scispace - formally typeset
Search or ask a question

Showing papers in "Journal of Physics: Condensed Matter in 1994"


Journal ArticleDOI
TL;DR: In this paper, the authors proposed a pseudowave function inside the cut-off radius with the concept of ultrasoft pseudopotentials introduced by Vanderbilt optimal compromise between transferability and plane-wave convergence.
Abstract: The construction of accurate pseudopotentials with good convergence properties for the first-row and transition elements is discussed. We show that by combining an improved description of the pseudowavefunction inside the cut-off radius with the concept of ultrasoft pseudopotentials introduced by Vanderbilt optimal compromise between transferability and plane-wave convergence can be achieved. With the new pseudopotentials, basis sets with no more than 75-100 plane waves per atom are sufficient to reproduce the results obtained with the most accurate norm-conserving pseudopotentials.

3,263 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of the electronic stopping power (Se) in metals irradiated by swift heavy ions was investigated in the framework of the thermal-spike model, and the effect on the lattice temperature induced by swift-heavy ion irradiation was calculated.
Abstract: In the framework of the thermal-spike model the present paper deals with the effect of the electronic stopping power (Se) in metals irradiated by swift heavy ions. Using the strength of the electron-phonon coupling g(z) with the number of valence electrons z as the unique free parameter, the increment of lattice temperature induced by swift-heavy-ion irradiation is calculated. Choosing z=2, the calculated threshold of defect creation by Se for Ti, Zr, Co and Fe is about 11, 27.5, 28 and 41 keV nm-1, in good agreement with experiment. Taking the same z value, the calculation shows that Al, Cu, Nb and Ag are Se insensitive. Moreover, in Fe, the differences in the damage created by U ions of different energies but exhibiting the same value of Se may be interpreted by a velocity effect. Using z=2, other calculations suggest that Be (Se>or=11 keV nm-1), Ga (Se>or=5 keV nm-1) and Ni (Se>or=49 keV nm-1) should be sensitive to Se but Mg should not. These examples put the stress on the effect of the physical parameters governing the electron-phonon coupling constant apart from z determination: the sound velocity linked to the Debye temperature and the lattice thermal conductivity. Furthermore, a simple criterion is proposed in order to predict the Se sensitivity of metals.

493 citations


Journal ArticleDOI
TL;DR: In this article, the transport coefficients of the Anderson model are calculated by extending Wilson's numerical renormalization group method to finite-temperature Green functions, and accurate results for the frequency and temperature dependence of the single-particle spectral densities and transport time tau ( omega, T) are obtained and used to extract the temperature dependence.
Abstract: The transport coefficients of the Anderson model are calculated by extending Wilson's numerical renormalization group method to finite-temperature Green functions. Accurate results for the frequency and temperature dependence of the single-particle spectral densities and transport time tau ( omega , T) are obtained and used to extract the temperature dependence of the transport coefficients in the strong-correlation limit of the Anderson model. Results are obtained for values of the local level position ranging from the Kondo regime to the mixed valency and empty orbital regimes. The low-temperature anomalies in the resistivity, rho (T), thermopower, S(T), thermal conductivity, kappa (T), and Hall coefficient, RH(T), are discussed in terms of the behaviour of the spectral densities. At low temperature all quantities exhibit the expected Fermi liquid behaviour, rho (T)= rho 0(1-c(T/TK)2), S(T) approximately gamma T, kappa (T)/ alpha T=1+ beta (T/TK)2, RH(T)=-Rinfinity (1- delta (T/TK)2). Analytic results based on Fermi liquid theory are derived here for the first time for beta and the numerical results are shown to be consistent with this coefficient. The range of temperatures over which universal behaviour extends is also discussed. Scattering of conduction electrons in higher-angular-momentum, l>0, channels is also considered and an expression is derived for the corresponding transport time and used to discuss the influence of the interference terms between the resonant l=0 and non-resonant l=1 channels on the transport properties. The presence of non-resonant scattering is shown to be particularly important for the thermopower at half filling, where the sign of the thermopower can depend sensitively on the non-resonant phase shift. Finally the relation of the results to experiment is discussed.

