Table of contents

The phase composition and morphology of black powder samples synthesized by pyrogenation of carbon nanotubes and colloidal Fe(OH)₃ were investigated using x-ray diffraction (XRD) and transmission electron microscopy. A new kind allotrope of carbon, 'new diamond' (n-diamond), was obtained in the final black powder when the treatment temperature was higher than 1000 °C. As the treatment temperature reached 1400 °C, the peak intensity of the n-diamond achieved its maximal value and the estimated yield rate was about 20%. The average size of the n-diamond nanometric particles was around 20 nm. Using the least squares refinement method and XRD pattern simulation technology, the crystal structure of n-diamond was studied.

Density functional modelling studies of the single vacancy in large Ge clusters are presented. We take a careful look at the origin of Jahn–Teller instabilities as a function of the vacancy net charge, resulting in a variety of structural relaxations. By comparing electron affinities of the vacancy with those from defects with well established gap states, we were able to estimate three acceptor states for the vacancy at E(−/0) = E_v+0.2 eV, E(=/−) = E_c−0.5 eV and  eV. As opposed to the Si vacancy, the defect in Ge is not a donor. We also show that these dissimilarities have fundamental consequences for the electronic/atomic picture of other centres, such as transition metals in germanium crystals.

The growth of thin platinum films on a nickel (100) substrate is investigated by helium atom scattering. Growth is shown to be disordered for all temperatures between 150 and 600 K, resulting in continual surface roughening and an absence of atomically flat terraces. We separate growth into three main regimes. First, at temperatures below 250 K, growth is three-dimensional and is described using a 'hit and stick' regime of limited adatom mobility. Between 250 and 370 K, deposited films remain three-dimensionally rough but involve intermixing of Pt and Ni. At temperatures above 370 K, large three-dimensional structures are absent and alloying is extensive, occurring via a sequence of thermally activated and well-defined alloy phases. Complete alloying occurs above 650 K.

The present paper deals with a systemic investigation of the interface electronic structure of as-deposited as well as annealed Ti/Ni multilayer (ML) samples up to 400 °C using core level and valence band (VB) photoemission techniques. For this purpose [Ti(50 Å)/Ni(50 Å)] × 10 ML samples have been prepared by employing an electron beam evaporation technique under ultrahigh vacuum conditions.

The depth profile core level photoemission investigation carried out on annealed ML samples indicates a gradual change in the nature of the electronic bonding at the interface with temperature. In particular the ML samples annealed at 300 and 400 °C clearly show shifts in the Ni 2p^3/2 and Ti 2p^3/2 core levels towards the higher binding energy side as compared to as-deposited samples, suggesting the formation of a TiNi alloy phase at the interface. The corresponding VB spectra also show appreciable changes and provide strong evidence for TiNi alloy formation. Further confirmation of this alloy phase formation is clearly reflected in the x-ray diffraction measurements carried out on these samples. The recorded x-ray diffraction patterns show a solid state reaction leading to amorphization when the ML sample is annealed at 300 °C and recrystallization to a TiNi alloy phase at the annealing temperature of 400 °C.

In order to determine the charge transfer between Ti and Ni atoms in the formation of the TiNi alloy phase, the 2p^3/2 core levels and the x-ray excited Auger regions of Ti and Ni were carefully investigated. The experimentally measured core level shifts for Ti and Ni were both found to be positive, leading to the conclusion that electronegativity criteria cannot be used to decide the direction of charge transfer in this case. The observed shifts in modified Auger parameters determined from recorded experimental data show a positive value for Ti and a negative one for Ni. This provides clear evidence that the direction of charge transfer is from Ni to Ti atoms during the formation of the TiNi alloy at the interface. The charge on ionized atoms calculated by using a simple electrostatic model indicates similar trends for the charge transfer deduced from Auger parameters and chemical shifts. In addition to this, areas under the core level peaks have been calculated by employing Shirley and Touggard background methods. The difference between the backgrounds, when normalized with respect to the elemental values, provides information about the density of states at the Fermi level (E_F). The density of states at E_F calculated in this way shows reductions in values for both Ti and Ni when the ML sample is annealed at different temperatures. This is in complete agreement with corresponding theoretically calculated densities of states.

