Table of contents

The crystalline and structural characteristics of aluminium oxide (Al₂O₃) thin films deposited by ultrasonic spray pyrolysis have been analysed using conventional and high resolution transmission electron microscopy. The aluminium oxide films were deposited on silicon (100) wafers, using a solution of aluminium acetylacetonate in dimethylformamide. The films were deposited with and without the addition of water mist generated in parallel to the spraying solution. The substrate temperatures during deposition of the films were in the range of 500-650 °C. The results indicate that the thin films deposited without water mist have an amorphous structure, while those films deposited with the addition of water mist show a two-face nature, formed by small crystallites embedded in an amorphous matrix. The crystalline phase has been determined, through the indexing of the electron diffraction patterns. It is found that it corresponds to 5Al₂O₃ · H₂O in a Tohdite hexagonal structure with unit cell lattice constants a = 5.575 Å and c = 8.76 Å.

The ground-state properties of the spin-1 pyrochlore antiferromagnet are studied by applying the VBS-like tetrahedron-unit decomposition to the original spin system. The symmetrization required on every vertex is taken into account by introducing a ferromagnetic coupling. The pairwise effective Hamiltonian between the adjacent tetrahedrons is obtained by considering the next nearest neighbour and the third neighbour exchange interactions. We find that the transverse component of the spin chirality exhibits a long-range order, breaking the parity symmetry of the tetrahedral group, while the chirality itself is not broken.

This paper emphasizes once more the many strands which go into creating the unique and complex nature of the mixed-valent HTSC cuprates, above T_c as below. Clearly it is not sensible to look in isolation to the lattice, magnetic or electronic aspects of the situation. Circumstances are too tightly coupled and interdependent. What one is in a position to do is to ascribe some pre-eminence in these matters. The line taken in this paper, as in its predecessors, is that charge constitutes the prime mover, but this taken within a chemical context, relating to bonding effects in the copper oxides and to a crucial shell-filling negative-U term of large magnitude. What dictates the uniqueness of copper in this is its particular position within the periodic table at the end of the 3d series. This provides access to (i) three stable valencies, (ii) to the limited metallicity of tight-binding, mixed-valent YBa₂Cu₃O₇, etc, (iii) to the p/d hybridized limitation of magnetic behaviour, and (iv) above all to the shell closure effects, with their dramatic energy adjustments of the relevant double-loading states when sited in high-valent local environment. This behaviour rests on the inhomogeneous electronic and structural conditions existing within these tight-binding mixed-valent systems. Normally such systems show strong positive-U behaviour and are magnetic. Here the shell-filling effects negate this U term to leave a net negative U_eff of -3 eV per pair. That positions the bosonic electron pairs degenerate with E_F. RVB spin coupling within the stripe phase geometry set up by the doped charge removes much spin-flip pair breaking, while the adopted 2D crystal structure with its saddle-point dominated FS provides the ideal k-space geometry from which to secure the negative-U driven pair formation. The Jahn-Teller effect associated with the d⁹ electron count is crucial in upholding the two-subsystem nature of the HTSC materials.

This paper looks at the support for this scenario that can be extracted from the recent work of Corson, Li, Mook, Valla, Norman, Varma, Gyorffy and many others. To the author it remains a mystery why other researchers have not examined the potential of this route for understanding the unique characteristics of the HTSC cuprates.

Magnetorheological (MR) fluids, which can rapidly be changed from a liquid state to a solid state and vice versa by a magnetic field, have the potential to revolutionize several industrial sectors. The key issue is to enhance their yield shear stress. This paper reviews the physical mechanism and microstructure of MR fluids. It finds that the weak points of the MR microstructure under a shear force are at the chains' ends. Hence, a general technique, a compression-assisted-aggregation process, is developed to change the induced MR structure to a structure that consists of robust thick columns with strong ends. The scanning electronic micrographic (SEM) images confirm such a structure change. With this approach, MR fluids become super-strong. The enhanced yield stress of MR fluids reaches 800 kPa at a moderate magnetic field.

