Table of contents

The structure of has been investigated by high-temperature neutron diffraction and molecular dynamics simulation. It was found that the apparent Mg - F bond contraction reported in previous work arises from large anisotropy in the distribution of ions. The cell parameter in the cubic phase is approximately 2% less than twice the average Mg - F bond distance. The consequences for computer simulation of perovskites of this dynamically induced reduction in cell volume are briefly discussed.

The electronic structure of n - i - p - i Si superlattices is investigated by the tight-binding renormalization method. Strong anisotropy of the hole masses in the in-plane direction is found and strong localization of the electron and hole wave-functions in the growth direction is reported. For the structure studied, a type-II configuration is found with nearly zero overlap between electron and hole wave-functions. The latter property is discussed in view of its possible device applications.

The equation of motion of a fluxoid with viscous forces is solved to calculate the effective London penetration depth which is found to vary as the square root of the magnetic field. The London penetration depth as a function of magnetic field in single crystals of is in reasonable agreement with the theory.

We study two coupled XY spin chains in the continuum limit with ferromagnetic interaction both along and between the chains. We find twist soliton solutions for the difference in orientation angle of the spins along the chains. If we allow the chains to be elastic, they will deform (move apart) in the region of the soliton in order to reduce magnetic energy. The extent of deformation is a result of a balance between the gain in magnetic energy and elastic energy cost. We also generalize these results to the case of a soliton lattice.

We report mesoscopic surface morphologies that occur on Nb surfaces during normal epitaxial growth on miscut sapphire. These features are remarkably effective as growth templates which give selectivity up to approximately : 1 between the two alternative stacking sequences of fcc metals, when grown heteroepitaxially on Nb.

We have measured the electronic resistivity and thermopower of liquid , and alloys for c < 0.4. In all of the alloys the resistivity is very similar, rising smoothly and steeply as c increases, but at all times remaining less than . The thermopower is also featureless and metallic. However, the temperature coefficient, , displays in the Cu alloys a minimum for (RE = rare earth), which furthermore is negative for ; a negative temperature coefficient is also seen for . We also present the results of an investigation of neutron diffraction scattering from liquid , and the results of a reverse Monte Carlo (RMC) analysis of the structure data, which shows evidence of ordering in the liquid, with characteristics similar to those of crystalline . We discuss several possible explanations for the negative temperature coefficient of both Ag and Cu alloys, and conclude that its origin lies in the presence of chemical short-range order.

The velocity autocorrelation functions and the corresponding memory functions for a set of liquid metals and Lennard-Jones fluids have been calculated using a mode-coupling theory. The data required for the theoretical calculations have been obtained from molecular dynamics simulations. The influence of both the short repulsive wall and the attractive well of the potential on the binary and mode-coupling terms of the memory function has been analysed. The mode-coupling theory has been tested by comparing the theoretical results with those directly obtained from computer simulations. The most marked discrepancies correspond to systems showing velocity autocorrelation functions with weak backscattering. In the case of the Lennard-Jones fluids, the binary term of the memory function is less well described using a gaussian function.

We present an ab initio Hohenberg - Kohn - Sham density functional study of structure and electrical conductivity in hot dense hydrogen. Our study covers the density range 0.40 - 1.34 ( = 1.5 - 1, P = 1.5 - 24 Mbar) at temperatures T = 800 - 3000 K. In this range both a molecular - atomic and an insulator - metal transition are expected to take place. Our results are compared with recent double-shock experiments in the hot fluid phase. We observe an increase in of an order of magnitude between and 1. At the lowest density, hydrogen is a molecular liquid below about 1100 K and continuously atomizes with increasing T. At the highest density, the sample is a monatomic liquid metal with high coordination number at all T. The metallic fluid is atomic, not molecular, for all densities investigated.

