Table of contents

A body centred cubic (BCC) form of elemental yttrium has been observed in rapidly quenched Y-Fe alloys. X-ray diffraction, ⁵⁷Fe Mossbauer effect and scanning electron microscopy measurements are reported. The lattice parameter of this previously unobserved form of yttrium (a₀=4.12+or-0.02 AA) is in good agreement with measurements on Y-Th solid solutions and the value calculated assuming an ideal BCC packing.

It has recently been proposed that some anomalies in the electrical transport properties of potassium below 4K can be explained in terms of an additional, indirect scattering from dislocations. However, it is shown here that the dislocation densities which have been assumed are several orders of magnitude larger than direct evidence suggests is possible.

The resistivity and thermoelectric power of metal-tellurium liquid alloys have been discussed for the case of small tellurium concentration. The nearly free electron model of the conduction band has been used. The rapid increase of resistivity in transition metal-tellurium alloys has been predicted.

The CW (steady-state) NMR observes the true T₂^-1 (=T₁^-1 for omega _e tau <<1) even for systems perturbed by the static quadrupolar interaction, because the CW line shape is equivalent to the Fourier transform of the relaxation function Psi _xx(t) which corresponds to the response to a radio-frequency (RF) pulse excitation of delta function form and not to a finite magnitude pulse smaller than the first-order quadrupolar splittings. The recent argument by Narath (1981) on the relationship between CW and pulse (transient) NMR experiments is shown to be incorrect.

The authors report small-angle neutron scattering measurements of pre-percolation clusters in Cu-16.7 at.% Mn at temperatures between 5 and 220K. They draw on the results of Monte Carlo computer experiments which show that pre-percolation clusters are diffuse and ramified, and fit their data by analogy with light scattering from Gaussian polymer chains. The average radius of gyration of a cluster at low temperatures is about 26 AA. By considering the topology of random lattice animals, they estimate that the average cluster contains about 170 Mn atoms.

The procedure for converting the hyperfine field (HFF) to constant volume is considered. It is shown that the equation invariably used for this purpose is valid as a first approximation only in those cases when the corrections are small. A new equation is suggested which is valid in the general case.

The authors have calculated the N_6,7 emission spectra from both Au and Pt sites in various Au_cPt_1-c alloys on the basis of the fully relativistic Korringa-Kohn-Rostoker coherent potential approximation (R KKR CPA) method. They argue that the corresponding experimental study would shed important light on the electronic structure of this interesting system.

Neutron inelastic scattering measurements have been carried out on two single crystals of the thermally quenched gamma phase (FCC) alloy Fe_1-xPd_x with x=0.28 and 0.37. The x=0.28 sample (crystal 2) exhibits a face centred cubic to face centred tetragonal (FCC-FCT) transition at T_m approximately 265K. The x=0.37 sample (crystal 1) does not undergo a structural transition. For both crystals the ( zeta zeta 0) TA₁ phonon branch, which corresponds to the elastic constant (C₁₁-C₁₂)/2, decreases in energy around the Gamma point as the magnetisation increases. The amount of softening of the elastic constant, Delta (C₁₁-C₁₂)/2, of crystal 2 is much larger than that of crystal 1 and has the same temperature dependence as the square of the magnetisation, M(T)². The temperature dependence of (C₁₁-C₁₂)/2 implies that this phonon softening is the cause of the FCC-FCT transition. The other branches display very little if any anomalous behaviour. A large phonon broadening has been observed in the soft phonon branch. Discussions of the broadening and the results of magnon measurements are also presented.

Microclusters of Pb and Bi, prepared by inert gas condensation, were studied by time of flight mass spectrometry. Cluster size distributions are presented for various metal and gas temperatures inside the condensation chamber. Increasing metal temperature as well as decreasing gas temperature result in a larger mean cluster size. Microparticles can be grown in a helium atmosphere whereas in other gases like Ar, Ne, Kr, N₂ and H₂ the particles become too large to be detected by the mass spectrometer. The cluster spectra show characteristic distributions, typical for each metal and independent of condensation conditions.

A method of describing the thermal fluctuations in the local atomic structure of liquid and amorphous metals in terms of the fluctuations in the atomic level stresses is proposed. The energy of the system is expressed in terms of these stresses in the elastic approximation. The temperature dependence of the second moments of the stresses is then calculated and shown to be linear with temperature. The glass transition is assumed to take place when the second moments freeze to certain values determined by computer simulation. It is shown that the pressure fluctuations and the shear fluctuations freeze at different temperatures. The glass transition, in the normal sense, is described by the freezing of the pressure fluctuation. This method provides a reasonable estimate of the glass transition temperature, and predicts it to be proportional to the product of atomic volume and bulk modulus. It furthermore provides an explanation of the different relaxation behaviours between the pressure and shear fluctuations.