Using an extended Hubbard model, we calculate the many-body states of various transition-metal (TM) ions in the full spherical symmetry, thus accounting for the electronic correlations in TM oxides (TMOs). The influence of the symmetry of the bulk (cubic) crystal and of an (001) surface is described by a ligand-field procedure. The Slater integrals are fitted to gas phase data, EELS experiments, and low-lying states from ab initio theory. We present spectra of second harmonic generation from the bulk states and from the (001) surfaces.

We show that the migration process of a 60° shuffle dislocation in an heteroepitaxial Ge/Si_0.5Ge_0.5(001) system can be analysed by classical molecular dynamics simulations. By following the misfit segment during its motion, we build a sequence of strain maps giving detailed information about the elastic-energy relaxation in the film. The atomic-scale mechanisms underlying the dislocation motion towards the interface are also monitored, showing, for instance, that kinks are actually present along the dislocation line.

Hybrid density functional theory has been used to investigate the initial surface reaction mechanism in atomic layer deposition (ALD) of Al₂O₃ on the hydroxylated GaAs(001)-4 × 2 surface. The precursors for ALD of Al₂O₃ are trimethylaluminium (TMA) and H₂O as the aluminium and oxygen sources. For the first half-reaction between TMA with the GaAs surface the calculated activation barrier is 15.4 kcal mol⁻¹, and the H₂O half-reaction proceeds by a mechanism similar to that of the first half-reaction, resulting in an activation barrier of 27.6 kcal mol⁻¹. Both half-reactions are thermodynamically favourable, exothermic by 52.0 and 43.6 kcal mol⁻¹ relative to the reactants, respectively.

Al and N K-edge x-ray absorption near-edge structure (XANES), scanning photoelectron microscopy (SPEM) and x-ray emission measurements were performed on AlN nanotips grown on p- and n-type Si substrates (p-AlN and n-AlN). Features and intensities in the Al and N K-edge XANES spectra of these AlN nanotips overall are similar. In contrast, the intensities of the valence-band SPEM spectra of p-AlN are apparently larger than those of n-AlN, which indicates that the valence-band density of states of p-AlN exceeds that of n-AlN. This result may be related to the observed enhancement of field-emission intensity of AlN nanotips grown on the p-type Si substrate.

Pressure–composition (P–C) hydrogen absorption isotherms have been obtained for MmNi_3.5Al_0.5Fe_0.5Co_0.5+0.5 wt% B and MmNi_3.5Al_0.5Fe_0.5Co_0.5 +25 wt% polyaniline (PANI) in the ranges 30–100 °C and 0.1–40 bar using a high pressure Seiverts' apparatus. The phase structure and microstructure morphologies of as-prepared and ball-milled materials have been analysed by XRD and SEM respectively. The effect of particle size, boron addition and PANI dispersion on the hydrogen absorption kinetics in MmNi_3.5Al_0.5Fe_0.5Co_0.5 has been studied and discussed. The diffusion coefficient and activation energy of dissolved hydrogen in MmNi_3.5Al_0.5Fe_0.5Co_0.5, MmNi_3.5Al_0.5Fe_0.5Co_0.5+0.5 wt% B and MmNi_3.5Al_0.5Fe_0.5Co_0.5+25 wt% PANI have been obtained from the studies of hydrogen absorption kinetics using the first order rate equation and the results have been discussed.

We consider the free energies of the single-period (SP) and double-period (DP) core reconstructions of the straight 90° partial dislocation in silicon. The vibrational contributions are calculated with a harmonic model. It is found that it leads to a diminishing difference between the free energies of the two core reconstructions with increasing temperature. The question of the relative populations of SP and DP reconstructions in a single straight 90° partial dislocation is solved by mapping the problem onto a one-dimensional Ising model in a magnetic field. The model contains only two parameters and is solved analytically. It leads to the conclusion that for the majority of the published energy differences between the SP and DP reconstructions the equilibrium core structure is dominated by the DP reconstruction at all temperatures up to the melting point. We review whether it is possible to distinguish between the SP and DP reconstructions experimentally, both in principle and in practice. We conclude that aberration corrected transmission electron microscopy should be able to distinguish between these two core reconstructions, but published high resolution micrographs do not allow the distinction to be made.