423 citations


Journal ArticleDOI
TL;DR: In this article, a thorough discussion of today's understanding of the growth of thin metal films and related atomistic mechanisms responsible for intrinsic stress is presented, where the intrinsic stress either originates from strained regions within the films (grain boundaries, dislocations, voids, impurities, etc.) or at the film/substrate (lattice mismatch, different thermal expansion, etc).
Abstract: It is well known that thin films develop large intrinsic stress during their preparation. The intrinsic stress either originates from strained regions within the films (grain boundaries, dislocations, voids, impurities, etc.) or at the film/substrate (lattice mismatch, different thermal expansion, etc) and film/vacuum interfaces (surface stress, adsorption, etc.) or is due to dynamic processes (recrystallization, interdiffusion, etc). Since the magnitude of most of these stress contributions is directly related to film morphology, important structural information can be extracted from measurements of the intrinsic stress. This article presents a thorough discussion of today's understanding of the growth of thin films and reviews the related atomistic mechanisms responsible for intrinsic stress. On the basis of these ideas recent experimental results on the intrinsic stress of UHV deposited polycrystalline and epitaxial thin metal films are discussed. Depending on the respective growth mode of the films-Volmer-Weber, Stranski-Krastanov and Frank-Van der Merwe modes-characteristic stress behaviours are observed. In situ intrinsic stress measurements are therefore a promising new technique to gain additional insight into film growth.

418 citations


Journal ArticleDOI
TL;DR: In this article, the photovoltaic effect in diode structures formed with thin films (100 nm) of the conjugated polymer poly(p-phenylene vinylene), PPV, sandwiched between electrodes of indium/tin oxide, ITO, and either aluminium, magnesium or calcium.
Abstract: We report measurements of the photovoltaic effect in diode structures formed with thin films (100 nm) of the conjugated polymer poly(p-phenylene vinylene), PPV, sandwiched between electrodes of indium/tin oxide, ITO, and either aluminium, magnesium or calcium. Under illumination incident through the ITO contact, large open-circuit voltages were measured, which saturated at approximately 1.2 V for Al and Mg devices, and approximately 1.7 V for Ca devices. Quantum efficiencies (short-circuit current/incident photon flux) of order 1% were measured at low intensities (0.1 mW cm-2). The spectral response of the photocurrent demonstrates that photon absorption near the electron-collecting electrode optimizes the photocurrent, indicating that device performance is limited by low electron mobilities in the bulk PPV. The photocurrent exhibits a weak temperature dependence, with an activation energy that is a function of the electric field in the polymer. We have used these measurements to estimate an exciton binding energy in PPV of approximately 0.4 eV.

342 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of rare-earth ion substitutions on magnetic and electrical properties of Ni-Zn ferrite was investigated. But the authors focused on the magnetic properties and not the electrical properties.
Abstract: The effect of Fe substitutions by rare-earth ions on magnetic and electrical properties of a Ni-Zn ferrite prepared by the classical method is investigated. A set of seven compounds with formula Ni0.7Zn0.3Fe1.98R0.02O4, where R identical to Yb, Er, Dy, Tb, Gd, Sm or Ce, was prepared. Emphasis is placed on current experimental results of bulk magnetic measurements and transport phenomena. The results obtained reveal that, by introducing a relatively small amount of R2O3 instead of Fe2O3, an important modification of both the structure and the magnetic and electrical properties can be obtained. We explain the influence of the rare-earth ions as an effect of the ionic radius. This assumption is supported by the lattice constant measurements. The best results from the viewpoint of magnetic and electrical characteristic acceptable for high frequencies were obtained for R ions with a large radius and with a stable valence of 3+ such as Gd in our work.

304 citations


Journal ArticleDOI
TL;DR: In this article, the surface energy of some defects for the (100), (110), and (111) surfaces of Cu, Ag, Au, Ni, Pd, and Pt were studied using effective medium theory.
Abstract: The energetics of some surface defects for the (100), (110), and (111) surfaces of Cu, Ag, Au, Ni, Pd, and Pt are studied using effective medium theory. Among the calculated data are the surface energy, the energetics of some reconstructions, the formation energy for steps and islands, and the activation energy for diffusion of adatoms.