The collective atomic dynamics of liquid has been investigated at 920 K using cold neutron inelastic scattering techniques. The isothermal and the adiabatic dispersion curves together with the damping function of the viscosity have been obtained from a fit of Lovesey's model to the measured total dynamic structure factor . The comparison of the total structure factor S(Q) and the dispersion obtained from the maxima of the current - current correlation function for liquid with the same functions obtained before for pure rubidium shows a small shortening and strengthening of the bonds in liquid with respect to the bonds in pure liquid Rb, even though the character of the alloy is still metallic in general at this small Sb concentration.

We have performed the first measurement of the Soret effect in a molten salt mixture under microgravity conditions during two different flights (D1 and D2) of the spacelab. After justifying our selection of the molten salt used (AgI/KI) and a description of the microgravity facility, we give the results of a three-dimensional simulation of combined convection phenomena and thermal diffusion effects. This simulation verifies the feasibility of the experiment and aids selection of the various geometric and physical parameters of the experiment.

The Soret coefficient deduced from these measurements is positive, corresponding to a heavy component migrating towards the cold end of the cell. Its value deduced from the thermopower is slightly different for the two flights ( for D1 and for D2) while the salt analysis after return to Earth gives the value but for an experiment duration corresponding to about one time constant. Its is concluded that useful measurements of thermodiffusion are really possible in good conditions in space, but blind measurements must necessarily be confirmed by optical measurements in order to demonstrate the possible convection of the liquid and to know the real level of separation attained, compared with the perfectly nonconvective situation.

We present the first calculations and computer simulations of the static structure and ionic transport properties of molten thallium halides near melting. The calculations have been carried out using the hypernetted-chain theory of liquids (HNC), and for the simulations we have used molecular dynamics (MD). The potentials used for the calculations have the same functional form as the semiempirical potential originally proposed by Vashishta and Rahman (Vashishta P and Rahman A 1978 Phys. Rev. Lett. 40 1337) for studying .

The total structure factors obtained from our calculations are in fair qualitative agreement with available neutron scattering data. The local structures of these melts exhibit a behaviour intermediate between those of the noble-metal halides and the alkali halides.

The mean square displacements, velocity autocorrelation functions and distinct correlation functions confirm further this intermediate behaviour suggesting a rather complicated diffusion mechanism where mass and size effects compete strongly. The results for the specific ionic conductivities are in good agreement with experiment if it is assumed that the ions, in their transport, have an integer charge of magnitude | Z | = 1, rather than the magnitude of the effective charges used in the potentials.

The immiscible alloys have been investigated by x-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM). The TEM results show that the smaller size nanocrystalline particles are obtained by milling for 160 h for Fe - Cu alloys with higher Cu concentration . The XAFS results indicate that the local structure of Fe atoms in gradually changes from a bcc structure to a fcc one while the Cu atoms maintain the original coordination geometry. In contrast, the local structures around both Fe and Cu atoms in alloys which have a larger average grain size indicated a bcc structure after 160 h milling. This suggests that in a mechanical alloying process the grain size strongly affects the alloying limit in the immiscible region. Possible mechanisms of bcc-to-fcc change in are discussed in relation to the interdiffusion and the internal pressure effect as a result of small grain size.

The structural and cohesive properties of more than thirty transition-metal sulphides of various stoichiometries and crystal structures have been investigated using density functional theory, with the aim of establishing a correlation between the strength of the metal - sulphur bond and the catalytic activities of these materials. It is shown that the local density approximation has a tendency to overestimate the strength of the bonding. The overbinding manifests itself in the prediction of too small atomic volumes and too large cohesive energies. Non-local corrections to the local exchange - correlation functional in the form of a generalized-gradient approximation correct the overbinding (albeit with a certain tendency to overcorrect, especially for the sulphides of the heavy transition metals) and result in accurate structural prediction and cohesive energies. A correlation between the sulphur - metal bond strength and the catalytic activities is established.