The BPP model for nuclear spin relaxation is modified to account rigorously for jumps of hydrogen within g site hexagons in the C15 AB₂ structure. The theory is applied to proton relaxation data for low H concentrations in TaV₂H_x and ZrCr₂H_x. It is shown that the data are consistent with the results of quantum diffusion calculations, diffusivity measurements and H–H interaction effects deduced from quasielastic neutron scattering experiments.

We combine the single-site dynamical mean field theory (DMFT) with the non-local GW method. This is done fully self-consistently and we apply our formalism to a one-band Hubbard model. Eventually at self-consistency the full self-energy and polarization operator of the system are retrieved. Some numerical results, in the metallic as well as the insulator regime, are presented and briefly discussed. Depending on the involved interaction (GW) parameters, substantial changes are found when the GW self-energy is incorporated. However, the main point of this work is to demonstrate the applicability of the method, not to make any strict comparison with exact results and experiments.

The optical properties of GdFe₃(BO₃)₄ under high pressures have been investigated experimentally and theoretically, and the results are compared with the properties of FeBO₃. A model of the GdFe₃(BO₃)₄ band structure is derived within the multielectron model taking into account the strong electron correlations. Crossover of the Fe³⁺ ion high-spin and low-spin states, collapse of the magnetic moment, the relaxation Coulomb correlations, and insulator–semiconductor transition are predicted. Optical transitions in GdFe₃(BO₃)₄ and FeBO₃ under high pressures were discovered.

Raman scattering was investigated in high-quality SrBi₂Ta₂O₉ (SBT) single crystals prepared by the self-flux solution method. The Raman spectra in the range 6–1800 cm⁻¹ were monitored as a function of temperature both below and above the ferroelastic–ferroelectric phase transition at about 600 K. The behaviour of the soft ferroelectric mode and of its damping coefficient near the phase transition indicates a strong coupling with an acoustical phonon. The results provide clear evidence for a crossing over from a displacive to an order–disorder component and they are discussed in terms of a complex sequence of the phase transitions in SBT.

A new method for the calculation of parameters is proposed. The method is based on determination of the glow curve maximum and effective values of the half-width and part of the half-width on the higher temperature side. A relation between the symmetry factor as a function of the corresponding constant α = n₀/(h+n₀) and the correction factor Δ is obtained. An approximate symmetry factor function is derived, which enables analytical calculation of the parameters: activation energy E, constant α, and pre-exponential factor s^(h). An iterative procedure is developed for more precise calculation of these parameters. The new method is checked for some characteristic values of the parameters. The connection between the models of general and mixed-order kinetics has been described theoretically.

In the perfect magnesium aluminate spinel structure all the tetrahedral sites are occupied by Mg²⁺ ions, while Al³⁺ ions occupy all the octahedral sites. Real MgAl₂O₄, however, exhibits cation disorder (inversion), so that some Mg²⁺ ions reside in octahedral sites with an equal number of Al³⁺ ions in tetrahedral sites. Atomistic simulation was used to correlate the degree of inversion with changes in lattice parameter. Results from several approaches, including a combined energy minimization–Monte Carlo technique (CEMMC), are compared with available experimental data. These show that the mean field method is not useful, while the defect volume approach can yield predictions that are useful in interpreting the CEMMC results, which agree most closely with experiment.

Powder x-ray diffraction patterns of magnetite (Fe₃O₄) have been recorded at 90 K, below the Verwey transition, at three wavelengths in the pre- to mid-edge region of the 7.1 keV Fe K absorption. Simultaneous fitting of these profiles has been used to refine the anomalous scattering coefficients for the octahedral B site iron atoms. The refined values give direct evidence for a significant degree (46%) of Fe²⁺/Fe³⁺ charge ordering in magnetite, and provide new constraints on the number of possible charge ordered models.

