Table of contents

The simple one-component plasma (OCP) model has been applied to thermodynamic properties of liquid 3d transition metals. For comparison, the results calculated by the hard-sphere (HS) model are also given. The electronic contribution to the entropy and heat capacity has been considered by a rectangular model of the density of states. In spite of the simplicity of the models the calculated values of the entropy, S, the specific heat at constant volume, C_V, and the compressibility _{chi T}, are in good agreement with experiment. This indicates that the OCP model, like the HS model, is well suited as a reference liquid for liquid transition metals.

The thermopower data in melt-spun glassy Mg-Zn alloys reported independently by Matsuda and Mizutani (1982) and Baibich et al. (1982) are in disagreement. The thermopower measurements were repeated on the McGill University apparatus using the Mg₇₀Zn₃₀ sample employed by Matsuda and Mizutani. The new results are in agreement with those reported by Baibich et al.

In amorphous alloys prepared by roller quenching, the substrate-induced shearing of the undercooled liquid results in the development of a preferred orientation of defects in the ribbon plane and elongated along the direction of substrate motion. This is supported by the authors' recent small-angle neutron scattering measurements on such materials indicating the presence of a preferred orientation of disc- or cigar-shaped density defects. The results seem to exclude the hypothesis of momentum-boundary-layer-controlled solidification. Consequences for magnetic properties are also considered.

The authors suggest that the potential around non-transition-metal solutes in Ni is similar to the screened Coulomb potential that has been used to rationalise the diffusion of solutes in noble metals, and the behaviour of Ge in Ni is analysed on this basis. For Zn in Al, a self-consistent calculation of the electronic structure indicates that the potential around Zn is screened within a short distance from the Zn site and is of an oscillatory nature near the nearest-neighbour position. Such a potential may explain the lack of systematic variation of the activation enthalpy for solute diffusion in aluminium with solute valence.

The isothermal irreversible relaxation and crystallisation of a Ni₃₅Ti₆₅ glass have been studied using the DSC technique at temperatures between 450 and 700K as well as the relaxation processes which occur close to the glass transition in amorphous Te₇₈Si₂₂. The thermal behaviour of the Te₇₈Si₂₂ samples may be described assuming a single activated process. The energy stored in this process decreases on approaching the glass transition temperature. A broad spectrum of activation energies was observed for Ni₃₅Ti₆₅. The total stored energy amounts to 40% of the energy of crystallisation. Four consecutive steps of crystallisation were observed.

The effects related to chemical short-range order (CSRO) are strongly pronounced in the neutron diffraction pattern Sⁿ(q) of Ni₃₅Ti₆₅. On annealing as-quenched amorphous Ni₃₅Ti₆₅ samples the author observed only minor variations of the CSRO. On heating an annealed sample the CSRO decreases. In both cases nearest neighbours remain unaffected.

The authors report and discuss measurements of low-temperature specific heat and thermal conductivity, upper critical field near T_c, electrical resistivity, mass density and X-ray diffraction, performed on an amorphous superconducting Zr₇₆Cu₂₄ alloy obtained by high-rate sputtering deposition at 77K. A thermal annealing of the samples in the amorphous state before crystallisation induces a large decrease of the very low-temperature specific heat related to the two-level system excitations, whereas the thermal conductivity increases in a correlative way. Both electron and phonon contributions to the specific heat in the normal state decrease and tend towards the values of liquid-quenched (LQ) alloys of corresponding composition. At the same time, the other properties studied are less affected, likewise in LQ alloys. The occurrence upon annealing of a chemical phase separation in the amorphous state is proposed to explain the results.

It is argued that 'charge transfer' is not a sensible concept to use in nearly-free-electron (NFE) alloys. Their charge density can be pictured as a set of neutral pseudoatoms. In the screened uniform charge model (SUCM) each pseudoatom consists of a part, called the associated volume, of the uniform (zeroth-order) charge density plus a screening charge. The screening for a particular ion depends on the associated volume and so screening charge transfer can occur between the different types of ion in an alloy. The causes of this are discussed and the behaviour in the dilute alloy limit is investigated. In general the size of associated volumes can be determined by minimising the total energy. A simple picture of the charge density in a pseudoatom is used to show the dependence of energy on pseudoatom size and to demonstrate the largest effects in heterovalent alloys. Some experimental measurements affected by screening charge transfer are suggested, and its manifestation in other models is discussed briefly.

