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)     

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.     

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.     

The superionic phase transition of fluorite-type crystals

The diffuse phase transition of fluorite-type superionic conductors will be studied in terms of the Bragg-Williams and the quasichemical approximation. The results are compared with experimental data taken from the literature. Improvements of the above-mentioned approximations, taking care of the long-range interactions, have been suggested. Good agreement between our theoretical results and experiment has been found. In addition, we have studied the effect of doping with trivalent cations. In agreement with experimental observations we find that the superionic phase transition shifts to lower temperatures with increasing concentrations of trivalent ions.     

Exoelectron emission accompanying the phase transformations in the AgI-type superionic conductors

Results of experimental investigations of the photostimulated exoelectron emission (EEE) accompanying the polymorphic phase changes in some superionic conductors (silver, copper and mercury iodides) are reported for the first time. The measurements of the temperature dependence of the intensity of photostimulated EEE were performed in air at atmospheric pressure, the (exo)electrons being detected with an open air point-counter with saturated ethanol quenching vapour. The DTA control measurements were performed at the same heating and/or cooling rate. A comparison of the results of the EEE and DTA measurements shows clearly that the polymorphic phase transitions and melting of investigated materials are accompanied by an enhancement (peak) of the EEE intensity. This enables the detection of the phase transformations in inorganic compounds by the EEE technique.     

Gating processes in Na-channel of biological membrane from the standpoint of superionic phase transition

The method of condensed matter physics is applied to reason out the problem of Na transport through a biological membrane. A similiarity of gating process in Na ion channel to the superionic phase transition is discussed. A possible microscopic mechanism is suggested.     

Behavior of the low-frequency conductivity of silver iodide nanocomposites in the superionic phase transition region

Physics of the Solid State - The behavior of the specific conductivity of composites based on silver iodide embedded in porous glasses with an average pore diameter of 7 ± 1 nm and in...     

Thermal history and phase transitions in the superionic protonic conductors CsHSO4 and CsHSeO4

Single crystals and polycrystalline samples of CsHSO₄ and CsHSeO₄ have been studied by differential scanning calorimetry (DSC) and IR and Raman spectroscopy at various temperatures. The influence of thermal history is evidenced and phase-transition diagrams are proposed. Ionic and protonic conductivity, phase-transition mechanisms, incommensurability and the nature of defects are discussed.     

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.     

Sedimentation in superionic conductors

The homogeneous distribution of mobile atoms in a simple of a nonstoichiometric superionic conductor with mixed (ionic and electronic) conductivity is changed when the sample is placed in a centrifugal field. The steepness of the resulting equilibrium gradient and the rate at which the equilibrium is attained depend upon the reduced mass of atoms and their ambipolar diffusivity.     

Transitions in superionic crystals

A theory of phase transitions in solid electrolytes is developed. Using mean field approximation the expression for the free energy and the equation of state are analysed and shown to be determined by two dimensionless parameters which depend on the properties of the crystal. The parameter plane has eight characteristic regions. In two regions phase transitions are absent. Single transitions occur in four regions. The specificity of unsymmetrical systems is revealed in the remaining two regions in both of which two phase transitions take place. Expressions for the temperatures of the corresponding transitions are presented. It is shown that with the increase of temperature the degree of cation disorder may either decrease or increase. In symmetrical systems double transitions are absent. Comparison of the theory with experimental data is presented.     

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)     

On the thermopower of superionic conductors

We investigate the thermoelectric power of a superionic conductor within the Brownian particle model. The dependence on temperature and friction is calculated numerically for the one-dimensional case. The results are compared to those of other models and approaches.     

Liquid-like defects in superionic fluorites

A model based on the presence of two-level systems identified with defect clusters is used to study different properties of superionic lead fluoride and calcium fluoride. The high degeneracy of the higher level indicates a liquid-like arrangement of anions within the cluster.     

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.     

Thermal conductivity in superionic conductors

The contribution of mobile ions to the thermal conductivity in superionic conductors is investigated by making use of the lattice gas model with a hopping term. The thermal conductivity is obtained as a function of an ion concentration and a repulsive interaction energy between the nearest neighbors. It is shown that the contribution of mobile ions to the thermal conductivity is of an Arrhenius type in the usual temperature region in which measurements are conducted.     

Neutron scattering in superionic conductors

The coherent dynamical structure factor S(q,ω) of one-dimensional superionic conductors is studied in a model of Brownian particles with harmonic interactions moving in a periodic potential. We present results for the q-dependence of the halfwidth of the quasielastic peak, which reflects the commensurability ratios of the periodic potential and the molten sublattice. Furthermore the influence of the periodic potential on the collective excitations of the mobile ions is discussed.     

Collective motion in superionic conductors

A theory of collective motion in superionic conductors is described by the use of a model of a crystalline cage immersed in a viscous liquid. The viscoelastic force and the interionic Coulomb force are considered as the cage—liquid interaction. The density-correlation functions and the frequency-dependent conductivity are calculated. The calculated conductivities for α-AgI are in good agreement with experiments. It is concluded that the structure in a.c. conductivity experimentally observed for α-AgI at frequencies below 10 cm^-1 can be ascribed to acoustic phonons.     

Hydrodynamics of superionic conductors

Using the Mori method we present a rigorous hydrodynamic theory for the collective excitations in superionic conductors at low frequencies and long wavelengths. By means of static and dynamic linear response theory all microscopic quantities of the theory are expressed as derivatives of the free energy or as transport coefficients. Explicit expressions for the dispersion and the damping of the modes and for the density correlation function are worked out for high-symmetry directions and numerically evaluated for-AgI. In particular we show that the two non-propagating modes can give rise to a small central component (width~k ², constant intensity) and a broad central component (constant width fork0, intensity ~k ²) in scattering spectra. Extrapolating our theory to larger wave vectors we offer a new explanation for the broad central component found recently in neutron scattering spectra of-AgI.     

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.