While the energy levels in Ho:YLF have been measured previously, they have not been as thoroughly investigated in the isomorphs, Ho:LuLF and Ho:GdLF. We report here the measurement of the energy levels of the trivalent lanthanide Ho³⁺ in GdLiF₄ (GdLF), YLiF₄ (YLF), and LuLiF₄ (LuLF). The measurement of the energy levels of Ho:YLF, although they have been measured before, are repeated here for self-consistent comparison to Ho:LuLF and Ho:GdLF. The Stark split levels for the first ten Ho manifolds in these materials have been measured, and the results have been fitted to a free-ion plus crystal-field Hamiltonian to generate a theoretical set of energy levels. Crystal-field parameters were varied to determine the best fit between experimental and theoretical energy levels. The energy levels of Ho:GdLF and Ho:LuLF are seen to be very similar to those in Ho:YLF. However, subtle changes resulting from replacing Y³⁺ with Gd³⁺ or Lu³⁺ in the fluoride crystal YLiF₄ result in shorter transition wavelengths in GdLF and longer transition wavelengths in LuLF. This has implications for Ho lasers operating at ∼2.0 µm. The energy levels for Ho:GdLF and Ho:LuLF determined here indicate that Ho:GdLF will have a larger lower laser level thermal population than Ho:YLF, while Ho:LuLF lasers will have a smaller lower laser level thermal population than Ho:YLF. This is consistent with the larger Stark splitting associated with the larger host ions that Ho substitutes for in these lithium fluoride materials. The intensity parameters are also determined from a Judd–Ofelt analysis and used to calculate radiative lifetimes and branching ratios for the first ten manifolds in Ho:GdLF, Ho:YLF and Ho:LuLF.

We report on the magnetic properties of an insulating cubic compound Cu₃TeO₆ studied by ac and dc susceptibility, torque magnetometry and neutron powder diffraction. A novel three-dimensional magnetic lattice composed of almost planar regular hexagons of Cu²⁺ S = 1/2 spins is present in Cu₃TeO₆. The magnetic susceptibility in the paramagnetic state obeys the Curie–Weiss law in the 200–330 K regime with Θ_CW = −148 K and at T_N = 61 K system undergoes an antiferromagnetic phase transition. Above T_N the susceptibility is isotropic. Below T_N a large anisotropy develops in fields H≥500 Oe. Torque measurements reveal the presence of antiferromagnetic domains below T_N. In a rather low magnetic field ( Oe) switching of domains is observed. The dynamics related to movement of domain walls is very slow at low temperatures (of the order of 10² s) and interferes with all torque measurements. The presence of domains is a consequence of the symmetry of the underlying magnetic lattice. Neutron powder diffraction reveals that antiferromagnetic long-range order is associated with the wavevector . The dominant component of the magnetic moment is along one of the space diagonals of the cubic unit cell, but it is not possible to resolve whether the structure is collinear or canted.

Highly brilliant synchrotron x-ray radiation was used to measure in situ the microstructural response of a ferroelectric capacitor subjected to bipolar rectangular pulses. High-resolution x-ray diffraction experiments were performed on a (111)-oriented PbZr_0.45Ti_0.55O₃ thin film with a composition in the morphotropic region and sandwiched between two platinum electrodes. From original real time measurements, the microstructural changes with electrical cycling have been evidenced and correlated with the observed polarization fatigue measured during the x-ray diffraction experiment. From concomitant variations of the diffracted intensity and the switching current maximum, several mechanisms have been discussed as a possible origin of the polarization fatigue: field induced phase transformation, oxygen vacancy self-ordering and widening of the internal bulk screening field distribution function during cyclic switching.

The spectral momentum densities of vanadium metal and V₂O₃ are measured by electron momentum spectroscopy. Results are compared with band structure calculations based on density functional theory (DFT). Qualitatively, the agreement between theory and experiment is good. The calculated total band width of vanadium metal (6.5 eV) is in excellent agreement with the observed one (6.5 ± 0.25 eV). The splitting between the outer and inner valence bands in V₂O₃ is 2 eV larger in the experiment than in the density functional theory calculation. The observed momentum distributions agree reasonably well with the calculated distributions with the exception of the intensity of the outer valence band relative to the inner valence band in V₂O₃: the outer valence band is less intense than calculated. The momentum density near the Fermi level in V metal resembles that of atomic V 3d orbitals. However, momentum profiles of the V 3d orbitals in V₂O₃ are much more sharply peaked than the atomic 3d orbital in both the theory and experiment. Correlation effects are discussed and theoretical problems in describing EMS data from narrow band systems are identified.