High-energy electromagnetic radiation scattering techniques have been used to measure the structural differences between four isotopic samples of methanol (CH₃OH, CD₃OD, CH₃OD and CD₃OH). The first series of experiments employed room temperature and ambient pressure. The carbon-oxygen intramolecular bond length was measured and found to depend more strongly on the isotopic substitution at the hydroxyl site than at the methyl sites. The oscillations in the isotopic difference of the x-ray structure factor, ΔS_X(Q), are shown at room temperature to be about 2% as large as the oscillations in the total structure factor. Our uncertainties are an order of magnitude smaller than those of previous gamma ray measurements (Benmore C J and Egelstaff P A 1996 J. Phys.: Condens. Matter8 9429-32). A second series of experiments was carried out at -80 °C at its vapour pressure in order to study the significant temperature dependence of these effects. The ΔS_X(Q) difference at -80 °C is shown to be up to three times larger than the room temperature difference. These studies showed that isotopic structural differences in methanol may be represented as temperature shifts that vary as a function of thermodynamic state and substitution site.

The high-precision structural measurements of several methanol isotopes described in paper I are Fourier transformed to obtain their corresponding pair correlation functions. At room temperature we have observed a structural isotopic difference depending on the methanol isotopes used ranging between 2 and 5% at intramolecular distances and between 5 and 8% at intermolecular distances relative to the magnitude of (g(r)-1) for CH₃OH. For methanol at -80 °C, a maximum effect of 20% has been observed. All these effects may be explained in terms of changes in ground state librational motions and perturbations to the hydrogen bonding structure. The effects are compared with structural changes caused by temperature shifts and are shown to agree with the reciprocal space studies in paper I.

With the help of an improved internal friction method, it was found that a peak could appear, as the temperature increased, above the liquidus in each of the internal friction curves of Pb-Bi alloys. The features of this kind of peak are similar to those found during the solid-liquid transition in the same experiments, and in accordance with the characteristics from solid-solid transitions verified by previous investigators. The results suggest that structural changes take place to some extent in the molten alloys as a function of temperature, which have been confirmed by the corresponding calorific peak in a differential scanning calorimeter. This phenomenon might help in understanding the nature of molten alloys and their melting mechanism as well. In addition, the internal friction technique proved to be quite sensitive to liquid structures and effective for studying liquid materials.

The traditional phenomenological asymmetry between melting and freezing has been studied by varying the crystal nucleation behaviour during freezing and the liquid nucleation behaviour during melting. The existence of a series of freezing peaks, whose shapes resemble normal melting peaks of bulk In, of In melt with pre-existing crystal nuclei has been demonstrated experimentally by means of accurate differential scanning calorimetry (DSC). A superheating peak has been investigated by using DSC for In nanoparticles embedded in an Al matrix. A phenomenological kinetic symmetry between melting and freezing has been demonstrated using DSC curves, which suggests that the melting and superheating behaviour of metal can be interpreted in terms of non-nucleation, heterogeneous nucleation and homogeneous nucleation models, respectively.

We study the slow relaxation of stress in polydomain acrylate liquid crystalline elastomers undergoing the alignment transition under an imposed extension. We analyse the long-time stress relaxation, the slow approach to the mechanical equilibrium and the role of time-temperature superposition. By building the master curves, we investigate extrapolated time intervals and show the presence of two distinct relaxation regimes. At the first stage, the fast power-law relaxation of stress, with the exponent 0.67, means that directional changes in nematic domains are dominant. At very long times, we find that a different, slow power law (with the exponent 0.15) becomes the dominant mode, similar to the classical results in isotropic rubbers. Model equilibrium stress-strain curves have been obtained by extrapolating the master curves. It appears that, at a true mechanical equilibrium, one finds no mesogenic effects in stress-strain, meaning that the non-trivial nematic effects could be transient, locked by network entanglements, but capable of completely relaxing by (very slow) rearrangement of network chains.

Quantum-size confinement of excitons in CuBr nanocrystals embedded in polymethyl methacrylate has been studied. Both the absorption band and the photoluminescence (PL) band of the Z_1,2 free excitons in the nanocrystals show a blue-shift due to a quantum-size effect, and the Stokes shift of the PL band becomes larger with decrease in the nanocrystal size at 77 K. These blue-shifts are interpreted by application of size quantization in the exciton dispersion on the basis of Cho's k-linear theory.