In order to investigate the high-temperature phase H, two methods were applied: addition of copper and deviation of the composition from stoichiometric Au₃Zn. A model for the two different types of interface between periodically ordered domains is presented and a trace analysis of rectilinear boundaries is carried out. Extra spots in the diffraction pattern are predicted by treating the formation of periodic domains as a twinning in the tetragonal lattice.

The fully relativistic KKR method is used to calculate the electronic structures of Au, Pt and the dⁿs¹ impurities in these metals, where n=7, 8, 9 and 10. In the case of the pure metals, the calculated theoretical N_6,7 spectra are in excellent agreement with experiment. In the case of the impurities, characteristic differences in the electronic structures of 3d, 4d and 5d impurities in Au and Pt are found. For Ni in Pt the present calculations are remarkably similar to the R KKR CPA calculations for Ni in Ni_0.30Pt_0.70.

In an experimental study of positron annihilation in cold-worked aluminium, deviations from the simple trapping model are observed. It is shown that these deviations can be attributed to the non-uniform distribution of positron traps over distances larger than the positron diffusion length. TEM examinations support this hypothesis. According to a modified version of the trapping model, measured annihilation parameters are correlated to the volume fraction occupied by zones with very high positron trap density, and to the capture rate in the less damaged zones.

Describes an approach (essentially a generalisation of Henein's model, 1979) to the electronic structure of coherent composition modulated (CCM) alloys which does not require a full band structure treatment of the modulation to arrive at an understanding of its essential consequences. The approach consists simply of considering the modulation as a perturbation on a suitable reference system whose electronic structure is more easily calculated. The electronic structure of the model is formally equivalent to that of a charge density wave system, with the exception that the gap is not pinned to the Fermi level. The wavelength at which a supermodulus effect has been measured in certain transition metal-noble metal CCM alloys is quantitatively in agreement with this model's explanation as arising from critical contact between the Fermi surface and mini-zone boundary. The model accounts for the supermodulus effect in two systems (Cu-Ni and Ag-Pd) with qualitatively different Fermi surfaces as well as for the lack of supermodulus effect in Cu-Au alloys.

A simple phenomenological model using the concept of a rigid shell moving relative to its nucleus for the way in which tightly bound d electrons respond to the lattice motion, is presented to study the lattice dynamics of noble and transition metals. The short-range part of the interaction is treated using the first two terms of Taylor's expansion of the potential energy and the conduction electron response is given by a screened potential. The model, which also satisfies the periodicity condition of a lattice and describes the ionic lattice as being in equilibrium in the electron medium without recourse to external forces, has been applied quite successfully to calculate the dispersion relations of the noble metals, i.e. copper, silver and gold.

The electronic structure of pseudobinary (T_1-xT_x')Fe₂ compounds is studied using simple models, disorder being treated using the CPA. Two models are proposed and applied to (Zr_1-xHf_x)Fe₂. The authors conclude that the 'reservoir' model is the more adequate for this series of compounds. The magnetisation and isomer shift data (measured at Fe sites) are calculated and the agreement with existing experiments is satisfactory. New experiments to confirm the predictions of the model are suggested.

The authors have used the fully relativistic version of the Korringa-Kohn-Rostoker coherent potential approximation to study the electronic structure of Au_cPt_1-c random substitutional alloys. They have calculated the averaged density of states, partial density of states, and the Bloch spectral functions at various concentrations covering the full range. Contrary to previous assumptions, the results show clearly that these alloys do not follow the predictions of the rigid band model. Their calculated electronic contribution to the specific heat agrees well with experiment. They predict large relativistic effects in the soft X-ray absorption spectra. They have found well defined Fermi surfaces for all the alloys and have studied their evolution with concentration.

Using the modified augmented plane wave (MAPW) method the momentum densities of all occupied electronic states of Cu were evaluated, even at large values of the momentum. From this information in the one-particle scheme the Compton profile is determined for different directions of the transmitted momentum and compared with recent experimental results. As these considerations are free of any adjustable parameters the limit of the independent-particle model may be studied.

