Structure and dynamics of liquid MgO under high pressure

Microstructure, dynamics, and diffusion mechanism in liquid MgO have been studied by molecular dynamics simulation. Models consisting of 2000 atoms were constructed under a wide range of pressure and at a temperature of 3800 K. The local structure is analyzed through the coordination number distribution and topology statistics of coordination units (basic structural units) MgO _x (x=2, 3, 4, 5, 6, 7). As regards the structural dynamics, the nearest-neighbor atomic exchange among coordination units, spatially heterogeneous dynamics, clustering, and structural stability (lifetime of basic structural units) are investigated in detail. Investigation of structural dynamics allows us to gain insight into various important atomic (molecular) properties and to clarify the diffusion mechanism in liquid MgO under high pressure.     

Chromium and cobalt diffusion in a ZnSe1 ? x S x solid solution

ZnSe_{1 ? x} S _x (1 ?? x ?? 0) crystals are grown from the vapor of binary components in a closed horizontal system. The ZnSe_{1 ? x} S _x crystals doped with chromium and cobalt are prepared by post-growth diffusion. Their absorption spectra are studied depending on the composition of the solid solution and the doping level. The diffusion coefficients of chromium and cobalt at the temperature 1040°C and their dependence on composition are determined.     

Composition dependence of the ionic diffusion coefficients in yttria-stabilized zirconias

The diffusion coefficient of the O²⁻ ions in yttria-stabilized zirconia (1−x)ZrO₂·xYO_1.5 has been investigated as a function of the concentration of Y³⁺ ion over a range x = 0.131–0.261 by using quasielastic light scattering. The diffusion coefficients were deduced from the analysis of the temperature dependence of the scattering intensity at 3 GHz. It is concluded that the diffusion coefficient reaches its maximum value at x = 0.165. The composition dependence of the scattering intensity suggests that the concentration of the moving carrier decreases as the Y³⁺ content increases in spite of an increase of the oxygen vacancies.     

A model for the temperature dependence of the viscosity in Cu–As–Se system

The microscopic bonding behavior of amorphous Cu_x(As₂Se₃)_1?x (x = 0.00, 0.01, 0.05, 0.10 and 0.20) is studied from the point of view of the viscosity. The model used in this analysis describes the temperature dependence of the viscosity in terms of the mean bond strength E₀, the mean coordination number Z₀, and their fluctuations ΔE, ΔZ of the structural units that form the melt. The model reproduces quite well the temperature dependence of the viscosity of Cu_x(As₂Se₃)_1?x observed experimentally. According to the theory, the fragility of the system increases as the fluctuation of the total bond strength increases. It is shown that the viscous flow is accompanied by the cooperative movements of the structural units and occurs by breaking selectively the weaker parts of the bonds between the structural units. This notion is related to the concept of Cooperatively Rearranging Region (CRR) in the theory of Adam–Gibbs. By analyzing the model, the fluctuation of the total bond strength in Cu–As–Se system is evaluated quantitatively, and the composition dependence of the fragility is discussed in terms of bond strength and the coordination number between the structural units. The analysis suggests that in the Cu_x(As₂Se₃)_1?x system, there must be a strong composition dependence in the bond strength and a weak composition dependence in the fragility and coordination number fluctuation.     

Fundamental optical properties of Cd1-xMnxSe single crystals

The absorption near the fundamental edge of Cd_1-xMn_xSe was measured in the composition range 0<x<0.3 at room and liquid nitrogen temperature with the electric field of the radiation parallel and perpendicular to the hexagonal axis. An exponential dependence of the absorption coefficient versus photon energy was found, and a linear dependence of energy gap E₀ on composition was obtained. The room temperature reflectivity measurements in the energy range 2.5–5.2eV, for two polarization of light were performed, and a linear dependence of the interband transitions energies vs. alloy composition was found.     

The relationship between chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice

The theoretical model developed by Lidiard was extended to describe the relationship between the chemical and tracer diffusion coefficients of aliovalent ions in an ionic lattice.It is shown that the relationship between the chemical diffusion coefficient, D, and the tracer diffusion coefficient, D¹, is D = 2D¹ if the migration of dimers is the principal mechanism of transport and for the migration of trimers D = 3D¹ if the concentration of impurity ion is relatively small. These relationships are valid regardless of the charge of the aliovalent or lattice ions.The chemical diffusion coefficients of Cr³⁺ in Cr-doped MgO were determined for three different temperatures, 1656, 1717 and 1768K, and for the concentration region 2.5×10^?2?2.8×10^?1 mole% Cr₂O₃. Using previously determined values for the tracer diffusion coefficient of ⁵¹Cr in Cr-doped MgO it was found that for the temperature and concentration region investigated D = (2.00±0.17)D¹ which indicates that diffusion proceeds primarily by the migration of dimers.     