A positron annihilation lifetime spectroscopic (PALS) study was carried out on a typical representative of simple hydrogen-bonded glass formers - glycerol - over a wide temperature region from 100 K up to room temperature. Several crossover temperatures, T_bi, were identified in the temperature dependences of the ortho-positronium (o-Ps) lifetime and the relative o-Ps intensity. The onset temperature of free-volume change, 137 K, was compared with the value of 135±3 K obtained from extrapolating thermodynamic data and with the temperatures from relaxation data. In order to determine the influence of the free-volume evolution on the relaxation dynamics above the glass transition temperature T_g, the free-volume fraction from the PALS data was related to the behaviour of the dielectric loss of glycerol. A new relationship between the temperature parameters of the onset of free-volume change and of the high-frequency wing of the dielectric loss is presented. Taking the PALS free-volume data as input for a free-volume model (Doolittle ansatz), it was found that the relaxation times τ_α can be reproduced above the bend temperature T_b2 = 241 K; the latter lies in the vicinity of crossover temperatures reported from mode coupling theory analyses of dynamic data. In the lower-temperature region T_b2>T>T_b1≅T_g, a contribution from thermally activated mobility has to be included to reproduce the dielectric relaxation time data.

Forty five years ago, an intense light scattering was observed at the α-β transition of quartz but the origin of this opalescence has remained mysterious for a long time. Recently Saint-Grégoire et al and Aslanyan et al have explained the origin of the opalescence by introducing new incommensurate (`inc') phases with ferroelastic properties in the transition region. In this paper we recall the main features of the α-β transition, of the inc phase and of the opalescence of quartz, which presents different properties in two regions of the α-inc phase boundary. We also describe the three typical structures observed in the phase boundary regions by electron microscopy. We present briefly the two previous ferroelastic models and we propose our own explanation for the origin of the opalescence. We discuss the relations of these three models with experimental results concerning thermal behaviour, microscopic structures and the origin of the refractive index variations. Most experimental results are in agreement, at least qualitatively, with our model where the two opalescence regions correspond respectively to the presence of inc rotation patches and of irregular Dauphiné microtwins, both in a non-equilibrium state.

Nanostructured zirconia doped with 5% yttria was prepared by a co-precipitate method. The structures were characterized by x-ray diffraction and extended x-ray absorption fine structure spectroscopy. The sample calcined at 300°C had an amorphous phase and a local structure like cubic zirconia. It crystallized into tetragonal nanocrystalline zirconia at 500 °C and the monoclinic phase appeared at 900 °C. The Y atoms merged into the zirconia network at 300 °C, with the oxygen vacancies located around the Zr atoms. The coordination number of the cation-cation reduction is due to the large amount of cations near the surface with fewer nearest neighbours when the grain size decreases.

Combined x-ray diffraction and x-ray absorption results are presented on the substitutional Ce valence transition in CeNi_xCu_5-x as a function of applied external pressure. No significant influence of pressure on the valence in the near-transition region of x is found in the crystal structure or in the Ce K-edge position. This result is discussed in the context of valence instabilities in intermetallic Ce compounds.

The kinetic of decomposition in the quasi-binary ionic system AgCl-NaCl has been studied by means of time-resolved neutron diffraction and small-angle neutron scattering. The coherent critical point is estimated as 447±3 K, 24 K below the upper critical binodal temperature. The chemical decomposition is almost completed in the first 200 s irrespective of the temperature. The mechanical relaxation of the lattice, however, takes place on a much longer timescale and is dominated by coherency strains leading to metastable intermediate states.

Deformation properties of multi-wall gold nanowires under compressive loading are studied. Nanowires are simulated using a realistic many-body potential. Simulations start from cylindrical face-centred cubic (111) structures at T = 0 K. After annealing cycles, axial compression is applied to multi-shell nanowires for a number of radii and lengths at T = 300 K. Several types of deformation are found, such as large buckling distortions and progressive crushing. Compressed nanowires are found to recover their initial lengths and radii even after severe structural deformations. However, in contrast to the case for carbon nanotubes, irreversible local atomic rearrangements occur even under small compressions.

A new variant of the recently developed energy-partitioning scheme is introduced, which allows us to give an interpretation of ab initio total energy results in a chemical language. In this scheme the energy of the bonds between atom-localized orbitals is represented by the covalent bond energy, which is invariant with respect to a constant shift of the effective crystal potential. This feature is a precondition for a comparison of the bond energies for various crystal structures within the framework of a band structure calculation. The implementation in a mixed-basis pseudopotential code is described, which requires the projection of the crystal pseudowavefunctions onto a minimal set of atom-localized non-orthogonal basis functions.