276 citations


Journal ArticleDOI
TL;DR: In this article, the authors present results of elastic band structure for two-dimensional composite materials, composed of periodical square arrays of parallel cylinders in a background, revealing, for the first time, the existence of several very large complete band gaps in the band structure of a material of practical interest such as a C fibre reinforced epoxy composite.
Abstract: We present results of elastic band structure for two-dimensional composite materials, composed of periodical square arrays of parallel cylinders in a background. We reveal, for the first time, the existence of several very large complete band gaps in the band structure of a material of practical interest such as a C fibre reinforced epoxy composite. Within these gaps the propagation of acoustic waves is forbidden. The influence of the geometry of the cylinders and the effect of the composition of the composite material on the band structure are studied. We also compare these results with those obtained for metallic composites such as W (Al) cylinders in an Al (W) matrix. The complete band gaps are observed in the cases of C cylinders in epoxy or W cylinders in Al, but not in the opposite situations. We discuss the existence of these gaps in relation to the physical parameters of the materials involved.

236 citations


Journal ArticleDOI
TL;DR: In this paper, the state of the art of the study of germanium-related color centers in silica glass and silica-based fibres is presented and the origins of the models and formation mechanisms and their weak and strong points are analysed.
Abstract: This review presents the state of the art of the study of germanium-related colour centres in silica glass and silica-based fibres. Most attention is concentrated on the dominant colour centres such as the Ge-related oxygen-deficient centres and paramagnetic Ge(n) centres, all of them having ultraviolet absorption bands. The hypothetical models and formation mechanisms proposed so far for these colour centres are discussed in detail. The origins of the models and formation mechanisms and their weak and strong points are analysed. The origin of the less well studied Ge-related colour centres in the visible spectral range (GeH, GeX, drawing-dependent defects) is also discussed.

223 citations


Journal ArticleDOI
TL;DR: In this paper, the authors report the fabrication of silicon-rich erbium-doped silica films that exhibit both 1535 nm fluorescence and visible photoluminescence.
Abstract: We report the fabrication by PECVD of silicon-rich erbium-doped silica films that exhibit both 1535 nm fluorescence and visible photoluminescence. Fluorescence spectra are presented along with absorption spectra that display a strong band edge in the blue, which we ascribe to the presence of Si microclusters. We are unable to observe characteristic Er3+ absorption bands and propose that excitation of the rare earth is via an energy transfer process from Si microclusters.

199 citations


Journal ArticleDOI
TL;DR: In this paper, the lattice parameters, atom positions and tilt angles of the oxygen octahedra were determined in the temperature range 12-1773 K. The results are discussed in the classification schemes developed for distorted perovskites.
Abstract: The rare-earth gallium oxide perovskites RGaO3 (R=La,Pr,Nd) are promising substrates for epitaxy of high-Tc superconductors. In this paper the crystal structures of orthorhombic and rhombohedral RGaO3 have been studied by neutron diffraction, which allows precise localization of light atoms such as oxygen, which are of importance in phase transitions. The lattice parameters, atom positions and tilt angles of the oxygen octahedra were determined in the temperature range 12-1773 K. Lanthanum gallate LaGaO3 shows a phase transition from an orthorhombic phase (Pbnm) to a rhombohedral phase (Rc) at 425 K. The results are discussed in the classification schemes developed for distorted perovskites.

Journal ArticleDOI
TL;DR: In this paper, the afterglow and thermoluminescence properties of several Ce3+ doped Lu2SiO5 crystals are reported, which are caused by the presence of charge traps in the crystals.
Abstract: The afterglow and thermoluminescence (TL) properties of several Ce3+ doped Lu2SiO5 crystals are reported. Both properties are caused by the presence of charge traps in the crystals. At least six different glow peaks are observed in the TL glow curve. Each is related to a specific charge trap. The parameters for these charge traps, such as the trap depth and the frequency factor, were obtained from first-order kinetics peak analysis of the TL glow curve. A charge trap with a depth of 1.0 eV is responsible for the afterglow observed at room temperature. Ce3+ ions appear to be the luminescence centres in the recombination process of the trapped charge carriers. It will be shown that optical excitation in the 5d levels of Ce3+ produces trap filling. The possible nature of the charge traps will be discussed.