Conduction band spectra of Mg metal have been obtained using X-ray and UV excitation. The spectra are compared with the conflicting band-structure calculations of Gupta and Freeman (1976) and Citrin et al. (1979). It is shown that the latter yields densities of states in better agreement with experimental observations.

The authors investigate the common qualitative idea that a solid is particularly stable if the electron states are filled up to some energy band gap. Such ideas have previously been advanced to explain the occurrence of a certain family of M₅X₄ compounds (M=transition metal, X=chalcogen) only for particular ranges of valence electron concentration (VEC). In these materials octahedral metal clusters condense to form infinite chains. The authors study the stability of such chains against break-up on two models, (i) a regular octahedral chain and (ii) the chain of the compound Nb₅Te₄, and discuss the question of why only the compounds with 2.4<or=VEC<or=3.6 exist.

A sum rule is presented for the change in energy produced by the presence of a substitutional impurity in a linear chain of atoms. The electronic structure is solved exactly within the tight-binding Hamiltonian and the cases of a diagonal and a non-diagonal impurity are discussed.

The authors report on ⁵⁷Fe Mossbauer isomer shift measurements of Fe_0.64Ni_0.36, Fe_0.68Ni_0.32 and Fe_0.60Ni_0.20 FCC Fe alloys in the temperature range from 13 to 1010K. The Weiss 2 gamma -states model and a simple 'volume model' have been fitted to the experimental data in order to explain the observed temperature dependence of the isomer shift. To separate the isomer shift from the measured centre-of-gravity shifts of the spectra, the authors used the Debye approximation of the second-order Doppler shift. The Debye temperatures were assumed to be dependent on temperature.

The results are presented of the effect of irradiation of bismuth with 1 MeV electrons on the low-temperature (4.2K) galvanomagnetic properties in fields up to 2.3 T. The data are analysed quantitatively assuming a rigid-band model. It is concluded that the Frenkel pairs created by irradiation act as donor impurities with an efficiency of 0.14 extrinsic electrons per pair, and substantially limit the carrier mobilities. These combined effects, taking into account the low intrinsic carrier density in bismuth, explain the observation that irradiation with 1 MeV electrons has a relatively much larger effect on the conductivity of bismuth than on that of any other metal.

Various galvano- and thermomagnetic transport properties of zinc have been measured in the Corbino geometry for magnetic fields parallel to the c axis in the range 0-2.5 T. The effects of magnetic breakdown of the basal-plane orbits have been observed in all transport coefficients with oscillations in the thermoelectric coefficient G=- epsilon "_xx/ lambda "_xx being particularly pronounced. Calculations of all the transport coefficients have been performed using the breakdown network model of Falicov et al. (1966). In general reasonably good agreement with experiment is obtained, demonstrating that the model can account successfully for all the low-field magnetotransport properties of zinc.

The electrical resistivity rho and the thermoelectric power Q of binary liquid Hg-Li alloys have been measured. The resistivity decreases with the addition of Li. This behaviour shows a systematic variation of rho with the addition of solute alkali elements to liquid mercury alloys. As for the other akali amalgams, Q shows a minimum at 5 at.% Li in the curve plotted as a function of the Li concentration at constant temperature. The variation of rho with the addition of alkali elements (including Li) can be well described by the Faber-Ziman theory (1965) based on the nearly free electron model, while the minimum in Q cannot. In order to understand the origin of the minimum in Q of liquid H-Li alloys, higher-order atom-atom correlation function effects seem to be very important, as stressed previously by the present authors for other alkali amalgams such as Hg-Cs alloys. The 'modified Miedema plot' appears to reveal that this higher-order correlation effect is caused by charge transfer from the solute to the Hg atom.

