relief profile has been simulated in the central part of the nanolattice for an elementary act of disordering in the lowest melting sublattice of F₁ ions. It has been found that the height E _m of barriers that prevent the motion of F₁ in the dielectric phase of LaF₃ crystal equals 0.37 eV and decreases to 0.15 eV in the superionic state. In addition, activation energy E _a of F₁ sublattice disordering in the dielectric and superionic states is equal to 0.16 and 0.04 eV, respectively. The profiles of the potential relief calculated on the xy and xz faces of the LaF₃ 3D nanolattice for the case when an F₁ ion moves along the x crystal axis in the dielectric state are presented. The corresponding energy barriers are 1.5–2.0 times lower than those at the center of the LaF₃ nanlattice.     

Effect of optical phonons scattering on electronic current in metallic carbon nanotubes

The effect of optical phonons scattering on electronic current has been studied in metallic carbon nanotubes. The current has been calculated self-consistently by total voltage equation and the heat transport equation. The total voltage equation consists of three terms, optical phonons collision term, acoustic phonon scattering term, and contact resistance one. Including LO, A₁, and E₁(2) phonons in collision term, we can reproduce the experimental I-V curves displaying negative differential conductance. Furthermore, one conclusion is made that the more optical phonons are scattered by electron, the lower current is in metallic carbon nanotubes. By comparing the current under different conditions, we can make another conclusion that there should be nonequilibrium optical phonons under high bias in spite of whether the metallic nanotube is suspended or not. This result agrees well with the others [M. Lazzeri, F. Mauri, Phys. Rev. B 73 (2006) 165419]. Based on these results, we do not only explain the experiment, but also propose to design a heat-controlling electronic transistor with metallic carbon nanotubes as its channel, in which the electronic current can be controlled by optical phonons.     

Bromide ion conductivities of eutectic mixtures of quaternary ammonium bromides possessing long alkyl chains

Several quaternary ammonium bromides possessing long alkyl chains and their mixtures were found to be bromide ion conductors. The ionic conductivities of quaternary ammonium bromides themselves were lower than 10^?9 S cm^?1 at a room temperature. On the other hand, the eutectic mixtures of the quaternary ammonium bromides showed large increase of ionic conductivity. The best bromide ion conductors were found for the eutectic of Q₅, Q₇, Q₈, and Q₁₂: 4×10^?8 S cm^?1 at 30°C, and 6.3×10^?6 S cm^?1 at 50°C. Addition of asymmetric quarternary ammonium bromides had a negative effect on the ionic conductivity. These results were explained by a space filling factor in the solid.     

Correlation between the quasi-elastic light scattering and the ionic-conductivity in superionic conductors

The quasi-elastic light scattering of the superionic conductors, (ZrO₂)_1?x (YO_1.5)_x (x = 0, 0.18, 0.33) has been studied in the region of frequency shift below 2.5 cm^-1. The quasi-elastic scattering far more intense than the Brillouin scattering has been found for a sample with x = 0.18. From the x-dependence of the intensity, it is shown that the ionic diffusion is responsible for the quasi-elastic scattering. The temperature variations of the intensity and the spectral shapes are interpreted in accordance with the macroscopic conductivity data, in terms of a model in which the barrier height distribution is taken into account.     

Empirical relationships for ion conduction based on vibration amplitude in perovskite-type proton and superionic conductors

For superionic and deformed perovskite-type H⁺-ion conductors, empirical relationships among atomic mass of host or substituted ion, H⁺-ion conductivity, σ_PR, activation energy, transition temperature T_c, etc. are proposed. We elucidate the roles of heavy host ion and specific crystal structures below and above the T_c on the H⁺-ion conduction by noticing a large amplitude of O-ion vibration mode arising from a large fourth order anharmonicity. We clarify the important role of strengthened ionic force for the host cation lattice at and above T_c on the H⁺-ion jumping, and interpret reasonably the σ_PR value depending on the concentration of doping ion, a large broadening of vibration band and an enhanced amplitude of OH-vibration mode, etc. We suggest the extension of this consideration to stabilized zirconium and superproton conductors, etc.     

Defect clusterization and transport properties of oxide and fluoride ionic conductors with fluorite structure: Quantum-chemical approach

Systematic studies of the effect of point defects on the electronic structure, chemical bond, and ionic conduction of classical ionic conductors MeF₂ (Me=Ca, Sr, Ba, Pb), ZrO₂-CaO, ZrO₂-Y₂O₃, and δ-Bi₂O₃ are carried out using the nonempirical LMTO method in the tight-binding approximation and using Hückel’s semiempirical method. The energies of formation and interaction of anti-Frenkel defects are calculated. The effect of defect clusterization in these compounds is detected. This effect is responsible for the emergence of the superionic state and anomalous physicochemical properties of the given solid electrolytes. The nature of instability of the fluorite structure is studied, and methods of its stabilization are proposed. The mechanisms and energetics of ionic transport in oxide and fluoride conductors with fluorite structure are investigated.     

Anomalous X-ray scattering studies on superionic glassy (As2Se3)-(AgX) systems (X = halides)

Anomalous X-ray scattering experiments for glassy room-temperature superionic conductors (As₂Se₃)_0.4 (AgI)_0.6 and (As₂Se₃)_0.4(AgBr)_0.6 were performed close to the As, Se, Ag, and Br K edges using a third-generation synchrotron radiation facility, ESRF. The differential structure factors, Δ_iS(Q), were obtained from detailed analyses, indicating that Δ_AsS(Q) and Δ_SeS(Q) of both the glassy superionic semiconductors are similar to those of glassy As₂ Se₃ except the prepeak in Δ_SeS(Q). The Δ_AgS(Q) spectrum of (As₂Se₃)_0.4 (AgI)_0.6 looks molten salt-like. However, the Δ_Ag S(Q) of (As₂Se₃)_0.4(AgBr)_0.6 glass have quite different features from that of (As₂Se₃)_0.4 (AgI)_0.6 glass in the low Q range, and the Δ_BrS(Q) has even a pre-shoulder around 13 nm^? 1 unlike molten salts. In the differential pair distribution functions Δ_ig(r) obtained from the Fourier transforms of Δ_iS(Q), the first peaks of Δ_Asg(r) and Δ_Seg(r) show no correlation with those of Δ_Agg(r) and Δ_Brg(r), and vice versa. From these results, it can be concluded that a pseudo-binary mixture of the As₂Se₃ network matrix and AgX-related ionic conduction pathways is a good structural model for these superionic glasses. Differences between the AgBr- and AgI mixtures were found in the second-neighbor structures around the Ag atoms, which may reflect those in the crystal structures of the AgX salts.     

Transport characteristics of phonons and the specific heat of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>:ZrO<Subscript>2</Subscript> solid solution single crystals

The temperature dependences of the specific heat and transport characteristics of phonons in single crystals of yttrium-stabilized zirconium dioxide Y₂O₃:ZrO₂ solid solutions have been studied. It has been shown that the temperature dependences of the specific heat at T > 5 K are almost identical at the degree of stabilization of a solid solution with an Y₂O₃ content of 5–20 mol %. Differences in the temperature dependences of the specific heat of samples from different sources at T < 5 K are due to the presence of low-energy two-level systems. The features of the transport characteristics of thermal phonons at liquid helium temperatures reflect not only the presence of two-level systems but also the scattering of phonons on low-dimensional domains of another phase coherently conjugate to the main phase of the Y₂O₃:ZrO₂ solid solution.     

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.