Journal ArticleDOI
TL;DR: In this paper, a three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behaviour in the vicinity of room temperature. But this phase behaviour can be understood in terms of a system of polydispersed spherical water droplets, each coated by a monolayer of AO, dispersed in a continuum of oil.
Abstract: The three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behaviour in the vicinity of room temperature. The phase diagram in the temperature-volume-fraction (of the dispersed phase) plant: exhibits a lower consolute critical point at about 40 degrees C and 10% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to the high-volume-fraction side at progressively lower temperatures. This phase behaviour can be understood in terms of a system of polydispersed spherical water droplets, each coated by a monolayer of AOT, dispersed in a continuum of oil. These droplets interact with each other via a hard-core plus a short-range attractive interaction, the strength of which increases with temperature. We show that Baxter's sticky-sphere model can account for the phase behaviour, including the percolation line, quantitatively provided that the stickiness parameter is a suitable function of temperature. We use the structure factors measured by small-angle neutron scattering (SANS) below the critical temperature to determine this functional dependence. We also investigate the dynamics of droplets, below and approaching the critical and percolation points, by dynamic light scattering. The first cumulant and time evolution of the droplet density correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena observed in an extensive set of measurements of electrical conductivity and permittivity close to percolation can also be interpreted through the same cluster-forming mechanism, which reproduces the most relevant features of the frequency-dependent complex dielectric constant of this system.

Journal ArticleDOI
TL;DR: In this paper, the effect of the Gamma -X crossover at low temperatures on the donor binding energies in single-quantum-well (QW) microstructures was investigated and it was found that, at a constant QW thickness, the predicted donor binding energy was enhanced with increasing pressure and then decreased when approaching the crossover pressure between the QW Gamma states and the barrier X states.
Abstract: The present work investigates the effect of the Gamma -X crossover at low temperatures on the donor binding energies in single-quantum-well (QW) microstructures. It is found that, at a constant QW thickness, the predicted donor binding energies are enhanced with increasing pressure and then decreased when approaching the crossover pressure between the QW Gamma states and the barrier X states. It is noticed that the pressure-dependent donor binding energies are mainly related to the changes in the dielectric constants and the effective masses of both materials.

Journal ArticleDOI
TL;DR: In this article, a simplified model of the band structure of kappa -(BEDT-TTF)2Cu(NCS)2 has been used to derive the bare band masses of the electrons from optical data.
Abstract: Magnetotransport measurements have been carried out on the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 at temperatures down to 500 mK and in hydrostatic pressures up to 16.3 kbar. The observation of Shubnikov-de Haas and magnetic breakdown oscillations has allowed the pressure dependences of the area of the closed pocket of the Fermi surface and the carrier effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature Tc. The effective mass measured by the temperature dependence of the Shubnikov-de Haas oscillations is found to fall rapidly with increasing pressure up to a critical pressure Pc approximately=5 kbar. Above Pc a much weaker pressure dependence is observed; Tc also falls rapidly with pressure from 10.4 K at ambient pressure to zero at around Pc. This strongly suggests that the enhanced effective mass and the superconducting behaviour are directly connected in this organic superconductor. A simplified model of the band structure of kappa -(BEDT-TTF)2Cu(NCS)2 has been used to derive the bare band masses of the electrons from optical data. Comparisons of these parameters with cyclotron resonance data and the effective masses measured in the present experiments indicate that the greater part of the enhancement of the effective mass necessary for superconductivity in this material is due to quasiparticle interactions, with the electron-phonon interactions playing a secondary role. The dependence of Tc on the effective mass may be fitted satisfactorily to a suitably parametrized weak-coupling BCS expression, although this cannot be taken as a definitive proof of the nature of superconductivity in organic conductors.

Journal ArticleDOI
TL;DR: In this article, the influence of local order around the titanium site on Ti 2p X-ray absorption spectra is studied for rutile, anatase, and brookite.
Abstract: The influence of local order around the titanium site on Ti 2p X-ray absorption spectra is studied for rutile, anatase and brookite. This is done by a configuration interaction based calculation of Ti L2,3 X-ray absorption edge shapes, explicitly taking into account the exact first-neighbour surroundings of the cation acting through crystal field splitting and hopping terms. The evolution of the peak splittings and intensities from rutile to anatase is well reproduced for the L2 edge and can be attributed to the small changes in the cation site symmetry. In contrast the splitting of the second peak of the L3 edge cannot be attributed to differences in the first coordination shell of the cation between the titanium oxides.