The authors calculate the electronic contribution to thermal diffusion using an expression which may be regarded as an extension of the Faber-Ziman theory of the resistivity of molten alloys and the Faber expression (1972) for the driving force in electromigration. The thermal driving force is directly related to the contribution of the ion to the thermoelectric power. To make contact with experiment the authors add to Q*^e, the electronic contribution to the heat of transport, an 'intrinsic' term Q*¹ calculated from dense-gas theory. The conclusions, in broad agreement with experiment, are that the electronic contributions are always in opposition: that in Na-Rb, K-Rb and Ag-Au Q*ⁱ outweighs Q*^e and the heavier ion always diffuses to the cold end of the specimen, and that in Na-K the magnitudes of Q*ⁱ and Q*^e are comparable and reversal of the direction of migration can take place as the concentration changes.

Thermoelectric power investigations of the metallic glasses from groups of transition metal-metalloid and transition metal-transition metal alloys are reported for the temperature interval 300-960K. It is shown that, apart from testing the applicability of various electron transport models, thermoelectric studies in the vicinity of the crystallisation threshold temperature supply valuable information about the crystallisation processes. In conclusion, the pattern of the thermoelectric power behaviour before and after crystallisation is presented.

Magnetisation measurements have been performed on bulk Chevrel-phase lead and tin molybdenum sulphides prepared under different conditions in order to study their critical current density J_c as a function of applied magnetic field and temperature. The influence on J_c of the grain size, of neutron irradiation and of doping the compound with inclusions or small insulating particles is discussed. The best result, J_c>or=10⁸ A m^-2 at 14 T and 4.2K, was achieved on a hot-pressed PbMo₆S₈ specimen having neutron-induced defects. The pinning force F_p as a function of reduced field b=H/H_c2 does not really obey any scaling law in these compounds due to possible inhomogeneities. However, the way to define the upper critical field H_c2 turns out to be important, since its extrapolation from the J_c(H) curves can restore, in the best cases, the scaling of F_p(b).

The authors have used Campbell's AC method to investigate the internal magnetic field profiles in PbMo₆S₈ cylindrical bulk samples. The response of the pinned flux line lattice to a small ripple field b₀ is observed to have an elastic term (linear in b₀) superposed on a dissipative term (quadratic in b₀), in opposition to the Bean critical-state model. The pinning force density, F_p, the penetration depth of the AC ripple field lambda ' and the interaction distance d are derived and compared with a general theory proposed by Matsushita (1979) for densely pinned superconductors. Good agreement is found, particularly in the case of a PbMo₆S₈ sample irradiated with fast neutrons.

Inelastic neutron scattering measurements have been performed on a Ni_0.784Mn_0.216 disordered alloy which is ferromagnetic below 330K. At temperatures lower than 40K the AC susceptibility measurements suggest the onset of a frustrated phase. Spin waves have been unambiguously observed at all temperatures below the Curie temperature 330K, and in particular below 40K. The inelastic scattering reveals a marked anomaly of the spin dynamics at about 100K below which a quasi-elastic line appears and grows in intensity as commonly observed in similar re-entrant systems. This anomaly is characterised by a large enhancement of the spin-wave damping, a marked change of the transverse static susceptibility which is higher in the 'spin-glass phase' than in the ferromagnetic phase and, at lower temperatures (below 70K), an increase of the stiffness constant. These experimental results are in contradiction with models suggesting a simple break-up of the infinite ferromagnetic network established at T_C.

The authors demonstrate that if one is willing to focus only on the coarser aspects of p(H) (the detailed shape of the hyperfine field distribution), such as its mean and its variance, then the Mossbauer spectra can be analysed consistently in a manner which retains all relevant parameter correlations even if no analytic relationships exist between the parameters. Measurements of room-temperature Mossbauer Zeeman spectra are reported for six different boron and phosphorus metalloid glasses and are analysed in this fashion. Complete results for mean values and variances of hyperfine fields, isomer shifts and pure quadrupole distributions are tabulated, the last two variances being reported for the first time in a ferromagnetic phase.

