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.     

Temperature dependences of phonon and mobile-ion spectra in superionic conductor CuBr

The temperature dependence of far-infrared reflectivity spectra in sintered CuBr has been measured from 18 to 570 K. Phonon and mobile-ion parameters are evaluated by the fitting procedure with the factored form of the dispersion relation. A strong fourth order anharmonicity is assigned. Estimated values of optical dielectric constant and optical ionic mobility μ_op decrease with increasing temperature within a limited temperature region. For μ_op value, the effects of mobile ion scattering by optical phonons and of the expansion of interionic distance are discussed with respect to the phonon damping constant and the ionic character of the interionic force.     

Structure-property relationships of fast copper ion conductor cubic CuI

Structure-property relationships of fast copper ion conductor cubic γ-CuI (300 K) and α-CuI (773 K) has been investigated using the time-of-flight (TOF) neutro powder diffraction over the Q-range up to 30 Å^? 1. Crystal structure of the both γ- and α-phases were refined using the conventional Rietveld methods combined with the maximum-entropy-method (MEM), whilst the local structures were analyzed by means of the pair distribution function (PDF) combined with the reverse Monte Carlo (RMC) simulation. The crystal structure analysis revealed temperature dependence of large thermal displacement of copper ions along <111> directions, with average copper ions density mostly distributed at the 8c center of tetrahedrally-coordinated by I ions. The PDF analysis estimated the new peak position of Cu–Cu pairs shifted from 4.27 Å in the γ-phase to 2.49 Å in the α-phase and estimation of coordination number decreases from 12.1 to 5.8, respectively. The local structure analysis found the new split peaks of g_Cu–Cu(r) at r about 2.47 Å and 2.72 Å in the α-phase that roughly correspond to the Cu–Cu diagonally distance between nearest neighboring tetrahedral corner 32f-32f sites and between tetrahedral center 8c-32f sites such as represented by the splitting Cu site model of Fm-3m.     

BaSnF4 fast ion conductor: Variations versus the method of preparation and anomalous temperature variation of the quadrupole splitting

A new method of preparation of high performance fluoride ion conductor, BaSnF₄, by water leaching of newly discovered barium tin(II) chloride fluorides, has been designed, and the materials have been studied and compared to the solid prepared by the usual dry method. The unit-cell parameters and crystallite dimensions were found to vary with the method of preparation. In addition, the crystallite dimensions were found to be highly anisotropic for the samples obtained by the wet method. The Mössbauer spectrum is made of a large tin(II) quadrupole doublet, and a broad tin(IV) oxide peak due to surface oxidation. The tin(II) spectrum is in agreement with covalently bonded tin(II) having a strongly stereoactive lone pair. An unusually high dependence of the quadrupole splitting at low temperatures was observed (5.8 times larger than for α-SnF₂).     

Temperature and frequency dependences of acoustic phonon lifetime in benzyl alcohol

The absorption coefficient and velocity of hypersound as functions of temperature were investigated by the spectra of molecular scattering of light in benzyl alcohol. Comparison with ultrasonic data made it possible to find the temperature and frequency dependences of acoustic phonon lifetime, which is important for interpretation of SMBS time dynamics.     

A model for the phonon frequency distribution function for crystalline polyethylene

A model for the phonon frequency distribution function of crystalline polyethylene has been developed considering the mass of the vibrating unit to be two methylene groups and distinguishing between the ‘hard’ longitudinal and transverse dispersive modes. It explains the temperature variation of specific heat of polyethylene and can be used for other studies.     

Variations of BaSnF4 fast ion conductor with the method of preparation and temperature