Magnetic field effect on the critical EPR-dynamics of the cubic ferromagnets CdCr2Se4 and CdCr2S4

We present measurements of the critical behaviour of the EPR linewidths at frequencies 9.21 and 35.5 GHz. In the exchange critical region above T_c (4πx ? 1) the strong field dependence of the linewidths is observed, even when the field variation of susceptibility x is small. This phenomenon is explained by the spin diffusion effect on the linewidth. The spin diffusion coefficients D for CdCr₂Se₄ and CdCr₂S₄ are determined from the linewidths data. The temperature dependence D is found to be consistent with the predictions of the dynamical scaling theory.     

A temperature-dependent Raman study of the xLiCl–(1–x)TeO2 glasses and melts

Raman scattering has been employed to study the temperature and composition dependence of the vibrational modes for the glass-forming oxyhalide mixtures xLiCl–(1–x)TeO₂ (x=0, 0.2, 0.3, 0.35, 0.5 and 0.67) in the glassy, supercooled and liquid state up to 600 °C. The analysis has shown that the network structure of the glass/melt is formed by mixing trigonal bipyramid and trigonal pyramid units. The change of LiCl content and/or temperature results to conversion of the trigonal bipyramid to trigonal pyramid units with a varying number of non-bridging chlorine and oxygen atoms. The fraction of the terminal oxygen atoms doubly bonded to tellurium versus temperature has been directly estimated from the Raman spectroscopic results with the aid of a structural model concerning the tellurite network systems. A well-resolved Boson peak (BP) dominates the low-frequency Raman spectra. The temperature dependence of the maximum of the BP has also been determined and discussed in the framework of its microscopic origin.     

Magnesia formed on calcination of Mg(OH)2 prepared from natural bischofite

Calcination of magnesium hydroxide, which was prepared from natural bischofite MgCl₂·6H₂O, leading to dehydration 2(MgOH) → MgOMg + H₂O, is accompanied by transition of phase not only to MgO but also to MgO_x at x < 1 (assigned to Mg₄O₃) at moderate temperatures. At higher temperatures, MgO_x is completely transformed into MgO. Magnesium hydroxide and oxide heated at different temperatures were studied using the TEM, XRD, IR, PCS, TG-DTA, nitrogen and argon adsorption methods. The electronic structure of MgO and Mg₄O₃ was studied using the ab initio quantum chemical method with periodic conditions. According to TEM images, the morphology of particles changing from Mg(OH)₂ laminae to aggregates of interpenetrated MgO cubelets and foils depend strongly on the calcination temperature. Significant changes in surface area are observed mainly at 325-470 °C on desorption of a major portion of eliminated water corresponding to 28.4 wt.% at its total amount of 30.9 wt.%. Pore size distribution (PSD) is sensitive to treatment conditions and the main PSD peaks shift towards larger pore size with elevating temperature. The characteristics of the surface hydroxyls as well as of the bulk MgO bonds depend on heating conditions, as noticeable changes are observed in the XRD patterns and the IR spectra of the samples undergoing the mentioned transformation of phase Mg(OH)₂ → MgO_x → MgO.     

Densities and surface tensions of SexTe1−x melts

The densities and surface tensions of Se_xTe_1?x melts have been determined from the melting points up to about 700°C. For Te-rich alloys a non linear behaviour has been found for the temperature dependence connected with a change in the coordination number.     

Phase states and dielectric properties of sodium-potassium niobate solid solutions

The temperature dependence of dielectric constant ? for single crystals of Na_1?xK_xNbO₃ (0.04?x?0.15) is studied for the first time. From the shape of the ?(T) anomalies corresponding to rotational phase transitions, the type of interaction between the order parameters and the polarization is determined. A phenomenological model is developed which adequately describes the experimentally observed sequence of high-temperature (T>300°C) phase transitions, the dielectric anomalies associated with these transitions, and the changes in the phase states under the action of external factors (pressure, electric field).     

Determination of concentration-dependent diffusion coefficients in amorphous Ni-P films studied by auger electron depth profiling

Studies of the interdiffusion in ion-plated Ni₁₀₀- _x P _x /Ni₁₀₀- _y /P _y thin films are described. The diffusion coefficients are evaluated from depth profiles obtained by using Auger electron spectroscopy (AES). The well-known Boltzmann-Matano method is applied for extraction of the diffusion coefficients from the depth profile data. The results show a strong composition dependence.     

Microstructural analysis of liquid and amorphous SiO2 nanoparticles

Microstructural properties of liquid and amorphous SiO₂ nanoparticles have been investigated via molecular dynamics (MD) simulations with the interatomic potentials that have weak Coulomb interaction and Morse-type short-range interaction under non-periodic boundary conditions. Structural properties of spherical nanoparticles with different sizes of 2, 4 and 6 nm obtained at 3500 K have been studied through partial radial distribution functions (PRDFs), coordination number and bond-angle distributions, and compared with those observed in the bulk. The core and surface structures of liquid SiO₂ nanoparticles have been studied in detail. We found significant size effects on structure of nanoparticles. Calculations also show that if the size is larger than 4 nm, liquid SiO₂ nanoparticles at the temperature of 3500 K have a lightly distorted tetrahedral network structure with the mean coordination number Z_Si-O≈4.0 and Z_O-Si≈2.0 like those observed in the bulk. Moreover, temperature dependence of structural defects and SiO_x stoichiometry in nanoparticles on cooling from the melt has been found and presented.     