New results for the uncoupled Hartree-Fock estimates of the 3d contribution to the ionic antishielding factor, gamma _infinity ^3d, are reported for Sc-Ni in the electronic configuration 3dⁿ4s¹. It is found that gamma _infinity ^3d decreases with increasing Z from -2.5 to -1.0 per 3d electron.

Spin-echo NMR measurements have been made, at liquid-helium temperatures, on ¹⁶⁵Ho in the ferrimagnetic alloy series Ho_0.03Gd_xY_0.97-xFe₂. Distinct spectra originating in domains and domain walls have been observed, both showing a non-linear variation of the hyperfine field with x. Results are analysed in terms of a model involving a non-linear dependence of the transferred hyperfine field from iron atoms on the number of gadolinium neighbours. The characteristics of the domain-wall resonances suggest that they originate from coupled nuclear-localised magnon modes.

Mossbauer spectra of dilute substitutional ⁵⁷Fe in VD_0.78 and NbH_0.84 were measured between 6 and 400K. The intensity of the Mossbauer patterns exhibits an anomalous decrease near 200 and 250K, respectively. This is attributable to the fluctuating lattice distortions caused by the diffusion of the interstitials and can be explained by an Arrhenius behaviour of the hydrogen jump rates with excitation energies of 0.14(2) eV in VD_0.78 and of 0.20(2) eV in NbH_0.84. For the displacements of the Fe probes by a nearest hydrogen neighbour values of 0.015(1) nm and 0.019(1) nm are derived for the two systems. The structure of the spectra and the temperature dependence of the centre shifts of the Mossbauer patterns reflect the hydrogen distribution near the Fe atoms, and thus properties of the different hydride phases as well as the interaction between the interstitials and the probes.

Fe and Sn Mossbauer effect measurements have been made on the amorphous alloys Fe₇₉SnB₂₀, Fe₆₉SnCr₁₀B₂₀, Fe₄₀Ni₃₉SnB₂₀ and Fe₃₅Ni₃₄SnCr₁₀B₂₀. Hyperfine field distributions, P(H), have been obtained from the Mossbauer spectra using Window's Fourier expansion method. For the Fe-site fields P(H) shows a single well defined peak in those alloys which do not contain Cr and a broadened bimodal distribution in those alloys which do contain Cr. The Sn-site P(H) consists of a broad single peak for all alloys. The results suggest that this distribution might actually be narrower in those alloys which contain Cr. These results are discussed in terms of the local environment of Fe and Sn atoms in amorphous transition metal alloys.

Mossbauer experiments on amorphous materials generally show an asymmetry often attributed to quadrupole splitting effects. On the other hand, some amorphous structures can be simulated with a soft-sphere relaxed model. The authors analyse this structure from the point of view of the electric field tensor, and look at the resulting distribution law for the quadrupole splittings and the contributions of dipolar terms to the internal field. By considering further a distribution of internal fields, a very simple model for analysing Mossbauer spectra is given and discussed.

Surface oxidation of three metglasses in the Cu-Zr system has been investigated by employing X-ray photoelectron spectroscopy and Auger electron spectroscopy with a view to comparing their oxidation behaviour with that of the corresponding crystalline states of the alloys. Surface oxidation of pure Zr metal has also been examined in detail using these techniques. Sub-oxides of Zr are formed during the initial stages of oxidation of Zr (at oxygen exposures <10L), while at higher exposures, ZrO₂ is formed together with the highest possible sub-oxide which the authors designate as 'ZrO'. The relative proportion of 'ZrO' goes through a maximum in the range 25-50 L. Both the glassy and the crystalline states of the Cu-Zr alloys exhibit preferential oxidation of Zr. The glassy alloys exhibit a higher rate of oxidation at intermediate exposures compared with the crystalline states of the alloys; the extent of oxidation at higher oxygen exposures is, however, higher for crystalline alloys. Interatomic Auger transitions have been found in the Zr+O₂ system as well as in Cu-Zr alloys.