Table of contents

Positron annihilation Doppler broadening measurements were made on quenched aluminium single-crystal and polycrystal samples as a function of isochronal annealing. Changes in the annihilation lineshape due to clustering of vacancies were observed. The measurements show that clusters begin to form very early in stage III and that the degree of clustering depends strongly on the density of vacancy sinks in the samples.

The measured phonon dispersion relations for PdD_0.63 at 150K are used to estimate the elastic constants and low-temperature Debye theta for this compound. The results explain the observations that the low-temperature lattice properties (e.g. specific heat, thermal expansion) of PdH_x alloys for x approximately=0.6 are very similar to those for pure Pd.

Experiments have shown that in the case of metals the temperature variation of the activation volume, per vacancy is due mainly to the rapid increase of the 'entropy volume' which corresponds to the pressure variation of the activation entropy. An explanation of this fact is proposed by expressing the thermal coefficient of the 'entropy' volume in terms of properties only of the bulk material.

Doppler-broadened annihilation lineshapes are measured for Ni, Fe and V between 90 and 330K. Annihilation behaviour does not follow volume expansion of the lattice.

The effect of the spin distribution of quasilocalised electrons on effective exchange interactions of RKKY type is considered. Some recent treatments of this problem are shown to be inadequate. A physically correct approach, which takes into account the correct l=2 angular momentum symmetry of the 3d wavefunctions and the strong resonant scattering, yields good agreement with experiment.

In some alloys of Ti or Zr and certain transition metals a low-temperature, metastable, trigonal phase known as omega -phase is formed during quenching. For large solute concentrations, the Bragg reflections characteristic of the ideal omega -phase do not develop, but diffuse X-ray or neutron diffraction is observed at wavevectors close to (but not coincident with) these Bragg wavevectors. A simple one-dimensional model, inspired by previous calculations of charge-density-wave transitions, is proposed to explain the qualitative features of the diffraction observed for the diffuse omega -phase. This phase is viewed as an aggregate of domains of ideal omega -phase. Essential features of the model are the coherency of the domains and the restrictions placed on the phase relationships between neighbouring domains.

The lattice parameter of Pd has been measured between room temperature and 6K and has been found to be a smooth function of temperature over this range. An investigation of the X-ray diffraction patterns of some beta phase Pd-H alloys and a mixed ( alpha + beta ) phase alloy and of their dependence on cooling through the 50K region, where anomalies in a number of physical properties of beta -phase Pd-H are known to occur, showed no discernible effect on the state of strain of the Pd lattice, i.e., no indication of homogeneous lattice strain in excess of 2.5*10^-5, nor evidence of line broadening indicative of inhomogeneous strains. Some consequences of this result for mechanisms which have been suggested to account for these anomalies are considered.

The possibility of simulating random substitutional alloys within the supercell model is investigated. Different model lattices are compared to the corresponding random systems in terms of their neighbour distributions. In particular the nearest-neighbour distribution has been used in this analysis. One advantage of the supercell model is that lattices corresponding to different degrees of short-range order can be generated. Special attention has been paid to these properties of the model lattices.

A simple phenomenological model has been developed for BCC transition metals which considers ion-ion interaction up to second neighbours and electron-ion interaction on the lines of Bhatia theory using an appropriate value for the screening parameter. Volume force has been averaged over the whole Weigner-Seitz sphere. In the present model the ionic lattice is in equilibrium in a medium of electrons. A computation of the phonon dispersion relations for chromium and vanadium gives reasonably good agreement with the experimental results.

Detailed measurements have been made of the anomalous low-temperature heat capacity and electrical resistivity of the intermetallic compound Al₁₀V. Both properties (and also the thermal expansion) are dominated by Einstein-like behaviour with a sharply defined characteristic temperature of 23+or-1K, and an excitation of the same energy has been seen with inelastic neutron scattering. An upper limit on the level width is 2K, so that hybridisation with band phonons appears to be unimportant. The compound is atomically well ordered, and there is one Al site whose environment is unusual and of the right kind to give a low-lying vibrational mode. A sample doped with Ga showed a second level, of energy about 10K, associated with preferential occupation of the special site by Ga. The structure that gives rise to this local mode in Al₁₀V appears to be accidental, a consequence of strong directional bonding between Al and V elsewhere in the unit cell. No other example is known of such a well defined local mode in a metallic solid.

The pressure dependence of the de Haas-van Alphen effect in potassium has been studied up to 4.6 kbar. The measured dHvA frequencies were lower than the free-electron scaling predictions for all pressures. The low pressure area compressibility of the Fermi surface was found to be 15% lower than the free-electron value. The results show the failure of the free-electron theory for potassium. The decrease in both the Fermi surface cross sections and the area compressibilities indicate the possible existence of a new ground state energy as predicted by the charge-density wave model. The observed orientation dependent Fermi surface distortions with pressure and the presence of beats in the dHvA oscillations are in qualitative agreement with the charge-density wave model.

The solution to a model problem representing positron trapping at defects in metals in the presence of positron-phonon scattering is presented. The model uses a separable form of the trapping interaction to allow the series expansion in the interaction to be summed exactly. This form of the trapping interaction is most suitable for well localised trapped states. A position representation is used, so that motion of the positron from point to point appears explicitly in the theory. In contrast to the predictions of classical theory, the aspects of positron motion that influence the trapping probability are relatively insensitive to positron-phonon scattering.

The electrical resistivity of dilute PtMn spin glass alloys (0.05-1.0 at.%) has been measured in a temperature range from 0.05 to 10K. All alloys show, in contrast to the maxima in Delta rho (T) normally observed in spin glasses, pronounced minima in the resistivity, with T_min varies as c. Below T_min, Delta rho varies 'proportional to T', above T_min, a positive logarithmic temperature dependence is observed. The total resistivity behaviour can be explained within the cluster model of Matho and Beal-Monod (1972). In addition, the variations of the single impurity limit R(T) for all magnetic Pt 3d alloy systems are reviewed and interpreted within the framework of a spin exchange (Kondo type) model.