Journal ArticleDOI
TL;DR: In this paper, the crystal structures of RBa2Cu3Ox (R=Y and rare earths; x=6 and 7) ceramic materials were investigated at 10 K by neutron diffraction and consistently analyzed concerning systematic trends.
Abstract: The crystal structures of RBa2Cu3Ox (R=Y and rare earths; x=6 and 7) ceramic materials were investigated at 10 K by neutron diffraction and consistently analysed concerning systematic trends. Other than for non-superconducting PrBa2Cu3O7 the lattice parameters and most interionic distances exhibit the well known lanthanide contraction behaviour, i.e., a linear relationship with the ionic radii of trivalent rare-earth ions. The only exceptions are associated with the apex oxygen O(1) ions: the chain copper Cu(1)-O(1) distances are constant within error limits, and the plane copper Cu(2)-O(1) distances are increasing across the rare-earth series. The much stronger increase of the distance Cu(2)-O(1) in the RBa2Cu3O6 series compared to the RBa2Cu3O7 series can be explained by the increase of Tc from 90 K for YbBa2Cu3O7 to 96 K for NbBa2Cu3O7. The smaller distance Cu(1)-O(1) for the RBa2Cu3O6 series compared to the RBa2Cu3O7 series may be related to the suggested double-well potential of the apical oxygen ion. For some interionic distances of PrBa2Cu3O7 approximately parallel to the h direction (i.e., the chain direction) we determine by extrapolation a valence of +3.4 for the Pr ions. This indicates for PrBa2Cu3O7 a highly anisotropic 4f-CuO2 valence band hybridization. An important structural property with respect to the superconductivity is the puckering of the CuO2 planes: the superconductivity is lost when the puckering angle exceeds a critical value of about 167.3.

Journal ArticleDOI
TL;DR: In this article, two solid-solid phase transitions at 232 and 248 K have been revealed and the pyridinium cation reorientations in all three phases characterized, the higher transition appears to be the Curie point of a new ferroelectric compound.
Abstract: Polycrystalline pyridinium perchlorate has been studied over a wide range of temperatures by differential thermal analysis, proton NMR spectra and relaxation, and dielectric permittivity measurements. Two solid-solid phase transitions at 232 and 248 K have been revealed and the pyridinium cation reorientations in all three phases characterized. The higher transition appears to be the Curie point of a new ferroelectric compound.

Journal ArticleDOI
TL;DR: In this article, the authors reported spin-wave results for the sublattice magnetization (to order O(1/S2)) and two spin stiffnesses and susceptibilities for the two-dimensional triangular Heisenberg antiferromagnet.
Abstract: We report spin-wave results for the sublattice magnetization (to order O(1/S2)) and two spin stiffnesses and susceptibilities (to order O(1/S)) for the two-dimensional triangular Heisenberg antiferromagnet. These stiffnesses and susceptibilities are used as input parameters in scaling functions for various observables. The scaling results for uniform susceptibility are compared with recent numerical data.

Journal ArticleDOI
TL;DR: In this article, a method to fit realistic g2(r) models to EXAFS spectra satisfying both long-distance behaviour and the compressibility sum rule is emphasized.
Abstract: Structural information on the pair distribution function g2(r) for single-component disordered systems is usually obtained from the experimental structure factor S(k) measured by diffraction techniques. Complementary short-range information can be provided by the analysis of the extended X-ray absorption fine structure chi (k) associated with a certain X-ray absorption edge. The intrinsic differences in the nature of the chi (k) and S(k) signals are discussed and particular effort is devoted to connecting the chi (k) signal with usual quantities familiar to the distribution function theory in disordered matter. An example of the short-range chi (k) sensitivity is presented showing signals associated with g2(r) functions of liquid Cu at 1150 degrees C and 1300 degrees C. The necessity to fit realistic g2(r) models to EXAFS spectra satisfying both long-distance behaviour and the compressibility sum rule is emphasized. A method to combine these constraints and previous information on g2(r) with available chi (k) data is proposed and applied to recent EXAFS data on liquid palladium.

