Preparation,electric conductivity and dielectrical properties of Cu6PS5I-based superionic composites

Composite samples based on microcrystalline Cu₆РS₅I superionic conductors, embedded in matrices of polyvinylacetate, epoxy and conducting glue, are produced. Temperature and frequency studies of complex electric conductivity and dielectric permittivity as well as dielectric loss of the obtained composites are carried out. The effect of differences in the composite production technology on the temperature of a second-order phase transition in the superionic phase, values of electrical conductivity, activation energy and dielectric permittivity is shown.     

Thermal conductivity of opal filled with a LiIO<Subscript>3</Subscript> ionic conductor

The temperature dependences of the thermal conductivity of a synthetic opal and opal-based nano-composites prepared by introducing a LiIO₃ superionic conductor into pores of the opal matrix from an aqueous solution or melt are measured by the hot-wire technique in the temperature range 290–420 K. It is demonstrated that the thermal conductivity of pure opal increases with an increase in the diameter of the SiO₂ spheres forming a face-centered cubic lattice of an opal and is determined by the total thermal resistance of interfaces between the spheres. Filling of opal pores with the ionic conductor leads to an increase in the thermal conductivity. The behavior of the thermal conductivity and its magnitude in opal-based nanocomposites depend to a large extent on the method of filling the matrix pores.     

Atomistic simulation of a superionic transition in UO2

The results of the atomistic simulation of a superionic transition and melting of uranium dioxide are presented. The temperature dependences of the concentration of defects in the oxygen sublattice and changes in the heat capacity and isothermal compressibility upon the superionic transition are calculated. It is shown that the curve of the superionic transition in the PT diagram can be described by the Ehrenfest’s equation. The possibility of describing the superionic transition within the framework of the theory of second- order phase transitions is discussed. Based on the results obtained, it is considered that this structural transformation can occur in other materials.     

X-ray study of the superionic phase transition in CuCrS2

The temperature disorder of copper ions in the CuCrS₂ superionic conductor was studied by X-ray analysis. It is shown that the average square deviations of atoms sharply increase and become comparable with the distance between neighbouring tetrahedral sites when CuCrS₂ transforms into the superionic phase. It is also established that the direction of maximum deviation of atoms is close to the direction connecting the tetrahedral positions.     

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)     

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.     

Entropy of crystallization of materials from a “molten” sublattice of superionic conductors

It is shown that the entropy of crystallization of materials (such as Cu, Ag, Au, and Li) in a “molten” sublattice of superionic conductors→crystal system may substantially exceed the entropy of crystallization of the same materials from a melt. The observed behavior explains the known tendency of various superionic conductors to form filamentary crystals (whiskers) in the solid phase, together with the completely different structure of these whiskers. Fiz. Tverd. Tela (St. Petersburg) 40, 227–228 (February 1998)     

Collective excitations in fast ion conductor superlattices

Within the framework of the electromagnetic theory, the collective modes in the superlattice system composed of superionic conductors and ionic crystals are studied. The superionic conductor is described by the hydrodynamical model in which the anion cage is immersed in the cation liquid. The behavior of the modes are analysed in terms of the coupling strength between excitations pertaining to different layers. The coupling strength is controlled by varying the slab thicknesses. An interesting behavior in which the diffusion mode transforms to the relaxation mode when the coupling strength is varied from strong to weak is obtained. Also, the effect of the coupling strength on the acoustical and optical modes are shown.     

Bonding in the superionic phase of water

The predicted superionic phase of water is investigated via ab initio molecular dynamics at densities of 2.0--3.0 g/cc (34-115 GPa) along the 2000 K isotherm. We find that extremely rapid (superionic) diffusion of protons occurs in a fluid phase at pressures between 34 and 58 GPa. A transition to a stable body-centered cubic O lattice with superionic proton conductivity is observed between 70 and 75 GPa, a much higher pressure than suggested in prior work. We find that all molecular species at pressures greater than 75 GPa are too short lived to be classified as bound states. Up to 95 GPa, we find a solid superionic phase characterized by covalent O-H bonding. Above 95 GPa, a transient network phase is found characterized by symmetric O-H hydrogen bonding with nearly 50% covalent character. In addition, we describe a metastable superionic phase with quenched O disorder.     