Absorption, fluorescence and decay properties of erbium-doped alkali tellurofluorophosphate glasses (RTFP) with the molar compositions of 50 (NaPO₃)₆–10 TeO₂–20 AlF₃–19 RF–1 Er₂O₃ (R = Li,Na or K) have been investigated in order to utilize these materials for the development of fibre amplifiers and lasers. The Judd–Ofelt (JO) model was applied to the absorption intensities of Er³⁺ (4f¹¹) transitions to determine the JO intensity parameters Ω_λ (λ = 2,4 or 6). These evaluated intensity parameters are used to calculate the spontaneous emission probabilities and branching ratios from the excited state J manifolds to the lower lying J^' manifolds of Er³⁺ emission transitions. The radiative lifetimes of these excited states are determined from their respective spontaneous emission probabilities. Visible and NIR emission spectra have been studied for all three Er³⁺-doped tellurofluorophosphate glasses. In particular, for the emission at 1.5 µm corresponding to the transition, radiative parameters like the full width at half maximum (FWHM), emission cross-sections (σ_e) and figure of merit (FWHM × σ_e) were determined and compared with other hosts. The measured lifetimes of the ⁴S_3/2 level are compared with predicted radiative lifetimes calculated from the JO theory. The potentiality of Er³⁺-doped tellurofluorophosphate glasses for devices such as lasers and optical amplifiers is discussed.

The study by x-ray diffraction and perturbed angular correlation spectroscopy of Sr_1−xBa_xHfO₃ compounds for x = 0.12, 0.25, 0.5, 0.75 and 0.88 is presented. The hyperfine parameters are analysed in terms of ionic radii, ionic or covalent bonds, and cation–oxygen distances. A simple explanation of the behaviour of these parameters with composition is provided.

We present a study of the magnetoresistance, the specific heat and the magnetocaloric effect of equiatomic RETMg intermetallics with RE = La, Eu, Gd, Yb and T = Ag, Au and of GdAuIn. Depending on the composition these compounds are paramagnetic (RE = La, Yb) or they order either ferro- or antiferromagnetically with transition temperatures ranging from about 13 to 81 K. All of them are metallic, but the resistivity varies over three orders of magnitude. The magnetic order causes a strong decrease of the resistivity and around the ordering temperature we find pronounced magnetoresistance effects. The magnetic ordering also leads to well defined anomalies in the specific heat. An analysis of the entropy change leads to the conclusion that generally the magnetic transition can be described by an ordering of localized S = 7/2 moments arising from the half-filled 4f⁷ shells of Eu²⁺ or Gd³⁺. However, for GdAgMg we find clear evidence for two phase transitions, indicating that the magnetic ordering sets in partially below about 125 K and is completed via an almost first-order transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets and rather pronounced for the ferromagnets for low magnetic fields around the zero-field Curie temperature.

Mössbauer and magnetization measurements have been performed on the La_0.5Sr_0.5Co_0.978⁵⁷Fe_0.022O₃ compound in the temperature range 100 K< T<293 K. It was shown that the most probable electronic state of the iron probe ions is a formal 4+ high-spin one. Such electronic Fe ion configuration is associated with the metallicity and confirms the delocalized e_g electron character. The paramagnetic single-line component together with a broadened magnetic splitting was observed in the Mössbauer spectra well below the magnetic ordering temperature. This peculiarity is interpreted in terms of a La-rich inhomogeneity containing Co³⁺ ions mainly in the low-spin state as well as irregular La³⁺/Sr²⁺ ion distribution in the surroundings of the iron probe ions. The mechanism of the magnetic interactions is discussed.

Much of the material in the introduction of this article was taken from the work of N A Hill without due acknowledgement. In particular, the last 12 lines of the second paragraph and all of the third and fourth paragraphs are taken verbatim from reference [41] (numbering as in the paper).

The authors apologise to N A Spaldin (nee Hill).

Reference

[41] Hill N A 2000 J. Phys. Chem. B 104 6694