Table of contents

The formation energy of a molecular vacancy in a crystal of anthracene was calculated using the atom-atom potential method. this energy was found to be 93% of the packing energy of a regular lattice. The relaxation of molecules surrounding the vacancy was very small and anisotropic. The migration of an anthracene molecule from one site to another vacant site was also studied in two directions. The calculated value of the self-diffusion energy for the (010) jump was 125% of the regular lattice energy, whereas that for the (001) jump was 138%.

The wavevector dependence of the critical fluctuations at the cubic-to-tetragonal structural phase transition in KMnF₃ doped with 1% and 10% of KMgF₃ has been studied using X-ray scattering techniques. It is found that the introduction of impurities lowers T_c and that the fluctuation-driven first-order transition becomes continuous when the impurity concentration exceeds a certain threshold. At temperatures far above T_c, the temperature dependence of the X-ray critical scattering cross section measured in the samples KMnF₃, KMn_0.99Mg_0.01F₃ and KMn_0.9Mg_0.1F₃ is described by the same critical exponents nu and gamma . Contrary to expectation, the magnitude of the anomalous narrow component observed in the critical scattering in pure KMnF₃ close to T_c is significantly reduced by the introduction of the impurities.

For pt.I see ibid., vol.1, p.3565 (1989). The frequency dependence of the critical fluctuations at the antiferrodistortive phase transition exhibited by the disordered crystals KMn_0.99Mg_0.01F₃ and KMn_0.9Mg_0.1F₃ has been studied using neutron scattering techniques. In pure KMnF₃ the soft R₂₅ mode is overdamped for temperatures below approximately (T_c+40) K. A similar behaviour is observed in the disordered crystals, but the damping constant is increased by the presence of the impurities. The amplitude of the central peak observed in the spectral response of pure KMnF₃ is not significantly increased by the addition of the impurities. This is consistent with measurements of the anomalous component observed close to T_c in complimentary X-ray scattering studies, which showed that this component was reduced by the addition of impurities. The results further indicate that although the temperature dependence of the anomalous scattering is similar in both the X-ray and neutron scattering experiments, the neutron central peak persists over the temperature range T_c to (T_c+20) K while the X-ray anomaly was only observed for a few degrees above T_c. Thus, while the anomalous scattering measured using the two techniques is related to the impurity concentration in the disordered crystals, the two features must be to some extent distinct.

With electron nuclear double resonance (ENDOR) the superhyperfine interactions of Fe³⁺ impurities with five shells of Li neighbours were measured in a congruent LiTaO₃ crystal. From the analysis of the ENDOR spectra it is found that Fe³⁺ is on a substitutional Li⁺ site on the threefold symmetry axis of the crystal. From the shape of the ENDOR spectrum it is concluded that most Fe³⁺ impurities are on very perturbed sites.

The dependences of intrinsic diffusion coefficients (D_Sb, D_Ag) and interdiffusion coefficients (D) upon temperature and composition in dilute AgSb alloys have been measured. The interdiffusion coefficients D(0) obtained with infinitely dilute solutions agree well with tracer data for D*_Sb(0) from the literature. The ratio of the intrinsic diffusion coefficients was determined by applying the thin plate method. Within the error margins the ratio D_Sb/D_Ag can be regarded as independent of concentration up to 3 at.% Sb. The values of D_Sb/D_Ag(0) with infinitely dilute solutions decrease linearly with increasing temperature. The jump frequency ratios w₄/w₀, w₂/w₁ and w₃/w₁ were calculated according to the five-frequency model on the basis of the vacancy flow factor L_AB/L_BB(0), the ratio of tracer diffusion coefficients D*_Sb/D*_Ag(0) and the linear enhancement factor b₁. From these the correlation factor f_Sb of impurity diffusion is obtained, which increases with temperature from 0.30 at 890 K to 0.49 at 1048 K, as expected. The binding Gibbs free energy delta g₁ of a vacancy-antimony pair has been estimated using four different methods of evaluation using the values measured in the present paper and diffusion data from the literature. The temperature dependence of delta g₁ is critically discussed.