New approach to treating the energetics of activation processes in the lattice of superionic conductors with intrinsic structural disorder

Using LaF₃-type superionic trifluorides as an example, it is shown that the intrinsic thermal structural disorder not affecting the symmetry of the lattice is embodied in light inelastic scattering spectra. Tools for analyzing the Arrhenius-like temperature dependence of the activation energy of disordering in the superionic conductor lattice are discussed. It is found that activation energy ΔEa found from the temperature dependence of the Raman line width is many times lower than energy Ea responsible for LaF₃ lattice disordering.     

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)     

Stabilization of LiBH4 superionic phase by halide doping and H disordering

We performed first-principles density-functional theory calculations to investigate the structural properties and the effect of halide ion doping to the superionic-conducting, high-temperature phase and the effect of halide doping on the phase of LiBH₄. It is computationally demonstrated that the superionic phase is stabilized owing to the halide doping with the large ions which fill the interlayer space of the superionic phase. The H-disordered phase is observed in the structure and is found to contribute to the stabilization of the superionic phase.     

Giant dielectric relaxation in TlGaTe<Subscript>2</Subscript> crystals

The conductivity and temperature-frequency dependences of the permittivity of TlGaTe₂ crystals have been studied. Strong dielectric relaxation has been revealed. It has been shown that the mechanism of dielectric relaxation is associated with hopping of Tl ions over vacancies in the thallium sublattice due to the transition of the system to the superionic state.     

Acoustic studies of phase transitions in crystals and nanocomposites

A brief review of acoustic studies of phase transitions in crystals (ferroelectric, ferroelastic, and superionic ones) and nanostructured composite materials made on the basis of porous matrices is presented. The studies were carried out at the Laboratory of Quantum Acoustics and Ultrasonic Spectroscopy of the St. Petersburg State University.     

Effect of low dimensionality on prefactor anomalies in superionic conductors

For a one-dimensional superionic conductor the jump rate inferred from NMR line narrowing is shown to have a prefactor orders of magnitude smaller than given by optical phonon frequencies. A reduced activation energy is also predicted.     

Coupled motions of ions and electrons in some superionic conductors: An ab initio molecular dynamics study

The dynamic and electronic properties of mobile ions in the superionic conductors CuI and Ag₂Se are studied by means of ab initio molecular dynamics simulations. The superionic behavior of these materials is successfully reproduced by our simulations. To investigate the bonding nature of these materials, the spatial distributions of the electrons and the time evolutions of the gross charge of the ions are calculated by the population analysis method. The time evolution of the electronic states around the mobile ions indicates that the local chemical bonding properties change accompanying the diffusive motion of mobile ions. It is also shown that the 4p-electron populations of Cu in CuI and 5p-electron populations of Ag in Ag₂Se change largely with time, while the s- and d-electron populations depend weakly on time.     

Dynamical structure, bonding, and thermodynamics of the superionic sublattice in alpha-AgI

We characterize the superionic phase transition and the lattice and electronic structures of the archetypal type-I superionic conductor alpha-AgI using extensive first-principles molecular dynamics calculations. We find that superionicity is signaled by a phase transition of the silver ions alone. In the superionic phase, the first silver shell surrounding an iodine displays a distinct dynamical structure that would escape a time-averaged characterization, and we capture this structure in a set of ordering rules. The electronic structure demonstrates a unique chemical signature of the weakest-bound silver in the first shell, which in turn is most likely to diffuse. Silver diffusion decreases upon melting, pointing to an unusual entropic contribution to the stability of the superionic phase.     

快离子导体玻璃滞弹性弛豫的红外发散响应

本文依据Ngai(倪嘉陵)的低频激发、弛豫、耗散理论和非晶态快离子导体的特点,提出了非晶态快离子导体的滞弹性弛豫理论。认为快离子导体玻璃中的超声吸收主要来源于与玻璃网络呈微弱联系的快离子的热激活弛豫及伴之而来的低能激发的损耗。理论成功地描述了超声吸收的频率、温度依赖关系,解释了弛豫时间分布理论所不能解释的快离子导体玻璃的实验特征。     

Low-frequency,coulomb-correlated density fluctuations in superionic conductors and their influence on the conductivity

The low-frequency density and current response of diffusing ions is calculated for superionic conductors. In particular, it is shown that the peculiar dispersion of the conductivity of α-AgI at low frequencies is due to the scattering of silver ions by slowly relaxing local density fluctuations caused by the other silver ions.