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.     

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.     

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.     

129I-Mössbauer study of diffusion effects in the superionic conductor Ag3SI

The superionic conductor Ag₃SI was studied by¹²⁹I-Mössbauer spectroscopy in the rhombohedral γ-phase and in the cubic β-phase at temperatures between 4.2 K and 180 K. In the low-temperature γ-phase, one observes well below the γ-β phase transition at 157 K a motional-narrowing-like decrease of the electric-quadrupole interaction, roughly linear with temperature, which points to a diffusional motion of the Ag⁺ ions already in the γ-phase. Diffusional effects are also reflected by anomalous decrease of thef-factor in the same temperature region, which originates from a restricted short-range diffusional displacement of the iodide ions induced by the hopping Ag⁺ ions.     

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)     

Phases and phase transitions of the superionic conductor Ag2HgI4 in the temperature range between 4.2 K and 370 K detected by diffuse reflectance spectrometry

Diffuse reflectance spectra of the superionic conductor Ag₂HgI₄ powder were recorded between 4.2 K and 370 K and transformed into the Kubelka-Munk function. Six parameters of the spectral band of this function, related to the absorption band of the particular phase, were considered. The graph representing the logarithm of the integrated Kubelka-Munk spectral band versus inverse temperature was most informative. The Arrhenius behaviour of this function, in a temperature interval, was related to the existence of a phase with constant lattice structure. The non-Arrhenius behaviour was tentatively attributed to a temperature sensitive phase. Thus fifteen different crystalline phases were found, only four of them having been, up to now, unquestionably admitted. In particular, evidence was given for the existence of two superionically conducting phases and two room temperature phases. All the graphs representing the chosen parameters versus inverse temperature pointed to the existence of two separate sequences of phases: one generated by cooling gradually the β-phase, stable at room temperature, to low temperatures (β sequence), the other by heating the β-phase above the σ → β (T)₊ transition point and cooling gradually the formed α'′-phase (α′ sequence). The concept of photon absorption thermal activation energy was introduced. Its largest value was of the order of magnitude of the electrical conductivity thermal activation energy of Ag₂HgI₄. Estimates of energy band gaps and their evolution with temperature were made. It is suggested that the polymorphism of the substance is the main reason for the hysteresis loop in the superionic conductivity transition region. The low temperature phases manifested luminescence caused by exciton decay.     

The effects of thermal activation on the complex conductivity of Ag2HgI4 in the microwave region

The complex conductivity of polycrystalline Ag₂HgI₄, a superionic conductor, has been measured as a function of temperature at 10, 24 and 70 GHz. Both conductivity and permittivity exhibited sharp changes at the β?α phase transition. The microwave conductivity of the β-phase was found to be insensitive to temperature changes and that of the α-phase has thermal activation energies lower than that of dc. The observed monotonic increasing conductivity, decreasing permittivity, together with thermal activation effects are indicative of hopping ionic transport.     

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.     

Low temperature electronic hopping in β-AgI

A frequency dependent conductivity typical of many disordered systems has been observed in the superionic conductor β-AgI at low temperatures. The frequency dependent conductivity is found to be electronic in nature with no evidence for a contribution from ionic hopping. The current results suggest the ionic hopping in superionics is classical in nature with no thermally activated tunnelling.     

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.     

New Developments in the Eight Vertex Model II. Chains of Odd Length

We study the transfer matrix of the 8 vertex model with an odd number of lattice sites N. For systems at the root of unity pointsη=mK/L with m odd the transfer matrix is known to satisfy the famous ‘‘TQ’’ equation where Q(υ) is a specifically known matrix. We demonstrate that the location of the zeroes of this Q(υ) matrix is qualitatively different from the case of evenN and in particular they satisfy a previously unknown equation which is more general than what is often called ‘‘Bethe’s equation.’’ For the case of even m where no Q(υ) matrix is known we demonstrate that there are many states which are not obtained from the formalism of the SOS model but which do satisfy the TQ equation. The ground state for the particular case of η=2K/3 and N odd is investigated in detail.     

Liquid-like Raman scattering by superionic materials

The Raman spectrum of a superionic conductor, α-AgI, is compared with the spectra of the melt of AgC$\text{l}$ and AgBr, in order to determine the responsible Raman processes. We conclude that because of the strong anharmonicity and high translational disorder in superionic conductors, spectra analyses based on the concept of harmonic lattice are unsuitable. A dynamical picture of ions similar to that of liquids appears to be more useful than the phonon concepts.     