The wavelength dependence of quantum interference corrections to the diffusion constant is calculated in two dimensions, in the weak-localisation regime, using models belonging to the three universality classes for localisation. In each case, the corrections at second order in perturbation theory are much larger for finite wavevector than expected from previous results at zero wavevector. In two cases (systems without time-reversal invariance and systems with spin-orbit scattering), this wavevector dependence determines the way in which perturbation theory breaks down as the cut-off length for quantum interference increases. The authors speculate that these results may signal crossover from simple diffusive behaviour to a critical regime characterised by novel variation of the diffusion constant with wavevector and frequency.

The contribution of normal electron-electron scattering to the electrical resistivity of K and Cu in the presence of an anisotropic relaxation time is calculated using the relaxation time approximation (RTA) and also, for the first time, using the formally exact variational method. As sources of anisotropic scattering the authors consider: (i) electron-dislocation scattering according to a model of Kaveh and Wiser (1986) for potassium, and (ii) a high longitudinal magnetic field B applied to a copper single crystal. It is found that, as compared with the variational method, the RTA overestimates the increase of the electron-electron scattering resistivity rho _ee=A_eeT² with anisotropy. The calculated enhancement of A_ee in copper by a magnetic field depends sensitively on the crystallographic orientation. Comparison with the experimental coefficients A_ee for B//(111) yields an Umklapp fraction of Delta approximately=0.04-0.06 for electron-electron scattering in Cu, which is smaller than Delta approximately=0.28 as previously calculated by Black (1978). The dislocation model cannot account quantitatively for the large variation in the experimental A_ee in potassium.

The (100) and (111) surfaces of gold reconstruct in such a way as to increase the density of surface atoms, this being thought to result from large tensile stresses in the ideal surfaces. The authors present self-consistent electronic structure calculations of unreconstructed Au(100) using the surface embedded Green function approach. The surface charge density shows increased bond-charge in the first and second layers which, by a Hellmann-Feynman theorem, leads to attractive surface stresses. This enhancement of surface bonds is shown to be a quite general property of s-band metals and is a consequence of surface band narrowing. The same features are included implicitly in the 'glue model' which has proved successful in describing gold reconstructions.

The first-ever measurements of the surface magnetisation of a metallic glass specimen using neutron reflectivity are presented. These have been made on commercial METGLAS 2605S2 metallic glass ribbon using the CRISP neutron reflectometer on the ISIS Spallation Neutron Source at the SERC Rutherford Appleton Laboratory. The reflectivity data were analysed by fitting a model to the data which has enabled an estimate of the magnetisation profile near to the ribbon surface to be made. Long-range variations (with a length scale approximately 80 AA) at the surface of the ribbon have been observed, in which the apparent magnetic moment value at the surface is approximately half that in the bulk. These results can be interpreted in terms of noncollinear moment arrangements which follow the irregularities at the surface.

The authors present the Brillouin spectra obtained from 22-tricosenoic acid, deposited onto a silicon substrate using the Langmuir-Blodgett technique, for hypersound propagation both parallel and perpendicular to the dipping direction. The spectra show the expected elastic anisotropy of the films and also demonstrate that the velocities increase with deposition surface pressure. Elastic constants are obtained for a film deposited at a surface pressure of 47.5+or-2.5 mN m^-1 by fitting the hypersound velocity data to a model of the film.

The authors report the observation in partially converted precursor-route poly(p-phenylene vinylene) (PPV) of a strong light-induced luminescence fatigue. Increasing the conversion towards the fully conjugated polymer results in a loss of the quenching effect. They discuss exciton annihilation at photoexcited charged polaronic states as a possible mechanism for this phenomenon.

The decomposition of a technical Al-based alloy (Zergal 4) has been studied by positron annihilation techniques. The observed dependence of annihilation characteristics on the aging time is attributed to a change in the positron state due to the transformation of Guinier-Preston zones into semicoherent eta ' particles. Predictions based on the Lifshitz-Slyozov-Wagner precipitation theory are in excellent agreement with the experiment data.

The authors show the existence of additional ferromagnetic phases and transition lines in the mean-field calculations of Benyoussef and collaborators (see J. Phys. C: Solid State Phys., vol.20, p.5349, 1987) for the phase diagram of a spin-1 Ising model in a random crystal field Delta _i with probability distribution P( Delta _i)=p delta ( Delta _i- Delta )+(1-p) delta ( Delta _i).

