Hydrogen location and diffusion in a two-dimensional metal: Proton magnetic resonance in metallic zirconium monohalide hydrides

Proton magnetic resonance has been studied in a new class of metal-hydrogen systems, ZrXH, based on the metallic double-layered monohalides of zirconium, ZrCl and ZrBr. The rigid lattice second moments of the proton resonance show that in the hemihydride phases hydrogen predominantly occupies alternate chains of tetrahedral sites within the double layer of close-packed Zr atoms. In the monohydrides all tetrahedral sites are occupied by hydrogen. Activation energies for hydrogen diffusion within the layers are derived from the temperature dependence of the resonance linewidth. A substantial anisotropy of the proton resonance shape is observed at high temperatures.     

Second-order inner displacement in the hexagonal close-packed structure

Expressions given recently by Ramanand et al. for second-order sublattice displacements in the hexagonal close-packed structure are shown to be in error. The use of inner elasticity theory leads to expressions that conform fully to the requirements of thermodynamics and crystal symmetry.     

Atomic displacements in the V<Subscript>52</Subscript>O<Subscript>64</Subscript> superstructure and the short-range order in superstoichiometric cubic VO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> vanadium monoxide with metal vacancies

Atomic displacements in the lattice of the tetragonal V₅₂O₆₄ superstructure have been experimentally determined. It has been found that atomic displacement waves, which are attributed to the formation of the short-range displacement order, appear in the vanadium and oxygen sublattices of this superstructure. It has been shown that the V₅₂O₆₄ superstructure is formed on the basis of disordered superstoichiometric cubic vanadium monoxide with the short-range order in the metallic sublattice. The character of the short-range order is such that vanadium atoms occupying tetrahedral positions are in the environment of four vacant sites of the vanadium sublattice. This means that the superstoichiometric VO_>1.0 vanadium monoxide has a cubic structure differing from the B1-type structure characteristic of most of the strongly nonstoichiometric cubic compounds MX _y (X = C, N, O) of transition metals.     

Electron density and electron redistribution in alloys—II: Electron density in alloys and intermetallic phases

Electron density for alloys which have close-packed metallic structures is calculated by assigning valence electrons to octahedral and tetrahedral interstices, a method which has been previously used for elemental metals. Some localization of electron density is proposed for β -phases when there is considerable difference in ion core sizes. This method of characterizing electron density in alloys can be used to derive structures with the amount of electron transfer if an assumption is made for the volume fraction occupied by each component of the alloy. In general, the electronic structure of intermetallic phases appears to be dominated by the correspondence of a definite number of valence electrons with the number of interstices in the metallic structure (the Hume-Rothery $\text{e}\text{a}$ ratios). The model used can also accommodate electron distributions which include both ionic and covalent components of electron density. This is the case for Laves phases and the metallic A-15 compounds. There is a preponderance of intermetallic phases where one component is a d-shell metal. Evidence is presented that in several such alloys there is a change in d-shell configuration of the elemental metal which serves to minimize size differences of the ion cores of the alloy.     

The stability limit of the fluid phase of polydisperse sticky spheres

It has been shown by Stell (1991, J. statist. Phys., 63, 1203) that at low temperature mono-disperse sticky spheres collapse to form coexisting close-packed solid and infinitely dilute gases. We show that polydisperse sticky spheres also collapse and calculate the collapse temperature. The polydisperse spheres separate into fractions with narrower polydispersities which can then solidify. This is perhaps the first example of a single-peaked polydisperse mixture phase solidifying and separating. It implies that a mixture of polydisperse large hard spheres with much smaller hard spheres does not show fluid—fluid coexistence.     

Formation of thermal vacancies on the Si sublattice of the intermetallic compound MoSi2

For a detailed understanding of high-temperature processes in complex solids the identification of the sublattice on which thermal defects are formed is of basic interest. Theoretical studies in intermetallic compounds favor a particular sublattice for thermal vacancy formation. In the present study we detect in ordered MoSi2 thermal vacancies with a low formation enthalpy of H(F)(V)=(1.6+/-0.1) eV, and we succeed in showing by experimental and theoretical efforts that they are preferentially formed on the Si sublattice. By these data self-diffusion in MoSi2 can be understood.     