A study of the electronic structure of about thirty transition-metal sulphides (TMS) of various stoichiometries and crystal structures is presented, supplementing recent studies of their structural and cohesive properties (P Raybaud, G Kresse, J Hafner and H Toulhoat, preceding paper). The electronic structure of the TMS is found to be determined by short-range interactions in the S 3p - TM d band complex, with the ligand-field splitting of the TM d states in the environment of the S atoms determining the structure of the d band. For the layered group VI disulphides, for and for the group VIII pyrites this leads to the formation of a gap at the Fermi surface. Semiconducting properties are predicted also for the monosulphides PtS and PdS and for and . We show that the semiconducting TMS have a higher catalytic activity for hydro-desulphurization than the metallic sulphides. We suggest a correlation between the catalytic activity and the characters of the highest occupied states (the frontier orbitals).

Experimental data suggest that in ionic solid solutions the bond-length mismatch is partially accommodated by microscopic lattice distortions. In this paper we study the structural properties of ionic alloys using a generalized Born - Mayer energy model accounting for the possibility of atomic-scale relaxations. Cubic supercells are used to simulate the real random alloys, providing statistical information about the atomic distribution and interatomic distances. The good agreement with the available experimental data and with the results of ab initio pseudopotential calculations performed for comparison for some selected systems indicate the validity of the model here employed. We also discuss the applicability to ionic alloys of the special quasi-random structures originally proposed for semiconducting alloys.

We report on a photoemission (PE) study of the region close to the Fermi energy in . The partially filled LUMO-derived band consists of two peaks separated in energy by . Variations in the PE cross sections cause these peaks to be resolved only for certain photon energies. The peak at higher binding energy predominates at very low x but its intensity gradually decreases when x increases, until for it has almost disappeared. This component is assigned to a room temperature dimeric or polymeric phase. The peak at lower binding energy is very similar to the LUMO-derived band of , but with a slight energy shift towards . We assign this component to an fcc phase with random occupancy of octahedral and tetrahedral sites.

The rate of electron tunnelling from a quantum well formed in a heterostructure and subjected to perpendicular dc electric and quantizing magnetic fields has been calculated. It has been shown that at a fixed value of magnetic field there exists a threshold value of electric field below which the tunnelling from the well is impossible. If the electric field exceeds the threshold value, the tunnelling rate increases with increasing electric field, but the dependence of the tunnelling rate on the electric field is not smooth because of new Landau levels being engaged, to which electrons can go over. It is exciting that the tunnelling rate in crossed electric and magnetic fields can be significantly higher than the rate in the presence of just the electric field. A physical explanation of this effect is given.

We present a rigorous dynamical relation for aging phenomena - the aging relation - for Ising spin glasses using the method of gauge transformation. The waiting-time dependence of the autocorrelation function in the zero-field-cooling process is equivalent to that in the field-quenching process. There is no aging on the Nishimori line; this provides arguments for dynamical properties of the Griffiths phase and the mixed phase. The present method can be applied to other complex systems with gauge symmetry such as the XY gauge glass.

The self-diffusion coefficient of , tetra-methylammonium , tetra-ethylammonium , tetra-propylammonium and tetra-butylammonium in solutions of the weak polymethacrylic acid (PMA) were measured with PFG NMR. No additional salt was present in any of the experiments. The polyion concentration and degree of neutralization were varied. The maximum relative counterion self-diffusion coefficient against polyion concentration, that was reported earlier, was observed for both alkali and tetra-alkylammonium counterions. We propose that the maximum is due to the combination of the obstruction by the polyion and the changing counterion distribution at increasing polyion concentration. An explanation of this proposal is offered in terms of the Poisson - Boltzmann - Smoluchowski (PBS) model for polyelectrolytes. Qualitative agreement of this model with experiment was found for the dependence of the counterion self-diffusion coefficient on the degree of neutralization of the polyion, on counterion radius and on polyion concentration, over a concentration range from 0.01 to 1 . Adaption of the theoretical obstruction, to fit the self-diffusion data of the solvent, also greatly improves the model predictions on the counterion self-diffusion.