Ionic conductors are solids that have a large number of defects and easy pathways that make it possible for ions to move over long distances in an electric field. In order to be mobile an ion must be small and have a low charge. The fluoride ion is the most mobile anion. The highest performance fluoride ion conductors contain divalent tin, and have a highly layered crystal structure related to the CaF₂ fluorite type. BaSnF₄ has the α-PbSnF₄ structure, which is a √2/2?×?√2/2?×?2 superstructure of the fluorite type, where the tetragonal unit-cell and the value of the a and b parameters being equal to half the diagonals of the (a,b) face of fluorite are due to the loss of the F Bravais lattice, and the Sn Sn Ba Ba order along the c parameter is at the origin of the doubling of the c parameter. The BaSnF₄ material was prepared first by Dénès et al. (C. R. Acad. Paris C, 280: 831, 1975), and its superionic properties were characterized by Dénès et al. (Solid State Ion., 13: 213, 1984). It was found to have a conductivity three orders of magnitude higher than that of BaF₂, with an ionic conduction rate τ _i?>?0.99. No BaSnF₄ was obtained by the aqueous medium, when aqueous solutions of SnF₂ and Ba(NO₃)₂ are mixed together; BaSn₂F₆ was obtained instead. In a new development of this work, BaSnF₄ has been obtained by the wet method for the first time. X-ray powder diffraction showed that the BaSnF₄ phase obtained by the wet method varies substantially from one sample to another: (a) signification variations of the c parameter of the tetragonal unit-cell reveals that the interlayer distance is sensitive to the leaching conditions, possibly because some of the leached ions remain in the interlayer spacing; (b) large variations of the crystallite dimensions and, as a result of the two-dimensionality of the structure, a strong crystallite dimension anisotropy are observed, with d∥?d⊥, where d∥ and d⊥ are the crystallite dimensions parallel to the four-fold main axis, and perpendicular to it, respectively, showing that the layers are very thin and the interlayer interactions are very weak. Variable temperature Mössbauer spectroscopy showed an unusual large variation of the quadrupole splitting with temperature. A tentative explanation based on unusually large bond angles has been proposed.     

Composition control and conductivity in Na/Zn-compensated β″-Na-gallate fast ion conductor

By controlled substitution of Zn²⁺ for Ga³⁺ and Na⁺ on the cation sites in β″-Na-gallate, the negative charge on the spinel block, and, hence, the number of charge compensating mobile Na⁺ ions in the conduction plane, can be changed. The mobile Na⁺ concentration can be varied from 1.5 to 1.67 ions per conduction plane. The ionic conductivity is largest at these two terminal compositions, and falls to much lower values between. A super-cell domain model is proposed to explain the behavior.     

声子频率对同位素掺杂硅声子散射的影响

点缺陷对声子的散射是影响电绝缘体热导率的重要机制之一,其中声子频率是影响声子散射的重要因素．本文主要研究声子频率对同位素掺杂硅声子散射的影响。首先产生一个窄频率范围的声子波包,然后使用分子动力学(MD)模拟声子在同位素掺杂硅中的散射过程,在原子尺度下清晰展示了声子对同位素掺杂的散射过程,并对能量的透射率和反射率进行分析。将模拟结果和已发表的理论结果相比较,在单个同位素掺杂缺陷下,在临近共振频率区域内用改进的R.O.Pohl公式成功的拟合了MD结果,这一结果会对在较宽频率包括非色散和色散声子范围内构造声子热传导公式有帮助.对于在较高的掺杂浓度下,声子频率对声子散射特性的影响还需要更进一步的研究。     

Physical properties of high performance fluoride ion conductor BaSnF4 thin films by pulsed laser deposition

This article presents the results on the growth and characterization of BaSnF₄ thin films on glass substrates prepared by pulsed laser deposition technique. The structural results of BaSnF₄ thin film carried out by glancing angle X-ray diffraction technique indicates the formation of the film with similar structure (tetragonal, P₄/nmm) to the bulk target material. The absorption coefficient and band gap of the film is determined by suitable analysis of the transmittance spectra. The transport properties of the thin films are studied using impedance spectroscopy in the temperature range of 323–573 K. The frequency-dependent imaginary part of impedance plot shows that the conductivity relaxation is non-Debye in nature. The scaling behavior of the imaginary part of impedance at various frequencies indicates temperature-independent relaxation behavior.     

The effects of incommensurability on the phonon modes in the one-dimensional superionic conductor K-hollandite

Polarized Raman and infrared spectra of the one dimensional (1-D) superionic conductor (K_2xMg_xTi(_8?x)O₁₆; x = 0.77) have been measured over the phonon frequency region 10–1000 cm^?1 as a function of temperature and pressure. The majority of the IR and Raman active modes predicted by group theory for the (Ti, Mg)O₆ framework were observed. The frequencies and their IR and Raman scattering cross-sections for the incommensurate lattice of K⁺ ions were calculated using a one dimensional linear chain model. This model assumes Coulomb interaction between nearest neighbors that are located in a sinusoidal potential due to the framework lattice. Several broad Raman bands were attributed to amplitudon type modes from the one dimensional incommensurate K⁺ ion sublattice. The IR active phason modes could not be identified unambiguously due to the underlying TiO framework vibrations which are known to possess large anharmonicity and oscillator strenghts.     