High chalcocite Cu2S: a solid-liquid hybrid phase

There are materials that exist in unusual solid-liquid hybrid phases, for example, the superionics at high temperatures of 700 °C. Using ab initio molecular dynamics, we show that the intensely studied Cu(2)S high chalcocite phase is actually a solid-liquid hybrid phase which exists in relatively low temperature (>105 °C). Its formation mechanism is different from the superionics. We also show that the previously proposed atomic structure for high chalcocite is incorrect, and the low chalcocite to high chalcocite transition should be described as a sublattice solid to liquid transition.     

Structure and lattice dynamics of titanium hydrides due to thermobaric treatment

Abstract

An in situ X-ray diffraction study of the high-pressure ζ-phase evidenced a fct Ti sublattice. In the fco X-phase quenched under pressure, H atoms are displaced to octahedral sites, and the energy of H optic peak (at ～ 75 meV) is half that in other Ti-H phases where H occupy tetrahedral sites. Phase transformations on heating of x-TiH(D)～0.75 were studied by neutron diffraction, small angle and inelastic neutron scattering (INS). Bound multiphonons were observed in the INS spectra of ordered γ-TiH(D).     

Mixed threefold and fourfold carbon coordination in compressed CO2

Carbon dioxide (CO2) has been recently reported to possess an amorphous form, named "carbonia," structurally similar to other group-IV oxide glasses. By combining ab initio constant pressure molecular dynamics, density-functional perturbation theory, and experimental IR spectra, we show that carbonia, and possibly also phase VI, is not SiO2-like, and that instead it is partially tetrahedral containing also a sizable amount of carbon in threefold coordination, but no sixfold octahedral coordination. Enthalpic considerations suggest that carbonia is a metastable intermediate state of the transformation of molecular CO2 into fully tetrahedral phases.     

Kinetic Considerations in the Formation of Electrical Active Grain Boundaries in Barium Titanate and Similar Perovskites

Grain boundaries in ceramic barium titanate and related materials can be engineered in order to obtain desired transport behavior. Our ability to do so is closely related to kinetic limitations during the preparation. The close-packed structure of perovskites excludes native or foreign interstitials in the bulk. (Interstitial protons are regarded as OH_O , using the Kröger-Vink notation). Antisites are also unlikely due to size, charge and coordination number mismatch. The possible point defects are, therefore, substitutionals and vacancies. The kinetic limitations of these species, and the results in terms of grain boundary engineering, are considered in this contribution.A clear distinction between three different conditions is made. At very high temperatures, it is assumed that all the relevant defects are mobile and can equilibrate, at least locally. Hence, their concentrations are all functions of the degrees of freedom of the system. At lower temperatures, the cation sublattice is frozen. Therefore, the concentrations of metal vacancies and substitutional cations are constants and, from local electrical neutrality point of view, a new parameter becomes important: the concentration of frozen charge. The concentrations of electronic defects and oxygen vacancies in this metastable state are functions of temperature, oxygen partial pressure and frozen charge. The normalized concentration of frozen metal vacancies is calculated as a function of the doping factor, f (defined as the ratio between the electron concentration at a given state and at a reference state), and a nonstoichiometry parameter. Around room temperature, the anion sublattice is also frozen, and only electrons and holes exhibit significant transport properties.     

Polar Kerr rotation spectra in yttrium iron garnet and lithium ferrite: A comparative study

Polar Kerr rotation (PKR) spectra in the range 2.0–5.7 eV at 295 K of lithium ferrite and yttrium iron garnet (YIG) single crystals are reported. The spectra show more details and cover a more extended energy region than those published so far. The interpretation of the spectra is based on the assumptions that 1) PKR is an odd function of the sublattice magnetic moments 2) the origin of PKR in both compounds is of similar nature, the differences due to the different crystal structures (garnet and spinel) being less important. The calculated PKR sublattice components show some similarities in their energy dependences, the magnitude of the tetrahedral sublattice component being higher than that of the octahedral one. The components were used in the calculation of the PKR spectra in diamagnetically substituted yttrium iron garnet (J. Appl. Phys.49, 2212, 1978). The results correctly predict the trends observed experimentally.     