The new variant of the energy partitioning scheme introduced in part I of this paper is applied to discuss the relative stabilities of the cubic L1₂ structure and the tetragonal D0₂₂ structure of TiAl₃ and ScAl₃ and the stabilization of the L1₂ structure of TiAl₃ by alloying with divalent transition metal atoms. Furthermore, a striking difference between the covalent bonding energies of TiAl₃ and ScAl₃ on the one hand and Ni₃Al on the other hand is found.

The images of three-dimensional 180° and non-180° (90°) domain configurations on the unpolished and unpoled (111) face of 0.92PZN-0.08PT rhombohedral relaxor-ferroelectric crystal have been obtained by environmental scanning electron microscopy for the first time. The size of the 180° domains ranges from 10 to 50 µm, and that of the non-180° domains from 1 to 10 µm. The boundaries of the crossed three-dimensional non-180° domain bands are nonparallel and have a triangular shape. This unique non-180° domain architecture is different from the 109° or 71° domain structure in normal rhombohedral ferroelectric materials, and is helpful for completely relaxing the additional stresses due to domain deformation in 0.92PZN-0.08PT crystals.

Raman spectroscopic measurements on negative-thermal-expansion (NTE) material zirconium tungstate Zr(WO₄)₂ at 20 K over the complete range of phonon frequencies yield 39 out of the predicted 54 optical phonons. The modes are assigned as lattice modes, and translational, librational and internal modes of the WO₄ ion. High-pressure measurements in a diamond anvil cell (DAC) have revealed that in addition to the low-frequency rigid-unit modes (RUMs) several other phonons, including the bending modes of the WO₄ ion, also exhibit negative Grüneisen parameter in the cubic phase. In the high-pressure orthorhombic phase above 0.3 GPa splitting of phonon modes is found to be consistent with the lowering of symmetry. Pressure-induced amorphization in this system at 2.2±0.3 GPa is argued to arise because a pressure-induced decomposition of the compound into a mixture of ZrO₂ and WO₃ is kinetically constrained. The temperature dependence of specific heat and thermal expansion coefficient are calculated and compared with reported results. In contrast to earlier models and calculations, which considered only the phonons below 8 meV (64 cm^-1) to explain the NTE, it is shown that modes much higher than 8 meV also contribute significantly to NTE in this material.

The hydrogen solubility properties of solid-solution ternary Pd_1-x-yDy_xNi_y (x = 0.020,0.040; y = 0.0555,0.111) alloys are investigated in the ranges 473⩽T (K)⩽873 and 0.05⩽P(bar)⩽25 respectively. From the lattice parameters of the alloys and the thermodynamic parameters of the alloy-hydrogen system, it has been found that the hydrogen solubility properties of these alloys are determined mainly by electronic effects. The relative partial molar enthalpy in the two-phase region implies that hydrogen atoms have a preference towards occupation of the interstitial sites surrounded mostly by Ni atoms.

We have measured the directional Compton profiles of a single crystal of hexagonal zinc along the [00·1], [10·0], [11·0] and [11·1] directions using high-energy (662 keV) gamma radiation from a ¹³⁷Cs isotope source. The experimental data have been compared with the corresponding theoretical Korringa-Kohn-Rostoker semi-relativistic calculations. The theory slightly overestimates the electron momentum densities at low momenta regions for all measured profiles. The directional difference profiles, both experimental and theoretical, show very small anisotropy of the electron momentum density in hexagonal zinc, at most half of that presented in the literature for cubic systems.

Different charged forms of substitutional oxygen in diamond are examined using ab initio plane-wave pseudopotential calculations. The results show that two defect levels are associated with substitutional oxygen and that the charged form of the defect depends sensitively on the Fermi level. One of the defect states lies well in the energy gap and the other is located near the conduction band edge. The positively charged form of the oxygen defect is suggested to be responsible for an optical absorption band occurring at 2.6 eV. It is shown that oxygen in the lattice vacancy exhibits an amphoteric behaviour in diamond.