The evolution of covalent bonding structures of liquid In–Sn alloys

The short-range orders of liquid In_xSn_100−x binary alloys have been investigated along the liquidus based on the experimental results obtained using X-ray diffraction. The covalent bonding structures characterized by the shoulder on the high-Q side of the first peak of S(Q) for all the In_xSn_100−x melts are detected. With increasing Sn content, there is a rather continuous change of the local structure possibly related to the presence of nearly covalent bonds in the liquid alloy. The peculiar temperature dependence of the viscosity is assigned to the evolution of tetrahedral units. The observed correlation between liquid structures and peculiar viscosity behaviors provides new insight into nucleation phenomena from microscopic viewpoint.     

Molecular dynamics simulations on local structure and diffusion in liquid TixAl1−x alloys

The microscopic structure and dynamics of liquid Ti_xAl_1-x alloys together with pure liquid Ti and Al metals were investigated by means of molecular dynamics simulations. This work gives the structural properties, including pair-correlation function, bond-angle distribution function, HA and Voronoi indices, and their composition dependence. The dynamical properties have also been studied. The calculated pair-correlation function, bond-angle distribution function, and HA and Voronoi indices suggest that the stoichiometric composition Ti_0.75Al_0.25 exhibits a different local structure order compared with other concentrations, which help us understand the appearance of the minimum diffusion coefficient at this composition. These results indicate that the mobility of atoms strongly depends on their atomic local structure.     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Magnetic microstructure of candidates for epitaxial dual Heusler magnetic tunnel junctions

Heusler alloys are considered as interesting ferromagnetic electrode materials for magnetic tunnel junctions, because of their high spin polarization. We, therefore, investigated the micromagnetic properties in a prototypical thin film system comprising two different Heusler phases Co₂MnSi (CMS) and Co₂FeSi (CFS) separated by a MgO barrier. The magnetic microstructure was investigated by X-ray photoemission electron microscopy (XPEEM). We find a strong influence of the Heusler phase formation process on the magnetic domain patterns. SiO₂/V/CMS/MgO/CFS and SiO₂/V/CFS/MgO/CMS trilayer structures exhibit a strikingly different magnetic behavior, which is due to pinhole coupling through the MgO barrier and a strong thickness dependence of the magnetic ordering in Co₂MnSi.     

Orientational disorder,glass/crystal transition and superionic conductivity in nasicon

The relations between the orientational disorder of SiO₄(PO₄) tetrahedra and fast sodium diffusion in superionic NASICON have been studied by conductivity (complex impedance method), DSC, X-ray powder difraction and vibrational spectroscopy (IR and Raman). Sol-gel routes allow to obtain pure glassy NASICON (Na_1+xZr₂Si_xP_3-xO₁₂x≅2) in the 500−700°C temperature range. Tetragonal zirconia nucleates above 700°C and disappears at about 900°C when the isolated tetrahedra framework is formed: a high orientational static disorder of tetrahedra exists and the symmetry is rhombohedral at all studied temperatures (20−600 K). Thermal treatment above 1100°C induces a drastic decrease of the static orientational disorder and nucleation of monoclinic zirconia. The resulting compound exhibits a monoclinic symmetry at R.T. and three phase transitions, two diffuse at about 60 K and 520 K and the 423 K monoclinic-rhombohedral transition associated with the superionic conducting state. An increase in dynamic disorder (broad Rayleigh wing up to 500 cm^-1 is simultaneously observed. The lower the static disorder at low temperature, the higher the dynamic orientational disorder and the phase transitions, and the lower the activation energy of conductivity at high temperature.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

Polyamorphism of liquid silica under compression based on five order-parameters and two-state model: a completed and unified description

The polyamorphism of liquid silica (SiO₂) at 3200?K and in a wide pressure range is investigated by molecular dynamics simulation. Results show that the structure of liquid SiO₂ consists of five order-parameters that do not depend on compression. Three order parameters that relate to the short-range order are SiO_x coordination units (x?=?4, 5, 6) and two order parameters that relate to the intermediate-range order are OSi₂ and OSi₃ linkages. The structure of liquid silica under compression can be described by the two-state model: low-density and high-density states. The low-density state is formed by clustering of OSi₂ linkages (in the low-density state, the short-range order (SRO) is mainly characterized by SiO₄ coordination units), conversely, clustering of OSi₃ linkages will form high-density state (in the high-density state, the SRO is mainly characterized by SiO₅ and SiO₆ coordination units). Under compression, in the liquid silica co-exist two phases: low-density and high-density phases. The size of phase regions significantly depends on compression.