This work reports a study of FIR reflectivity in betaine calcium chloride dihydrate (BCCD). The spectra are fitted by using the Kurosawa model for the dielectric function. The fits allow the observation of the temperature dependence of the vibrational parameters of the more important polar modes. Particular attention is paid to the lower-frequency polar modes, which are very sensitive to the sequence of phase transitions in this compound. The lower frequencies of the z(yy)x-Raman spectrum, measured between room temperature and T = 20 K, allow a tentative identification of the symmetry of the low-energy modes that become infrared active at lower temperatures. It is shown that these modes correspond very likely to external modes, where the BCCD molecules behave as rigid units. In the ferroelectric phase, the IR spectrum is compatible with the orthorhombic space group .

The study of the pyroelectric effect along the [100] direction, in a sample cut as a plate parallel to a natural crystalline face to avoid misorientation, does not reveal any trace of an x-component of the spontaneous polarization, previously reported. This result seems to confirm the absence of intrinsic monoclinic distortions in the ferroelectric phase of BCCD.

A reversible pressure-induced phase transition in lanthanum nickel ferrate manifests itself in the infrared spectrum of the transition metal - oxygen stretching modes by the emergence of new peaks at pressures greater than . Analogies to this transition are made by considering charge transfer in dilanthanum cuprate and its modification by partial substitution of copper ions by chromium ions.

A mixture of 0.8 mole fraction He was investigated by high-pressure Raman spectroscopy. The vibron in fluid is observed up to . Spectra of external and internal modes of the van der Waals compound , which is formed above 9 GPa, were obtained up to 40 GPa. The similarities of the spectral features to those of imply a close relationship between the structures of and . The enhanced -intensity ratio suggests that He may substitute or displace molecules from the a site in the -phase. The observed splitting of the main vibron of isotopic species indicates that in addition to factor-group interactions site effects are responsible for the observed splittings of the main vibron of .

Optical properties of ordered and disordered FeAl alloy films have been measured in the 0.5 - 5.0 eV energy range. The influence of the order - disorder structural transition on the optical properties of FeAl compounds has been studied. Experimental results for ordered FeAl are compared with the results of ab initio semi-relativistic extended linear augmented-plane-wave calculations, and explained in terms of the electronic energy band structures.

The two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) from KCl and KI single crystals at low temperatures has been measured. The intensities of the narrow peaks resulting from the Bloch modulation of the delocalized positronium (Ps) are studied. The intensities of the peaks except for those at the momenta corresponding to the (000) and (200) reciprocal-lattice vectors are very low for both KCl and KI. The experimental results for KCl are compared with the calculations made by Zhang and Song. Calculations with simpler approximations are also performed for both KCl and KI.

The three-dimensional momentum density of annihilating electron - positron pairs has been studied for a single Al - Li - Cu icosahedral quasicrystal. A direct Fourier transform method is employed to reconstruct the three-dimensional momentum density from measurements of the two-dimensional angular correlation of positron annihilation radiation (2D-ACAR). The crystallographic anisotropy in the momentum density is observed to be very small. The asphericity of the Fermi surface is not found explicitly within the experimental resolution in the momentum space. The features of the three-dimensional electron - positron momentum density agree with those obtained by means of Compton profile measurement. It is suggested that a strong lattice - electron interaction at the Fermi level occurs in this icosahedral phase.

The ferroelastic and antiferroelectric transition (between phases called P and R) in samples, pure and doped with Mn (0 - 3.3 mol%), was investigated. The phase diagram was obtained from DTA tests. The doping resulted in lowering of the P - R phase transition temperature. The shift of the P - R phase transition temperature was observed also in dielectric permittivity measured under compression. Changes in enthalpies obtained from DTA were compared to the latent heat values calculated independently from the Clapeyron - Clausius equation. Values of thermodynamic parameters of the P - R phase transition were evaluated. The EPR test showed that the Mn dopant causes disorder and local strains. Athermal martensitic behaviour of the P - R phase transition in was enhanced by applied compression and the dopant subsystem.