Mixed-spin ising model with one- and two-spin competing dynamics

In this work we found the stationary states of a kinetic Ising model, with two different types of spins: sigma=1/2 and S=1. We divided the spins into two interpenetrating sublattices, and found the time evolution for the probability of the states of the system. We employed two transition rates which compete between themselves: one, associated with the Glauber process, which describes the relaxation of the system through one-spin flips; the other, related to the simultaneous flipping of pairs of neighboring spins, simulates an input of energy into the system. Using the dynamical pair approximation, we determined the equations of motion for the sublattice magnetizations, and also for the correlation function between first neighbors. We found the phase diagram for the stationary states of the model, and we showed that it exhibits two continuous transition lines: one line between the ferrimagnetic and paramagnetic phases, and the other between the paramagnetic and antiferrimagnetic phases.     

Effect of dopant polarizability on oxygen sublattice order in phase-stabilized cubic bismuth oxides

Bismuth oxide doped with isovalent rare earth cations retains the high temperature defective fluorite structure upon cooling down to room temperature. However, these doped materials undergo an order-disorder transition of the oxygen sublattice at about 600 °C. When annealed at temperatures less than the transition temperature the oxygen sublattice continues to order, and consequently oxygen ion conductivity undergoes a decay. Modeling of ordered structures based on TEM diffraction patterns indicates a 〈111〉 vacancy ordering in the anion sublattice. Neutron diffraction studies show additional structural changes in the oxygen sublattice due to ordering. These studies indicate that the ionic conductivity is dependent on the distribution of oxygen ions between the regular 8c sites and the interstitial 32f sites in the fluorite structure. Earlier neutron diffraction studies indicate that short range ordering of the anion sublattice is related to the polarizability of the cations. In this study we relate the stability of the disordered structure and the formation of long range order to the polarizability of the dopant cations, in terms of the time constant for conductivity decay and the dielectric constant. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Helimagnetism of Fe: high pressure study of an Y2Fe17 single crystal

In this Letter we present direct observation of the Fe helimagnetism in an Y2Fe17 single crystal under pressure. Combined neutron diffraction and magnetization measurements under pressure showed that the collinear ferromagnetic phase of Y2Fe17 is substituted by the pressure induced helical incommensurate phases. The complex pressure-temperature-field behavior of the pressure induced helical magnetic phases is attributed to intrinsic properties of the iron sublattice that gives a valuable contribution to the discussion about dominating theoretical models of magnetism in gamma-Fe.     

Dynamic compensation temperature in kinetic spin-5/2 Ising model on hexagonal lattice

We present a study of the dynamic behavior of a two-sublattice spin-5/2 Ising model with bilinear and crystal-field interactions in the presence of a time-dependent oscillating external magnetic field on alternate layers of a hexagonal lattice by using the Glauber-type stochastic dynamics. The lattice is formed by alternate layers of spins σ=5/2 and S=5/2. We employ the Glauber transition rates to construct the mean-field dynamic equations. First, we investigate the time variations of the average sublattice magnetizations to find the phases in the system and then the thermal behavior of the dynamic sublattice magnetizations to characterize the nature (first- or second-order) of the phase transitions and to obtain the dynamic phase transition (DPT) points. We also study the thermal behavior of the dynamic total magnetization to find the dynamic compensation temperature and to determine the type of the dynamic compensation behavior. We present the dynamic phase diagrams, including the dynamic compensation temperatures, in nine different planes. The phase diagrams contain seven different fundamental phases, thirteen different mixed phases, in which the binary and ternary combination of fundamental phases and the compensation temperature or the L-type behavior strongly depend on the interaction parameters.     

Bose−Einstein condensates with tunable spin−orbit coupling in the two-dimensional harmonic potential: The ground-state phases,stability phase diagram and collapse dynamics

