Table of contents

An icosahedral phase has been observed for the first time in the equilibrium immiscible Fe-Cu system. The new metastable phase was formed by directed xenon ion mixing of Fe/Cu multilayers at room temperature and without any amorphous-to-quasi-crystalline phase transition. The atomic concentration of this phase was determined by in situ energy-dispersive spectroscopy (EDS) to be around Fe₆₀Cu₄₀.

In quenched as well as in irradiated W⁵⁷Co single crystals, the authors observed directly the trapping of vacancies at ⁵⁷Co probe atoms by the appearance of an additional absorption line in the Mossbauer spectra. This new line is characterised by a quadrupole splitting of 0.96 mm s^-1 and an isomer shift of 0.187 mm s^-1 relative to ⁵⁷Co on substitutional lattice sites. By isochronal annealing of irradiated and quenched samples, they obtained a consistent description of long-range vacancy migration in tungsten between 640 and 920 K in correspondence with the results of other authors.

The electronic structure of itinerant d electrons in the intermetallic compound NbFe₂ with the C14 Laves phase structure is calculated in the tight-binding approximation. By using the calculated density of states curve and taking into account the effect of spin fluctuations, the temperature dependence of the paramagnetic susceptibility is calculated. A fair agreement between the calculated and observed results is obtained.

Electron correlation effects in transition metals with model rectangular and semi-elliptic local density of states are studied within the Hubbard model by the self-consistent second-order perturbation theory in U/W (where U is the Coulomb integral and W the bandwidth). It is shown that a satellite structure on the one-particle spectrum predicted by the nonself-consistent second-order perturbation theory disappears when the problem is treated in a self-consistent way.

The synthesis of a range of superconducting copper perovskites of the general formula Y_nLa_2-n+xBa_4-xCu₆O_14+y (n=0, 1, 2) is reported. These materials have been characterised in terms of their lattice parameters, oxygen stoichiometry and superconducting critical temperature, T_c. Substitution of yttrium by lanthanum in YBA₂Cu₃O_{7+or- delta} results in only minor changes in physical properties whilst replacement of barium by lanthanum causes a rapid drop in T_c. These results are interpreted in terms of the distribution of oxygen within the structure. It is concluded that all superconducting samples are essentially orthorhombic.

Using a conformal mapping technique, the image force expressions for the screw dislocation near an elliptic hole and a semi-elliptic (edge) hole have been derived. By letting the short axis of the ellipse approach zero, the finite length crack limit solution has been obtained. This is found to be in agreement with the result obtained by a method which uses a dislocation model for the crack.

Measurements of the anelastic piezoresistance effect associated with O-D relaxations in Nb have been carried out to examine its applicability to the study of relaxation phenomena associated with hydrogen and its isotopes. The existence of an anelastic piezoresistance effect for deuterium trapped at O solutes in Nb has been established. The Arrhenius temperature dependence of the O-D relaxation times agrees well with that determined by internal friction techniques.

The influence of the zero-point motion of the light interstitial impurities H, D and T in Pd on the de Haas-van Alphen scattering quantities is investigated computationally. The scattering of Bloch electrons caused by the impurities is described by a KKR-Green function formalism which contains the multiple-scattering effects due to host embedment. The vibrational motion of the hydrogen isotopes is taken into account by considering instantaneous scattering from displaced interstitials. It is found that the zero-point motion has a small influence on the scattering. The authors therefore conclude that, contrary to previous conjectures, the isotope effects measured in the de Haas-van Alphen spectroscopy of Pd alloyed with H, D and T cannot be attributed to different zero-point motion amplitudes of the hydrogen isotopes.

Earlier methods for the description of alloys with strong interatomic correlations are used to study H-H interactions and the thermodynamical properties of hydrogen in systems of the NbH_x type. Explicit expressions are given for configurational contributions to the thermodynamics in the disordered phase. These expressions are used to estimate the parameters of the H-H interactions from experimental data. It has been found that in the vicinity of some x=x_s approximately=0.6 the concentration dependence of the partial enthalpy of hydrogen in NbH_x changes sharply. A model is suggested which relates this anomaly to a change of the electronic state of hydrogen in an alloy at x approximately x_s. The calculations of the thermodynamical properties suggest a significant value of the configurational heat capacity C_conf and its strong dependence on x and T, as well as a considerable temperature dependence of the partial enthalpy and entropy of hydrogen, h^H and s^H.

A microscopic theory of phase equilibrium in transition-metal alloys is presented. The approach is to combine a tight-binding cluster-Bethe-lattice method treatment of the internal energy with a cluster-variation method treatment of the configurational entropy. One of the great advantages of this approach is the inclusion of short-range order both in the internal energy and entropy calculation. The calculations are parameter free and use only the results of elemental electronic structure calculations as input, i.e., bandwidth and one-site energy. Calculations of the enthalpies of formation of Cr-Mo, Cr-W, Mo-W, Mo-Ta, Mo-Nb and Ta-W alloys are presented. The model is also used to calculate the miscibility gaps displayed by the Cr-Mo, Cr-W and Mo-W systems.