The results of band structure calculations for HCP Co in ferromagnetic and paramagnetic phases are presented together with a self-consistent FCC Co band structure. These calculations were performed by means of a linearised KKR method (model Hamiltonian) for lattice constants appropriate to atmospheric pressure and 290 kbar. On the basis of the calculations the authors offer a new model for the Fermi surface of ferromagnetic HCP Co. The stability of the paramagnetic state is studied for both HCP and FCC phases under pressure. Also, the results of band structure calculations are analysed by comparing the theoretical band structure with experimental measurements of the photoelectron spectrum. The calculated joint density of states for spin sub-bands in the ferromagnetic phase of HCP Co has shown agreement between the experimental data and the theoretical predictions of peculiarities in the energy dependence of the optical conductivity.

The band structures of both paramagnetic and antiferromagnetic phases were calculated in the framework of local density (or spin density) approximations of the exchange-correlation potential. In order to compute the antiferromagnetic phase of Cr the perfect nesting model was assumed. These calculations were performed as a function of the lattice parameter and also self-consistently using the linear hybridised KKR method. The ground-state and quasi-particle properties are discussed in comparison with the suitable experimental data. The pressure dependence of the magnetic moment, the nesting wave vector, the antiferromagnetic gap and the Fermi surface extremal cross sections are also discussed.

The Fermi surfaces for two BCC solid solutions in the Zr_cNb_1-c alloys have been investigated within the KKR CPA framework. The alloy compositions studied correspond to e/a ratios of 4.8 and 4.4 respectively. In this concentration range a change in the Fermi surface topology should be expected on the grounds of anomalies in the experimental elastic constants for such systems. The calculations do not give clear evidence that such a change does indeed occur. In fact the authors find that the influence of the disorder is more pronounced with respect to the Nb-Mo alloys and the claimed transition from the jungle-gym to the superjungle-gym structure involves heavily damped states. It turns out that such a smoothness is also reflected in the computed electronic contribution eta to the electron-phonon mass enhancement factor as a function of the concentration.

The contribution of electron-electron scattering to the point-contact spectroscopy signal at small V is shown to increase as ln(l/a) at large l, where l is the mean free path and a is the orifice size. Calculations are presented which give an indication of the value of l/a at which this contribution should be observable in potassium.

Electron transport properties have been studied in a number of Hume-Rothery type amorphous thin film alloys prepared by the vapour quenching method. Alloy systems employed were the ternary systems based on the Ag-Cu binary eutectic. Amorphous phases stable at room temperature were found in the following composition ranges: 0<or=x<or=0.8 in (Ag_0.5Cu_0.5)_1-xMg_x, 0<or=x<or=0.7 in (Ag_0.5Cu_0.5)_1-xAl_x and 0<or=x<or=1.0 in (Ag_0.5Cu_0.5)_1-xGe_x. In all amorphous alloys except those with Ge content higher than x=0.3, the Fermi diameter 2k_F, deduced from the Hall coefficient, agrees well with the corresponding free electron value. The formation of a stable amorphous phase in a wide composition range with an added third element, having a valence higher than one, allowed a wide range of well-defined 2k_F values to be mapped out and correlated with the composition dependence of rho _300K and the TCR. This provided a unique opportunity for testing the validity of the Ziman theory for the electron transport property of an amorphous alloy near room temperature. It was confirmed that the 2k_F/K_p ratio indeed closely controls the behaviour of the TCR, and that the TCR changes its sign at 0.98, in good agreement with the Ziman theory. However, it was also revealed that a negative TCR continues to exist even when the 2k_F/K_p ratio exceeds 1.15.

Electrical resistivities of liquid Li-Sn, Na-Sn and Na-Pb alloys have been measured. The resistivities exhibit distinct maxima close to 'stoichiometric' compositions: Li₄Sn, Na₄Sn, Na₄Sn₃ and Na₄Pb. At the same compositions the temperature derivative d rho /dT is negative and -d rho /dT assumes extraordinarily large values. The effects are attributed to strong chemical interactions between the components. These interactions tend to reduce the density of states at the Fermi level.

The authors evaluate the anisotropy in the residual resistivity of potassium which results from the hypothesis that the conduction electrons are in a static charge density wave state. The anisotropic relaxation times due to impurity scattering on the Fermi surface of potassium are evaluated by means of an interactive procedure which allows a numerical solution of the linearised Boltzmann equation. The impurity-scattering potentials are obtained from pseudopotential form factors which are locally rescreened and corrected for lattice distortion around impurities. They obtain an average value of the anisotropy in the residual resistivity which, for parameters characteristic of potassium in the charge density wave hypothesis, only slightly exceed the unity.

An investigation of the nature and properties of interface plasmons at metal-metal junctions has been completed. It is shown that interface plasmons possess two characteristic momenta (and wavevectors), one being two-dimensional and the other three-dimensional. The three-dimensional momentum may be so large as to rule out the possibility of observing interface plasmons at some interfaces. A diffuse density profile at the interface can produce a complex structure in the interfacial density of plasmon modes. However, there is always strong structure with a zero two-dimensional wavevector at the interface plasma frequency. For three metal systems it is shown how the structure, associated with the individual interfaces, arises in the interfacial density of plasmon modes.