Journal ArticleDOI
TL;DR: In this article, the generalized Landau free energy functional is set up and four applications made and the origin of metastable tweed microstructures is shown in the dense medium of strong embryonic tweed-like microdomains existing as fluctuations at temperatures as high as 2Tc and above.
Abstract: Structural phase transitions are considered in which the effective ordering interaction J(Rij) arises from local stresses induced by the ordering in cell i and propagated elastically to a distant cell j. The generalized Landau free energy functional is set up and four applications made. Firstly the origin of metastable tweed microstructures is shown to lie in the dense medium of strong embryonic tweed-like microdomains existing as fluctuations at temperatures as high as 2Tc and above. Secondly the tweed-like pattern originates from the very anisotropic interaction associated with domain walls, and four cases are distinguished. Thirdly it is shown that the Landau-Ginzburg theory of the width and shape of a domain wall can be carried over in some cases but not others. Fourthly the magnitude of critical fluctuations is considered together with corresponding corrections to the Landau theory of the phase transition in the four cases.

Journal ArticleDOI
TL;DR: For short interatomic separations, molecular-type binding is dominant; the model requires only one-centre spherical calculations for adding the density effect to the binding energy of the molecule in vacuum.
Abstract: We describe approximate methods for calculating the binding energy of two protons in an electron gas. For short interatomic separations, molecular-type binding is dominant; the model requires only one-centre spherical calculations for adding the density effect to the binding energy of the molecule in vacuum. For large separations, the pair potentials are obtained using a superposition of atom-in-jellium densities in a density functional including a new form of the kinetic energy which has several exact asymptotic behaviours. The binding energy curves are presented, for interatomic distances covering both the molecular and the asymptotic regions and for electron densities in the range 2.00

Journal ArticleDOI
TL;DR: In this paper, a self-consistent band structure calculation was performed for 2H-NbSe2 in the normal noncharge-density-wave state using a full potential linearized augmented plane wave (FLAPW) method, which accounts well for the experimental angular dependence of the quantum oscillations in terms of a small flat hole Fermi surface around the Gamma point.
Abstract: Quantum oscillations have been investigated far into the mixed state of the type II superconductor 2H-NbSe2. The de Haas-van Alphen effect is found to persist to fields as low as approximately 0.3BC2 and to temperatures of approximately 0.003TC. A self-consistent band structure calculation, performed for 2H-NbSe2 in the normal non-charge-density-wave state using a full potential linearized augmented plane wave (FLAPW) method, accounts well for the experimental angular dependence of the quantum oscillations in terms of a small flat hole Fermi surface around the Gamma point. This feature derives mainly from chalcogen p bands and has a mass-enhancement factor much smaller than expected from the specific heat. Quantum oscillations in the presence of the vortex lattice are observed to experience an additional damping, from which the order parameter is estimated to be Delta (0)=0.6+or-0.1 meV.

Journal ArticleDOI
TL;DR: In this article, first-order Raman scattering of the soft ferroelectric F1u mode (TO1) and of the hard TO2 and TO4 optic modes evidences the existence of Ca2+-centred polar microregions.
Abstract: First-order Raman scattering of A1g and Eg modes originating from the antiferrodistortive F2u soft mode confirms the cubic-tetragonal (Oh-D4h) phase transition in Sr0.993Ca0.007TiO3, at T0=125 K. Within the cubic and the tetragonal phase first-order Raman scattering of the soft ferroelectric F1u mode (TO1) and of the hard TO2 and TO4 optic modes evidences the existence of Ca2+-centred polar microregions. Their size increases rapidly on cooling towards the ferroelectric ordering temperature, Tc=18 K. Splitting and incomplete softening of the F1u components is consistent with an order-disorder transition into an orthorhombic low-temperature phase with C2v symmetry.

Journal ArticleDOI
TL;DR: In this article, the T-P phase diagram for solid C60 at pressures up to 20 kbar in the temperature range extended to 700 K was presented, and a new phase region was outlined.
Abstract: The earlier high-pressure study of C60 concentrated on the orientational transition and room-temperature transformations observed in diamond anvils. This work presents a part of the T-P phase diagram for solid C60 at pressures up to 20 kbar in the temperature range extended to 700 K. A new phase region is outlined herein. The new phase is retained to ambient conditions and preliminarily characterized. The new phase is more dense than the initial C60 by at least 8.5% and less compressible by approximately 1.5 times. Its crystal structure is very different from the well known face-centred cubic and simple cubic phases, and its recovery to a cubic phase is associated with an endothermal heat effect of approximately 23 J g-1. The nature and the thermodynamic stability of the new phase are discussed in connection with polymerization of C60 molecules, which seems likely.