The authors comment on the work of Hadjicontis et al., J. Phys. F: Met. Phys., vol.13, p.1635 (1988) g=cB Omega is a good approximation for the temperature function of the diffusion coefficient D. B(T) Omega (T), however, does not explain the entire curvature of ln D against 1/T and thus is no proof for the contribution of only one defect to D. The application of this relation to the impurity diffusion leads to an additional error due to an incorrect assumption with respect to the impurity diffusion entropy.

The incommensurate structure with an unusual diffraction pattern in a rapidly solidified V₆Ni₁₆Si₇ alloy is analysed and its reciprocal 'lattice' indexed using 18 basis vectors. This structure has some unique features, which are closely related to both conventional incommensurately modulated structures and quasi-crystals. However, it appears to differ from the latter two types of aperiodic structures in some important aspects.

The authors present energy-loss spectra (ELS) in the range 0-20 eV taken as a function of temperature (90-800 K) and primary energy (100-400 eV) for a Ni(430), or (⁷/₂-(110)*¹/₂-(100)), surface. A strong primary-energy dependence of ELS and pronounced differences within the ELS of the low-energy electron diffraction beams were observed. These characteristics arise from the presence of ordered atomic steps on the surface, as shown by a model calculation. They also observed temperature-dependent variations of the ELS, and interpreted them as being induced by a structural transition of the surface.

Using general arguments borrowed from magnetism, the author shows that it is short-range order parameter fluctuations close to T_c that lead to the observed behaviour of chi at a Peierls transition; in the quasi-static approximation d_chi /dT should vary as the lattice specific heat. The addition of impurities produces rounding of the d chi /dT cusp when the true phase transition is destroyed by disorder. The theory compares favourably with experiment.

Neutron crystal-field spectroscopy as a probe of the local symmetry at the rare-earth site in non-stoichiometric RBa₂Cu₃O_7-x(0<x<1) is introduced. The oxygen vacancy concentration x includes charge redistributions that are modelled by virtual distortions of the oxygen ordering in the CuO₂ planes. The model assumptions are quantitatively accounted for by neutron spectroscopic data obtained for HoBa₂Cu₃O_7-x (x=0.2, 0.8). Some implications of the observed charge redistributions on the superconducting properties are discussed.

The temperature dependence of electrical conductance has been measured on single crystals of the newly discovered oxide compound RE₂Ba₂CuPtO₈ (RE identical to Y, Er, Ho). This Pt- containing oxide is orthorhombic and has a characteristic double-zigzag chain of -O²-Pt⁴⁺-O²-Cu²⁺-O²- in columns running parallel to the b axis, which is the growth direction of the rectangular-shaped long-bar single crystals. All these compounds are found to be semiconductors with energy gaps of about 0.2-0.4 eV. They exhibit conductances proportional to exp(-T^-12/) rather than the thermally activated exp(-T^-1) dependence. Such behaviour is discussed in terms of two models, namely one-dimensional variable-range hopping and the lattice-vibration-induced band-broadening effect.

The authors present a series of calculations for positron states and lifetimes in perfect and defected YBa₂Cu₃O_7-x using an atomistic model. A simple relationship is found between the lifetime and the oxygen deficiency. The results suggest that the long-lived component resolved in lifetime spectra is associated with metal vacancies and/or metal-oxygen-vacancy complexes, while oxygen vacancies are found to be only shallow positron traps. The authors give an interpretation of positron lifetime experiments in terms of defects, oxygen-deficient domains and structural changes in the material.

The authors report here for the first time the positron annihilation study of quasicrystalline Al₈₆Mn₁₄ alloy. their study confirms calorimetric results reported earlier, which indicated that, before the crystallisation, the quasicrystalline phase in this alloy goes through a structural relaxation process. The underlying cause of the substantial drop in the line-shape parameter S during the crystallisation process is also considered.

The authors show that the point-source electron beams recently obtained by Fink (1986) have a spatial coherent length of the order of centimetres. The narrow opening of the beam suggests that the source itself is also coherent.