Magnetisation, diffuse neutron scattering, ⁵⁷Fe Mossbauer measurements and nuclear magnetic resonance (NMR) studies were performed on the pseudobinary (Fe_1-xMn_x)₂Y and (Fe₁-_xMn_x)₂B intermetallic compounds. It is shown that in both systems the Mn atoms possess magnetic moments and that all these magnetic moments are ferromagnetically coupled. The concentration dependence of the individual Fe and Mn moments has been derived from a combination of magnetisation, neutron and Mossbauer data. The mean Fe moments show a strong decrease with increasing Mn content; the Mn moments appear to be less concentration dependent.

The magnetic excitation spectrum in HoCo₂ is calculated using a combined model of localised moments on the holmium atoms and itinerant d electrons on the cobalt atoms. The crystalline electric field acting on the holmium atoms is taken into account and the random phase approximation is used. The calculated results of the magnetic excitation spectrum for HoCo₂ at 6 and 18K are in fairly good agreement with recently observed inelastic neutron scattering measurements obtained by Castets et al. (1982).

Fe_100-xP_x(13<or=x<or=20) amorphous ribbons were prepared by rapid quenching from the melt. The Delta E effect and the magnetostriction were measured in order to discuss the magnetoelastic properties. Young's modulus at zero field is almost independent of the temperature, indicating the Elinvar characteristics over a wide temperature range. On the other hand, Young's modulus at saturated field shows some anomalous temperature dependence and its magnitude is larger than the paramagnetic Young's modulus extrapolated linearly above the Curie temperature. The room-temperature values of Young's modulus for Fe-P amorphous alloys are smaller than those for Fe-B amorphous alloys. The concentration dependence of the saturation magnetostriction lambda _s at room temperature indicates a broad maximum around 16 at.% P. The values of lambda _s is slightly smaller than it is for Fe-B amorphous alloys, even though the Delta E effect of the former alloys is several times smaller than that of the latter alloys.

The conduction electron spin resonance (CESR) linewidth of polycrystalline pure silver foils and some of its dilute alloys have been measured from 4.2 to 25K at 40 MHz, 9.27 and 20.9 GHz. The spin-flip scattering cross sections are given for copper, palladium, platinum and gold in silver; these are found to be frequency independent. There is however, a frequency-dependent contribution to the linewidth for all purities which is found to be linearly dependent upon frequency and inversely proportional to the sample thickness. It is suggested that this frequency- and thickness-dependent term is dominated by surface roughness and, as such, is due to high-angle scattering at the surface. The mechanism provides rapid mixing of states on the Fermi surface necks, those with large spin-orbit coupling with the very nearly free electron states on the 'belly'. With careful removal of the residual linewidth contributions caused by impurities, defects and the surface, the intrinsic temperature-dependent portion of the CESR linewidth has been extracted. At very low temperatures this varies as exp(-T/120) which suggests once again the importance of high-angle scattering brought about by the few remaining high-energy phonons found at these temperatures. As the temperature is increased, normal spin-phonon scattering of the 'belly' electrons start to take over, thus overriding the significance of the high-angle scattering leading then to a T⁴ dependence, in approximate accordance with expectations from phonon scattering theory.

Pulsed NMR measurements of the proton spin-lattice relaxation time, T₁, in yttrium dihydride containing controlled amounts of gadolinium are reported over the temperature range 140-1250K. Addition of gadolinium produces a temperature-dependent relaxation contribution which is proportional to gadolinium concentration and gives rise to a subsidiary minimum on the low-temperature side of the main minimum due to proton-proton dipolar interactions. The results are interpreted in terms of a magnetic dipolar interaction between the Gd³⁺ paramagnetic ions and the protons. The possibility that some of the diffusion data deduced from previous NMR measurements has been affected by the unsuspected presence of paramagnetic impurities is discussed.

The temperature dependence of the quadrupole interaction of ¹⁸¹Ta in an antimony lattice has been studied using the time-dependent perturbed angular correlation method. The quadrupole interaction frequencies nu _Q at 78, 293, 523 and 773K have been measured to be 499.5+or-1.3, 474.4+or-4.4, 454.9+or-3.8 and 425+or-4.8 MHz respectively. The effective EFG at 293K is (7.82+or-0.56)*10¹⁷ V cm^-2 with the temperature coefficient alpha _T=7.6*10^-6K^-32/.