The low-temperature specific heat and electrical resistivity of the polycrystalline non-stoichiometric manganites La_0.95-xSr_xMnO₃ have been investigated in the doping region x = 0.00-0.30. The specific heat has terms proportional to T and T³. The resistivity of the samples decreases as T^1/2 with increasing temperature, goes through a minimum and then increases proportionally to T³. The temperature T_min , corresponding to the minimum of the resistivity, shifts with Sr content as T_min ~x^-2/5.

Results of experimental observation of high-field thermal-diffusion autosolitons (ASs) in an electron-hole plasma (EHP) in p-Si samples are reported. The EHP was photogenerated by light and heated by an electric field directed along the ⟨100⟩ axis. It is shown that the EHP is stratified with the formation of a `comb' of five or six static ASs, if the EHP concentration and applied voltage exceed some threshold values, correspondingly. The electric field strength inside the ASs lies between 2000 and 6000 V cm^-1, whereas on the outside it spans from 50 to 100 V cm^-1. The high strength of the field inside the ASs is caused mainly by the high temperature of the carriers in these domains which may reach up to 1000 K. The multi-valley structure of Si energy bands is qualitatively reflected in both the AS formation dynamics and their further evolution affected by the inter-valley electron transitions in the high-field conditions inside the ASs.

The contribution of the electron-phonon interaction to the energy of a unidirectional charge-ordered state (stripe phase) of two-dimensional electrons in GaAs heterostructures is analysed. The dependence of the energy on the direction of the electron-density modulation is calculated. It is shown that in electron layers situated close to the (001) surface the interference between the piezoelectric and the deformation potential interaction causes a preferential orientation of the stripes along the [110] axis.

We study the temperature dependence of the mobility for elastic scattering in quantum wells. Due to anomalous screening in two-dimensional systems the mobility decreases linearly with temperature. The parameter for this linear temperature dependence is a function of well width and of carrier density. It is expressed in terms of the density dependence of the form factor for finite width effects and by the local-field correction for many-body effects (exchange and correlation). We argue that alloy-disorder scattering should lead to the linear temperature dependence of the mobility.

We study the energy spectrum of the two-electron spherical parabolic quantum dot using the exact Schrödinger, Hartree-Fock and Kohn-Sham equations. The results obtained by applying the shifted-1/N method are compared with those obtained by using an accurate numerical technique, showing that the relative error is reasonably small, although the first method consistently underestimates the correct values. The approximate ground-state HF and local-density KS energies, estimated using the shifted-1/N method, are compared with accurate numerical self-consistent solutions. We make some perturbative analyses of the exact energy in terms of the confinement strength, and we propose some interpolation formulae. A similar analysis is performed for both mean-field approximations and interpolation formulae are also proposed for these exchange-only ground-state cases.

We report here ab initio density functional theory study of the electronic band structure and electric field gradient (EFG) in MgB₂ under pressure. The band structure calculations are in agreement with other recent calculations. The superconductivity in MgB₂ is related to and dominated by the existence of boron σ p_x,y-band holes at the Γ point, with negligible contribution from the Mg ions. The character of the σ band is unchanged even after application of pressure, although there is a shift of position and an increase of dispersion. The calculated density of states decreases with pressure which, in conjunction with the Bardeen-Cooper-Schrieffer theory, agrees with the trend of the experimental T_c versus pressure data. The broad bump in T_c(P) data observed by Tissen et al near 9 GPa is not indicated in the present band structure study. The EFG at the B site is nearly constant as a function of pressure and that of Mg changes by ~34% over the pressure range considered. The present result indicates that the B electronic system does not change much under pressure up to ~38 GPa, which confirms one reported study but disagrees with the other.

We investigate the quasiparticle spectrum near surfaces in a two-dimensional system with d-density-wave order within a mean-field theory. For Fermi surfaces with perfect nesting for the ordering wavevector (π,π) of the d-density wave, a zero-energy bound state occurs at 110 surfaces, in close analogy with the known effect in d-wave superconducting states or graphite. When the shape of the Fermi surface is changed by doping, the bound-state energy moves away from the Fermi level. Furthermore, away from half-filling we find inhomogeneous phases with domain walls of the d-density-wave order parameter. The domain walls also support low-energy bound states. These phenomena might provide an experimental test for hidden d-density-wave order in the high-T_c cuprates.