Evolution of the ion distribution function during HF heating at twice the ion cyclotron frequency

We derive the equation governing the ion distribution function when a toroidal plasma is heated by a radiofrequency wave at $\text{ω = 2Ω}{}_{\mathrm{<mtext>ci</mtext>}}$. A numerical integration of this equation shows that a suprathermal anisotrpic tali develops in the distribution function, as experimentally observed on the ST Tokamak.     

Temperature dependent phonon dynamics in the quasi-one-dimensional systems

The dynamical properties of phonons coupled linearly to the electrons with a planar Fermi surface, are examined at finite temperature. The additional phonon branch, found previously from the T = 0 calculations, should not be observable in the Nb₃Sn-like compounds, which undergo the cubic-to-tetragonal transition. This additional branch exists however in the weak coupling limit, where, unexpectedly, the soft mode becomes well defined only close to the transition temperature for the Peierls instability     

Doping effect on crystal structure and conduction property of fast oxide ion conductor LaGaO3-based perovskite

In order to discuss oxide ion conduction mechanism for LaGaO₃-based perovskite compounds, doping effects were investigated using two kinds of solid solutions whose oxygen vacancy concentrations are the same: one is La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 with A-site and B-site substitutions and the other is LaGa_0.8Mg_0.2O_2.9 with only B-site substitution. Conductivity measurements showed that La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 had a circumstance whereby oxide ion could more easily diffuse in the perovskite structure than in LaGa_0.8Mg_0.2O_2.9. Structural analyses using neutron diffraction found out the following three differences: the first finding was that the saddle point formed by two A-site cations and one B-site cation in La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 was larger than that in LaGa_0.8Mg_0.2O_2.9 due to larger displacements of A-site and B-site cations; the second was that the doubly doping with Sr and Mg was more effective for reduction of GaO₆ octahedral tilt angles than the doping with Mg; the last was that La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9 had larger oxygen displacement than LaGa_0.8Mg_0.2O_2.9. It was considered that these structurally related parameters dominated the high oxide ion conduction in LaGaO₃-based perovskite compounds.     

Origin of the acoustic phonon frequency shifts in semiconducting nanoparticles

The surface and size effects on the acoustic phonon properties of semiconducting nanoparticles, such as ZnO, are studied using the s-d model and a Green's function technique. We have shown that the electron-phonon and anharmonic phonon-phonon interactions play an important role in ZnO nanoparticles and must be taken into account in order to explain the experimental data. Due to surface and size effects on the electron-phonon constants, the acoustic phonon frequency and their damping increase with decreasing of particle size.     

Temperature dependent phonon confinement in silicon nanostructures

Temperature dependent variation in Raman line-shape from silicon (Si) nanostructures (NSs) is studied here. Asymmetry and red-shift in room temperature Raman spectrum is attributed to phonon confinement effect. Raman spectra recorded at higher temperatures show increase in FWHM and decrease in asymmetry ratio with respect to its room temperature counterpart. Theoretical Raman line-shape analyses of temperature dependence of phonon confinement is done by incorporating the temperature dependence of phonon dispersion relation. Experimental and theoretical temperature dependent Raman spectra are in good agreement.     

The frequency dependence of phonon interactions at Terahertz-frequencies studied by time-resolved phonon spectroscopy

Phonon properties concerning propagation, frequency spectrum and lifetime in the Terahertz range have been investigated. The phonons have been produced by square wave Joule heating of thin constantan films evaporated onto a strongly doped ruby. Cross sections for phonon scattering at the Cr³⁺ impurity ions of the ruby were determined as a function of frequency. The theoretical estimate for mass defect scattering given by Klemens [9] is found to agree reasonably with the experimental results. Lifetime measurements indicate that phonons in ruby preferentially propagate in the transversal mode. Phonon frequency spectra in the constantan films were measured under experimental conditions, where the strength of electron phonon coupling is significant; thus electron-phonon relaxation times could be obtained as a function of frequency.     

Effect of the superconducting transition on phonon energies and lifetimes in lead

We have performed a high-resolution inelastic neutron scattering study of certain transverse phonons in lead. We find that neither the energies nor the linewidths change appreciably with the onset of superconductivity. This finding contradicts a recent experimental report, but is in accord with theoretical expectations.     

Temperature dependence of phonon linewidth in β-AgI

The temperature dependence of the linewidth and frequency of a TO one-phonon mode of β-AgI is measured using far infrared and Raman spectroscopy. Near the β-α transition an anomalous damping is observed. A very simple model is proposed to explain this damping by interactions between phonons and ionic carriers.