We study the ground-state phases, the stability phase diagram and collapse dynamics of Bose−Einstein condensates (BECs) with tunable spin−orbit (SO) coupling in the two-dimensional harmonic potential by variational analysis and numerical simulation. Here we propose the theory that the collapse stability and collapse dynamics of BECs in the external trapping potential can be manipulated by the periodic driving of Raman coupling (RC), which can be realized experimentally. Through the high-frequency approximation, an effective time-independent Floquet Hamiltonian with two-body interaction in the harmonic potential is obtained, which results in a tunable SO coupling and a new effective two-body interaction that can be manipulated by the periodic driving strength. Using the variational method, the phase transition boundary and collapse boundary of the system are obtained analytically, where the phase transition between the spin-nonpolarized phase with zero momentum (zero momentum phase) and spin-polarized phase with non-zero momentum (plane wave phase) can be manipulated by the external driving and sensitive to the strong external trapping potential. Particularly, it is revealed that the collapsed BECs can be stabilized by periodic driving of RC, and the mechanism of collapse stability manipulated by periodic driving of RC is clearly revealed. In addition, we find that the collapse velocity and collapse time of the system can be manipulated by periodic driving strength, which also depends on the RC, SO coupling strength and external trapping potential. Finally, the variational approximation is confirmed by numerical simulation of Gross−Pitaevskii equation. Our results show that the periodic driving of RC provides a platform for manipulating the ground-state phases, collapse stability and collapse dynamics of the SO coupled BECs in an external harmonic potential, which can be realized easily in current experiments.     

Visualization of conduction channels and the dynamics of ion transport in superionic conductors

A method for visualizing conduction channels is proposed. This method is based on graphical analysis of conduction channel fragments which belong to a Voronoi-Dirichlet elementary polyhedron and lie outside the rigid sphere centered at a fixed-sublattice ion that is located at the geometric center of the elementary polyhedron under consideration. Taking into account the weak nonrigidity of spheres and root-mean-square displacements of ions in the fixed sublattice makes it possible to construct a channel as the surface of the mobile ion density. The most probable regions of mobile ion motion are quantum-mechanically interpreted as channel walls, which is confirmed by constructing the equipotential surfaces of interionic potential for α-AgI. It is found that the Andersson mathematical dynamics and the dynamics of ion transport in AgI lead to the same pattern of the motion. The symmetry rules are used for predicting the directions of motion along the allowed vibrational coordinates of tetrahedral and octahedral α-CuI fragments.     

Observation of condensed phases of quasiplanar core-softened colloids

We experimentally study the condensed phases of repelling core-softened spheres in two dimensions. The dipolar pair repulsion between superparamagnetic spheres trapped in a thin cell is induced by a transverse magnetic field and softened by suitably adjusting the cell thickness. We scan a broad density range and we materialize a large part of the theoretically predicted phases in systems of core-softened particles, including expanded and close-packed hexagonal, square, chainlike, stripe or labyrinthine, and honeycomb phase. Further insight into their structure is provided by Monte Carlo simulations.     

Mn-stabilized zirconia: from imitation diamonds to a new potential high-Tc ferromagnetic spintronics material

From the basis of ab initio electronic structure calculations which include the effects of thermally excited magnetic fluctuations, we predict Mn-stabilized cubic zirconia to be ferromagnetic above 500 K. We find this material, which is well known both as an imitation diamond and as a catalyst, to be half-metallic with the majority and minority spin Mn impurity states lying in zirconia's wide gap. The Mn concentration can exceed 40%. The high-Tc ferromagnetism is robust to oxygen vacancy defects and to how the Mn impurities are distributed on the Zr fcc sublattice. We propose this ceramic as a promising future spintronics material.     

Atomic-vacancy ordering in the carbide phase ζ-Ta4C3t-x

The structure of the nonstoichiometric trigonal (rhombohedral) carbide ζ-Ta₄C_3t-x forming in the tantalum-carbon system is studied by neutron diffraction, x-ray diffraction, and metallography. The unit cell parameters of the trigonal (space group R-3m) carbide ζ-Ta₄C_3?x (TaC_0.67) are found to be a _h = 0.3123 nm and c _h = 3.0053 nm. It is shown that the metallic close-packed sublattice of the ζ-Ta₄C_3?x phase consists of alternating blocks in which the metallic atoms are arranged in the same manner as on the fcc sublattice of the cubic carbide TaC_y and the hcp sublattice of the hexagonal carbide Ta₂C, respectively, and that the former sublattice is intermediate between the latter two sublattices. We are the first to establish experimentally that the distribution of carbon atoms and structural vacancies in the ζ-Ta₄C_3?x carbide is ordered and to calculate the distribution function of carbon atoms over the nonmetallic-sublattice sites involved in the ordering. The results obtained for the ζ-Ta₄C_3?x phase are used to refine the phase diagram of the Ta-C system.