Table of contents

Neutron and X-ray diffraction measurements have been made on a series of amorphous CuTi alloys produced by mechanical alloying (MA). Parent titanium powder was used that was free from hydrogen. The copper-rich alloys were found to be fully amorphous after 16 hours of MA, but the reaction was incomplete in the titanium-rich alloys after the same period and they consisted of intermetallic phases. This is in contrast with the results for a series of samples examined by the authors previously, which had small contaminations of hydrogen, but which all transformed to an amorphous state after the same period of MA. This suggests that the presence of hydrogen stimulates the amorphization reaction and that in its absence the reaction is inhibited in the titanium-rich samples. It is also found that the degree of chemical short-range order present in the fully transformed amorphous samples is also influenced by the hydrogen contamination in the parent titanium.

The method of isotopic substitution in neutron diffraction is applied to study the coordination environment of Cl^- in a 0.999(3) molal solution of NaCl in fully deuterated ethylene glycol (EG). It is shown that the EG molecules act as monodentate ligands to give a Cl(EG)₄^- complex that comprises Cl^- . . . D-O hydrogen bonds which are approximately linear and of length 2.14(3) AA. The results are compared with those obtained for the Cl^- coordination complex in aqueous and methanolic solution. It is found that there is a reduction from approximately=6 to approximately=4 in the number of ligands binding to Cl^- on changing the solvent from water to EG and that, for all three solvents, the Cl^- . . . D-O hydrogen bond is typically linear. The effect of a small hydrogen imbalance between the solutions used for isotopic substitution experiments is briefly considered.

The appropriate generalization of the Clausius-Clapeyron equation to describe the first-order melting transition of two-dimensional Wigner electron crystals is used (a) within a phenomenological framework and (b) in conjunction with microscopic theories relating to anyons and composite fermions, to represent the main features of the melting curve as a function of Landau level filling factor nu .

Amorphous Se_1-xSb_x alloys were prepared using a conventional rapid quenching technique. It is found that antimony can only be substituted up to approximately 10 at.% in selenium to produce Se-Sb glassy alloys. The photoacoustic technique is employed for the first time to determine the optical energy gaps of Se_1-xSb_x glasses. The optical energy gap of amorphous selenium is found to be 1.99+or-0.02 eV. It reduces drastically on substitution of 2 at.% Sb after which its variation with x is small. The activation energies of Se_1-xSb_x glasses have been determined from the temperature dependence of their electrical resistivities. The activation energy as a function of x also shows a sudden decrease in the range 0.01<or=x<or=0.02. Plausible explanations have been suggested based on the Street-Mott model for charged defect states in amorphous chalcogenides.

The self-trapped exciton (STE) luminescence at 300 nm with a decay time of 620 ns in pure BaF₂ can be suppressed by La³⁺ or Nd³⁺ doping. Crystals were studied with La concentrations ranging from 0.2 mol.% to 22 mol.% and Nd³⁺ concentrations between 0.15 and 11 mol.% X-ray induced emission spectra at temperatures from 90 K to 430 K and decay-time spectra are presented. Information about trapping of electrons by La³⁺ ions and the activation energy for thermal quenching of STE luminescence is obtained from a fit of the experimental data to a model calculation. The conclusions obtained for La³⁺ -doped crystals can also be employed for explaining similar aspects of the STE luminescence observed for the Nd³⁺ -doped BaF₂ crystals.

The authors apply a previously developed perturbative formalism for anharmonic shell models to the calculation of phonon shifts and widths in a one-dimensional model of a solid with polarizable ions. They analyse these properties for different k-points in the Brillouin zone and also as a function of temperature for fixed k. They compare the results for several values of the core-shell coupling constant g with those obtained by using a rigid-ion model (g to infinity ). They show that the electronic polarizability of ions leads to significant modifications of the k-dependences as well as of the ratio between the damping and shift. They give an interpretaton of existing discrepancies between theory and experiment for anharmonic phonons in silicon. Finally, they mention some further applications.

The energy spectrum and the optical absorption coefficient alpha ( omega ) of a hole polaron in a PtCl chain are numerically calculated on the basis of the Baeriswyl-Bishop model. In intermediate coupling, there appear some shallow gap states as well as deep gap states, both of which give contributions to the absorption. The three-band structure of the absorption in the near- and mid-infrared region is consistent with the experimental data on (Pt(en)₂)(PtCl₂(en)₂)(ClO₄)₄. The theory also shows that in the far-infrared region there is another absorption near 500 cm^-1, which is associated with the shallow gap states.

The electronic transport properties of a linear chain have been analysed. The chain conductance, the electrochemical potential variations along the chain, and some defect scattering effects have been discussed using the non-equilibrium Keldysh formalism. For the elastic regime the authors show that there is a deep link between the chain conductance and the electrochemical potential drop along the chain. For the inelastic regime they discuss how the quantum interference associated with the electronic transport is damped by the inelastic scattering.

Neutron diffraction with polarization analysis was performed on a single crystal of CuO, oriented with its alpha * and c* axes in the diffraction plane. It was verified that in the commensurate phase I(0-212.7 K; k=(¹/₂, 0, -¹/₂)), the magnetic moments were indeed along b*, but it was found that in the incommensurate phase II (212.7-232.5 K; k=(0.506, 0, -0.483)) there also exists a component of the magnetic moment along b* in disagreement with an earlier study by Forsyth et al. (1988). The data are in agreement with the very recent results of Brown et al. (1991). The authors propose a helimagnetic arrangement in the incommensurate phase II, with the magnetic moments rotating in the plane (b*; 0.506 alpha *+1.517c*). They present a short account of their experimental results, a study of the stability of the magnetic structure in phase II and the symmetry analysis of the possible magnetic space groups.

The magnetic susceptibility and the magnetization of polycrystalline CdGd₂Se₄ were investigated. The data indicate that this compound exhibits an antiferromagnetic transition at 9.5 K. The effect of the dipolar interactions and the exchange coupling between the gadolinium ions in the crystal lattice were studied by analysis of the temperature dependence of the EPR linewidth.