RbIO3-HIO3和CsIO3-HIO3两个赝二元系的研究

用X射线衍射及差热分析法研究了两系统的相平衡。确定了RbH₂(IO₃)₃的结构(三斜晶系,点阵常数:a=8.338?,b=8.244?,c=8.254?,α=60.66°,β=85.58°,γ=66.01°,z=2)。密度D_m=4.61g/cm³。CsIO₃-HIO₃系中有两个化合物:CsH(IO₃)₂(为质子导体)和3CsIO₃·17HIO₂。 关键词：     

Electronic structure of NiAl

Using the local approximation for exchange-correlation effects we employ the linearized augmented-plane-wave method (LAPW) to compute self-consistently the band structure, equilibrium lattice constant, and x-ray spectra of the B2-phase of the NiAl intermetallic alloy. The results are compared to other theoretical and experimental data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fiziki, No. 7, pp. 41–45, July, 1987.     

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.     

A scanning tunneling microscopy study on the effect of post-deposition annealing of copper phthalocyanine thin films

Using scanning tunneling microscopy, overlayer lattice structures and thin film morphology of copper phthalocyanine (CuPc) have been investigated while varying posterior, annealing temperatures. The room temperature deposition of CuPc on Ag(1 1 1) resulted in ordered domains, which were three molecular layers high and separated with disordered regions. The ordered surface also exhibited an oblique lattice with the lattice constants of 13.3 Å and 16.4 Å and the angle of 74° between the two. By annealing the thin films systematically to 580 K, the symmetry of overlayer lattice increased from the oblique to the square lattice, commonly observed on various substrates. Further annealing to 780 K desorbed much of the CuPc molecules. However, the remaining molecules reacted and formed dendrite-like chains, in which each CuPc molecule was linked through its isoindole rings. A possible model for the formation of dendritic chains through polymerization is discussed.     

Poissonian Behavior of Ising Spin Systems in an External Field

We apply the Stein–Chen method for Poisson approximation to spin-half Ising-type models in positive external field which satisfy the FKG inequality. In particular, we show that, provided the density of minus spins is low and can be expanded as a convergent power series in the activity (fugacity) variable, the distribution of minus spins is approximately Poisson. The error of the approximation is inversely proportional to the number of lattice sites (we obtain upper and lower bounds on the total variation distance between the exact distribution and its Poisson approximation). We illustrate these results by application to specific models, including the mean-field and nearest neighbor ferromagnetic Ising models.     

Families of commuting transfer matrices and integrable models with disorder

A family of commuting transfer matrices is shown to be associated to each symmetry transformation of a given Yang-Baxter algebra. This applies in lattices models and field theory.The Yang-Baxter algebra remains unchanged when an arbitrary parameter μ_l is associated to each lattice site. We generate in this way integrable one-dimensional hamiltonians with long-range couplings and disorder given by the <{;μ₁<};. These operators are lattice versions of the non-local charges in sigma models. As a simple example we get a Dzialozhinski-Moriya interaction with an arbitrary coupling per site from the six-vertex model. A similar model with a disordered magnetic field follows too. Their exact solution by an algebraic Bethe ansatz is presented. We derive the excitations spectrum in terms of the density of parameters (μ).As another application, the total spin S² is computed for a XXZ Heisenberg chain (μ_l ≡ 0) as a function of the anisotropy Δ (− ∞ < Δ < + ∞).     

Ferroelectric properties of the LaF<Subscript>3</Subscript> superionic conductor nanocluster

We report on the results of quantum-chemical calculation of the lattice energy in a LaF₃ superionic crystal of size 3.5 × 2.0 × 2.2 nm that contains 1200 ions with different structural configurations of fluorine ions. It has been shown that the most energetically disadvantageous configurations of fluorine ions correspond to arbitrarily (randomly) disordered nanolattices in the case when fluorine ions of all three types (F₁, F₂, and F₃) participate in their melting. It has been found that unidirectionally disordered nanolattices that contain a large number of defect dipoles of the anion vacancy–interstitial fluorine atom type with parallel dipole moments is energetically more advantageous than nanolattices with randomly disordered structure. It has been proposed that the electric field produced by a large number of parallel defect dipoles is formed in disordered LaF₃ nanolattices even at room temperatures. This makes it possible to classify microscopically small LaF₃ crystallites as promising functional materials that can be used, e.g., in modern solid-state technologies.     

Phonon density of states in γ-, β-, and α-AgCuS

The ternary superionic conductor AgCuS was studied by means of inelastic neutron scattering at temperatures of 150 to 398 K. The experimental time-of-flight spectra were recalculated into the generalized phonon density of states G(ɛ) in an incoherent approximation. It is shown that G(ɛ) of AgCuS has a nontrivial temperature dependence over the low-energy range. The relation between the temperature dependence of G(ɛ) and the existence of a low-energy mode in AgCuS is discussed. The text was submitted by the authors in English.