Local electronic structures near the interstitial atoms C, N and O in FCC Fe were calculated self-consistently, using the DV-X alpha cluster method. From the analysis of the bond order it was found that the covalent bond strength between atoms varied in the order, Fe-C>Fe-N>Fe-O. This was also confirmed in the electron density distribution map where strong p-d sigma -type bonds are predominantly present between Fe and C atoms, whereas no such bonds exist between Fe and O atoms, and instead weak p-d pi -type bonds are observed in the map. Also, charge transfer takes place from an iron atom to an interstitial atom and is more enhanced in Fe-O than in Fe-C. In other words, the trend towards ionic bonding is higher in Fe-O than in Fe-C even in FCC Fe. This tendency of metal-metalloid bonding appears to be maintained even in the compounds Fe₃C, Fe₄N and FeO. Further, the local electronic structure was found to be modified by the lattice distortion around an interstitial atom.

Superlattice films consisting of ferromagnetic Fe and antiferromagnetic Mn were prepared by ultra-high-vacuum evaporation. The alternate stacking of BCC Fe (110) and alpha -Mn (110) lattice planes when the superlattice periods are in the range from 40 to 80 AA and the Fe and Mn layer thicknesses are equal was confirmed by X-ray diffraction. The films have fibrous textures which are composed of coherent grains of approximate dimensions of 100 AA in the film plane and 200 AA along the growth direction. The distributions in the growth orientation are within 10 degrees . Mossbauer spectra indicate that alloy layers are formed at the interface regions and that the degree of atomic intermixing is rather extensive at the interfaces, where Fe atoms are deposited on Mn layers. The thicknesses of the alloy layers are in the region of 10 AA. When Fe layers thinner than 15 AA are sandwiched by Mn layers, the structure is changed significantly because of alloying. The Mossbauer spectra show unresolved magnetic splitting, with the magnetic hyperfine field widely distributed in the range from 0 to 200 kOe (100 kOe on average). The magnetisation curve strongly depends on the external field and also on the thermal history. The properties of Fe-rich layers are similar to those of the Fe-Mn amorphous alloy and the magnetic behaviour of the multilayers is interpreted as mictomagnetism.

The ferromagnetism of compositionally modulated (CM) Mn/Ge films with a layer thickness d ranging from 3 to 300 AA has been studied. When the layer thickness of Mn (d_Mn approximately=d_Ge) is more than about 15 AA the magnetisation per unit volume of Mn (M) increases with decreasing d_Mn and a linear relationship between M and d_Mn^-1 is found. On the other hand, M is about 90 G for films with d_Mn<10 AA. The dependence of M on d_Mn in Mn/Ge CM films has been compared with the results for Fe-, Co- and Ni-based CM films and can be explained by the formation of a ferromagnetic MnGe alloy layer at the boundary between the Mn and Ge layers. The rapid increase of the rotational hysteresis loss and of the coercive force found below 20 K is attributed to the ferro-antiferro interaction between these Mn-Ge alloy layers and the Mn layers.

The electronic structures of the compounds Fe₂P, Co₂P and CoMnP are calculated by the LMTO method within the LSD approximation, not only for the orthorhombic structure but also for the hexagonal structure. The paramagnetic DOS are very similar for both structures. The values of the paramagnetic DOS at the Fermi level allow us to predict that Co₂P exhibits ferromagnetism only in the hexagonal structure and that Fe₂P exhibits ferromagnetism in both crystal structures. The theoretical results for the magnetic properties of CoMnP are in agreement with the experimental results. The site preference is also discussed together with the electron transfer from P atoms to 3d atoms.

Magnetic susceptibility, thermal expansion and NMR have been measured for the pseudobinary Laves phase intermetallic compounds Y_1-xSc_xMn₂ with x<or=0.08 and Y_1-xLa_xMn₂ with x<or=0.25. The substitution of Sc for Y in YMn₂, leading to a shrinkage of the lattice, makes the Mn moment unstable and stabilises the paramagnetic state down to the lowest temperature. On the other hand, the Mn moment is stable for the compounds substituted by La with a larger atomic volume than that of Y. The analysis of thermal expansion curves has revealed that longitudinal spin fluctuations with giant amplitudes are thermally excited in paramagnetic Y_1-xSc_xMn₂.

The magnetisation of amorphous Gd_0.70Pd_0.30H_0.20, prepared by sputtering was measured between temperatures of 100 and 300 K in fields of up to 5 kOe. While the Curie temperature T_C was found to be significantly lower than that observed for undiluted, liquid-quenched amorphous Gd_0.70Pd_0.30, the magnetic response in low fields was found to be essentially identical, indicating that the critical behaviour of this amorphous system does not change upon dilution with hydrogen. A detailed examination of the low-field magnetic response is compatible with an equation of state revealing that the transition, occurring at a temperature of 106.3 K, is paramagnetic to ferromagnetic in nature.

The temperature dependence of positron annihilation parameters in In-Pb alloys has been studied by the long-slit angular correlation and Doppler broadening techniques. A strong correlation between the results and underlying changes in the lattice constants has been found. Both the parameters characterising the positron states and the measured values of the monovacancy formation energy show the effect, which is most marked in the In-rich face-centred tetragonal (c/a>1) phase.