We performed a first-principles calculation of the electronic band structure of MgB₂, a superconductor discovered recently, by employing a full-potential linear muffin-tin orbital method. In advance, the influences of the lattice parameter on the electronic structure are investigated with different volumes and c/a ratios. The effects of substitution on the electronic structure are discussed in relation to the variation of the lattice parameters and electron filling which is investigated via a Mg pseudo-atom. The variations of the two-dimensional covalent σ-band and three-dimensional metal π-band are investigated in detail at several high-symmetry points of the Brillouin zone (BZ). These results show that the variations of the electronic structure of MgB₂ are mainly determined by the effect of the c/a ratio. Considering the variation of the Fermi surface at the M and Γ points of the BZ, the corresponding electronic topological transition (ETT) induced by the lattice and doping are discussed. The ETT at the M point should be related to the structural phase transition in Al_xMg_1-xB.

Expansion of the superlattice of boron layers, with AB₂ structure, due to different intercalated A atoms has been studied to understand the emergence of high T_c superconductivity in the diborides. The structure of these metal heterostructures at the atomic limit (MEHALs) (with A = Al, Mg, Ti, Hf, Zr) has been measured by synchrotron x-ray diffraction. The increasing atomic radius of the intercalated A ions induces an increase of (1) the separation between the boron layers and (2) the tensile micro-strain ε of the B–B distance within the boron layers. The results show that the superconductivity in these MEHALs appears in a critical region in a phase diagram controlled by two variables, the micro-strain and the charge density (ε, ρ).

We consider a simple cubic magnetic cluster with spin-1/2 ions at its eight corners. The superexchange Hamiltonian employed in this paper involves nearest, next-nearest (nn), and next to next-nearest (nnn) neighbour interactions. These competing exchange interactions are found to generate a number of phases with ground states: (a) ferromagnetic; (b) nn-neighbour dimers; (c) regular antiferromagnetic; (d) nnn-neighbour dimers; and (e) nnn-neighbour forming triplets. We have also found fully polarized and partially polarized lowest spin excitations in the broken symmetry phases. The effects of the low-energy characteristics in various phases have been quantified by computing thermodynamic properties, such as magnetization and susceptibility.

Manganates of the compositions Nd_0.5-xLa_xCa_0.5MnO₃ and Pr_0.5-xLa_xCa_0.5 MnO₃, with the average A-site cation radius, ⟨r_A⟩, in the range 1.17-1.20 Å, are charge ordered at ordinary temperatures (T_CO~240 K), and undergo a re-entrant transition to a ferromagnetic state on cooling (T_C<T_CO). The ferromagnetic Curie temperature, T_C, of the re-entrant transition increases markedly with x or ⟨r_A⟩ in the two series of compounds. The plots of T_C and T_CO against ⟨r_A⟩ show that the two curves intersect around a ⟨r_A⟩ value of 1.195 Å, below which T_CO>T_C. Site disorder due to the size mismatch of the A-site cations, σ², has a marked effect on the re-entrant transition temperature, T_C. Thus, in a series of manganates of the type Ln_0.5A_0.5MnO₃ with a fixed ⟨r_A⟩ value of 1.185 Å, the T_C decreases markedly with increase in site disorder, suggesting that the re-entrant transition can be entirely suppressed at a sufficiently high value of σ². Between T_CO and T_C, the CO and FM states are likely to coexist, the coexistence temperature regime decreasing with increasing ⟨r_A⟩, and increasing with σ² at a fixed ⟨r_A⟩.

Starting from an antiferromagnetic Heisenberg Hamiltonian for the 15 spin-1/2 ions in V₁₅, we construct an effective spin Hamiltonian involving eight low-lying states (spin-1/2 and spin-3/2) coupled to a phonon bath. We numerically solve the time-dependent Schrödinger equation of this system, and obtain the magnetization as a function of temperature in a time-dependent magnetic field. The magnetization exhibits unusual patterns of hysteresis and plateaus as the field sweep rate and temperature are varied. The observed plateaus are not due to quantum tunnelling but are a result of thermal averaging. Our results are in good agreement with recent experimental observations.

The spectral density functions for the relaxation of nuclear spins due to time-dependent electric quadrupole interactions are studied for the diffusion of atoms on a crystal structure. It is shown that, for an arbitrary concentration c of diffusing atoms, the spectral density functions and consequent relaxation rates scale with concentration as c(1-c). The detailed forms of the functions can be obtained simply from analysis of the low spin concentration limit, unlike the analogous case for magnetic dipolar relaxation. Some applications to metal-hydrogen systems are described.