Journal ArticleDOI
TL;DR: In this article, the equilibrium structure of aqueous Na-Cl electrolytes in the concentration regime between 1 and 5 mol l-1 is studied by means of a molecular dynamics computer simulation using interaction-site descriptions of water and the ionic components.
Abstract: The equilibrium structure of aqueous Na-Cl electrolytes in the concentration regime between 1 and 5 mol l-1 is studied by means of a molecular dynamics computer simulation using interaction-site descriptions of water and the ionic components. The electrostatic interactions are treated both with the newly developed charged-clouds scheme and with Ewald summation. In the case of a 5 mol l-1 electrolyte, the results for pair correlations obtained by the two methods are in excellent agreement. However, the charged-clouds technique is much faster than Ewald summation and makes simulations at lower salt concentrations feasible. It is found that both ion-water and ion-ion correlation functions depend only weakly on the ionic concentration. Sodium and chloride ions exhibit only a negligible tendency to form contact pairs. In particular, no chloride ion pairs in contact are observed.

Journal ArticleDOI
TL;DR: For 30 diamond and zinc-blende-structure semiconductors, the bond length d, bond polarity alpha p, bulk modulus B, elastic shear constants (c11-c12)/2 and c44, bond-stretching force constant alpha, bond-bending force constant beta, internal displacement parameter zeta, effective atomic charge Z*, transfer parameter beta *, transverse charge eT*, and piezoelectric charge ep* are calculated from bond orbital calculations based on the tight-binding method as mentioned in this paper.
Abstract: For 30 diamond- and zincblende-structure semiconductors, the bond length d, bond polarity alpha p, bulk modulus B, elastic shear constants (c11-c12)/2 and c44, bond-stretching force constant alpha , bond-bending force constant beta , internal displacement parameter zeta , effective atomic charge Z*, transfer parameter beta *, transverse charge eT*, and piezoelectric charge ep* are calculated from bond orbital calculations based on the tight-binding method. The results are compared with previous theoretical calculations and experiments.

Journal ArticleDOI
TL;DR: In this article, a review of recent work on bilayer bulk phases is given, ranging from vesicular through planar to connected (cubic and L3) phases.
Abstract: A brief review of recent work on bilayer bulk phases is given, ranging from vesicular through planar to connected (cubic and L3) phases. The emphasis is on understanding the dilute phases. A new model is proposed for the transition from the L3 or sponge phase to a vesicular phase.

Journal ArticleDOI
TL;DR: In this article, a model for the evolution of the grain-boundary structure with misorientation angle was proposed, which regards the approach and confluence of the line defects forming the boundary, and results in a decrease of the resonance part of the total scattering cross section.
Abstract: The partial-wave method is used for the calculation of the residual electrical resistivity caused by dislocations and grain boundaries in various metals. The dislocation core is regarded as a resonance scattering line defect containing surplus free volume. Scattering by elastic distortions of the lattice is neglected. An estimation of the effective carrier concentration in transition metals is performed on the basis of Mott's theory. The proposed model for the evolution of the grain-boundary structure with misorientation angle regards the approach and confluence of the line defects forming the boundary, and results in a decrease of the resonance part of the total scattering cross section. The high-angle grain boundaries are assumed to consist of cylindrical voids. Taking into account the values of the steady Burgers' vectors, quite satisfactory agreement with the available experimental data is obtained.

Journal ArticleDOI
TL;DR: In this paper, the thermal conductivity of nominally pure KTaO3 exhibits an unexpected strong phonon scattering around 8 K. The question arises whether this scattering is related to the distorted microregions observed by other experiments.
Abstract: The thermal conductivity of nominally pure KTaO3 exhibits an unexpected strong phonon scattering around 8 K. The question arises whether this scattering is related to the distorted microregions observed by other experiments. We have investigated a set of undoped single crystals grown by different methods. By comparison with doped specimens, it is suggested that acoustic phonons are coupled to the soft optic mode via the local disorder. From complementary dielectric permittivity and electron paramagnetic resonance experiments, oxygen vacancies and rhombic Fe3+ appear to be incontestable candidates to account for the distorted microregions.