The temperature dependence of the electrical resistivity, thermal resistivity, and thermoelectric coefficient G(T) have been measured in high-purity aluminium between 1.5K and 6.5K. A quadratic temperature dependence was observed for these quantities below 4K rather than the cubic dependence usually expected from the breakdown of Matthiessen's rule.

The electrical resistivities, rho , of liquid La, Ce, Eu, Gd and Yb have been calculated on the basis of the t matrix formulation of the Ziman nearly-free electron formula. Experimental liquid structure factors at different temperatures have been used for calculating the variation of rho with temperature. The calculated values of rho , its temperature coefficient, and the thermoelectric power for all metals show good agreement with the available experimental data. The authors interpret the observed high values of rho in terms of large p and d phase shifts and the negative temperature coefficient of rho for Eu as due to the proximity of the Fermi momentum to the position of the first peak of its structure factor.

The high field Nernst-Ettingshausen coefficient of a pure polycrystalline foil of Al from 1.8K to 5K has been measured. The experimental value of the diffusion component of this coefficient yields a transport heat capacity gamma ^t which is the same as the experimental electronic heat capacity gamma ^c with a probable uncertainty of less than 3%. This result demonstrates that the electron-phonon renormalisation is identical for both the specific heat and Nernst-Ettingshausen coefficients to within an uncertainty of 10%. The experimental phonon-drag component of the Nernst-Ettingshausen coefficient is positive and varies approximately as T³ in the temperature range studied. Both the sign and temperature variation are taken as evidence of the presence of substantial electron-phonon Umklapp scattering.

In the range 77K<or=T<or=220K the mobilities in Bi have been calculated under the assumption that the scattering mechanism is intervalley scattering caused by optical and acoustical phonons for which the effective matrix elements have been found by Lopez (1968). For the electrons both the nonparabolic and the parabolic model are used. It is found that the acoustical phonon always gives results that are much larger than the measured mobilities, whereas the optical phonon gives results that agree very well with the measured values if one uses the nonparabolic model with the temperature-dependent band parameters found by Vecchi and Dresselhaus (1974). For the parabolic model the agreement is not so good. The thermopower is also calculated using the same scattering mechanism and the nonparabolic model. The result is that the thermopower for the electrons and the holes has the same sign (negative). The total thermopower is found to be nearly constant in the temperature range considered, but the values deviate by about 40% from the experimental values.

A model of local structural states (phases) in the superconducting TI phase of the nonstoichiometric alkali tungsten bronzes M_xWO₃ (M an alkali) is shown to be consistent with the dramatic increase of the superconducting transition temperature as x approaches the TII phase boundary value. This model also predicts the existence of (x dependent) anharmonicity and heat capacity anomalies and it is consistent with the fact that the cubic phase of Na_xWO₃ is nonsuperconducting.

The paper discusses the relaxation of the magnetisation, M, and the order parameter q, in spin glasses with arbitrary spin S, within the mean field approximation. It is shown that the rate of the relaxation of the order parameter is slowed down as the spin glass freezing temperature is approached from either above or below. The relaxation of M for T>T_f and the fast relaxation of M for T<T_f show no critical behaviour near T_f.

The paper studies the behaviour of the Mn spin for dilute Mn in Pt as a function of temperature and applied field using the technique of nuclear orientation. In the preferentially oriented cubic polycrystalline foil a preferential nuclear spin ordering axis has been observed in the absence of an applied field. A measure of interaction of the spin with its environment has been obtained which may be modelled by a molecular field with mean value 440 G, and having a distribution in magnitude and direction. The saturation hyperfine field for Mn in Pt was found to be -365+or-15 kG.

The Mossbauer spectra of BCC Fe-V alloys in the concentration range 40-80 at.% V have been obtained for both the ordered and disordered states. No broadening of the absorption line has been detected in an 80 at.% V alloy even at liquid He temperature indicating that the alloy is nonmagnetic. Broadening of absorption lines due to the magnetic hyperfine field has been observed for the alloys containing less than 70 at.% V. The spectra obtained at 5K were analysed to yield the hyperfine field distribution curves. It was found that the hyperfine field is notably reduced by atomic ordering. This change in hyperfine field is explained as the result of the change in Fe atomic moment which increases with increasing the number of Fe nearest neighbours. Profiles of the hyperfine field distribution curves are explained on the same basis. The dependence of the Fe atomic moment on the near-neighbour Fe composition is compared with the result of CPA calculations. Correlation between the lattice constant and the magnetic moment is discussed.

The magnetic properties of the Laves phase system Y(Fe_1-cAl_c)₂ have been studied by means of bulk magnetisation measurements in fields up to 72 kOe for 0<c<or=0.98 and by Mossbauer effect techniques for 0<c<or=0.326. The mean magnetic moment and Curie temperatures show a rapid nonlinear decrease with increasing c values and go to zero near c=0.25 while the high-field susceptibility shows a pronounced maximum at this concentration. The Mossbauer spectra lead to hyperfine fields whose values remain nearly constant in the concentration range investigated, correlated to iron sites with different environments. The results are well described in a local environment model where the iron atoms carry a moment if surrounded by n>or=n₀=15 iron neighbours; this moment remains constant and equal to that of YFe₂ at least up to 32.6 at.% Al. This model accounts for the variation with c of the spontaneous magnetisation related to the magnetic atoms, the high-field susceptibility which arises from the nonmagnetic Fe atoms, and for the hyperfine field distributions.