Features of the diffuse phase transition in lead magnesium niobate and strontium barium niobate, typical relaxor ferroelectric materials, were studied as a function of temperature and frequency. A new empirical equation for a phenomenological description of the temperature dependence of the dielectric permittivity (ε') peak is proposed. In fact, the proposed equation provides an excellent fitting of the experimental curves at temperatures into and above the dielectric dispersion region, enabling us to calculate some characteristic parameters of the phase transitions in ferroelectric materials.

The pressure-induced sequence of phase transitions of the BaF₂ fluorite was studied within the shell-model approach. This fluorite crystal presents two pressure-induced phase transitions at approximately 3 and 15 GPa. The interatomic potentials were calculated by using relevant physical properties measured at ambient conditions. These potentials were used to minimize the lattice enthalpy at high hydrostatic pressures. By comparing the enthalpies and lattice parameters of the three possible structures, it was possible to describe the complete phase transition sequence of the material. The model reliability is confirmed by a comparison with experimental results reported at ambient conditions, which are in good agreement with the model predictions.

SiO₂/Si/SiO₂ nanometer double barriers (SSSNDB) with Si layers of twenty-seven different thicknesses in a range of 1-5 nm with an interval of 0.2 nm have been deposited on p-Si substrates using two-target alternative magnetron sputtering. Electroluminescence (EL) from the semitransparent Au film/SSSNDB/p-Si diodes and from a control diode without any Si layer have been observed under forward bias. Each EL spectrum of all these diodes can be fitted by two Gaussian bands with peak energies of 1.82 and 2.25 eV, and full widths at half maximum of 0.38 and 0.69 eV, respectively. It is found that the current, EL peak wavelength and intensities of the two Gaussian bands of the Au/SSSNDB/p-Si structure oscillate synchronously with increasing Si layer thickness with a period corresponding to half a de Broglie wavelength of the carriers. The experimental results strongly indicate that the EL originates mainly from two types of luminescence centres with energies of 1.82 and 2.25 eV in the SiO₂ barriers, rather than from the nanometer Si well in the SSSNDB. The EL mechanism is discussed in detail.

Erratum

In the 10 September 2001 issue of Journal of Physics: Condensed Matter volume 13 number 36, a cluster of papers appeared under the heading `Battery materials'.

It should have been pointed out that these papers were from a symposium on battery materials at the American Crystallographic Association meeting held in St Paul, Minnesota, in July 2000.

The Preface to this issue was omitted. It should read as follows:

Preface

The battery materials symposium was held in St Paul, Minnesota in July of 2000 as part of the American Crystallographic Association annual meeting. The symposium was organized by Jacqueline A Johnson of Argonne National Laboratory (ANL) who was Chair of the Amorphous Materials Special Interest Group (AMSIG) within the ACA. She would like to thank all the speakers who participated so enthusiastically in the symposium and the numerous sponsors who kindly contributed to it's success: ACA (American Crystallographic Association), NASA (National Aeronautics and Space Administration), NIST (National Institute of Standards and Technology), NSF (National Science Foundation) and PRF (Petroleum Research Fund).

Jacqueline A Johnson

Argonne National Laboratory

Also, the paper `Nuclear magnetic resonance studies of nanocomposite polymer electrolytes' by S H Chung et al was omitted from the cluster. It follows this erratum.

The origin of the ionic conductivity enhancement in polymer electrolytes that occurs on adding inorganic oxide powders was explored by ¹H and ⁷Li nuclear magnetic resonance. Ionic and molecular self-diffusion coefficients determined by pulsed field gradient spin-echo measurements demonstrate that lithium ionic diffusivity is enhanced in the composites, but this enhancement is not attributed to polymer segmental mobility. Two different systems were investigated: a high-molecular-mass poly(ethylene oxide)-LiClO₄ complex with nanoscale TiO₂; and a low-molecular-mass poly(ethylene glycol)-LiClO₄ solution with Al₂O₃. In the latter case the effect of varying the alumina surface acidity or basicity was considered.

The PDF file contains web links to all the articles in this volume.