Table of contents

The high-pressure structural behaviour of two berkelium-californium alloys was studied up to about 47 GPa. The alloys had been formed through the ingrowth of ²⁴⁹Cf into ²⁴⁹Bk by beta ^- decay. The berkelium-rich alloy (about 35% Cf) exhibited the same phases as pure berkelium metal, the californium-rich alloy (about 60% Cf) the same phases as pure californium. An approximately linear increase of transition pressures with californium content was observed. The results are compared with data obtained previously for the high-pressure behaviour of the pure americium, curium, berkelium and californium metals. As in the pure metals, the transformation of the alloys to the alpha-uranium-type structure, accompanied by a sudden volume decrease, is interpreted as the onset of 5f electron itinerancy.

The authors report the growth of high-quality bicrystal superlattices formed from alternating layers of HCP Ru and FCC Ir metals. Ir layers which are thick compared with the Ru do grow in the FCC structure, whereas thin Ir layers take the HCP structure, probably owing to strain effects.

The contribution of normal electron-electron scattering to the anomalous surface impedance is evaluated taking the spatial anisotropy due to the presence of a thin skin depth in the metal into account. The authors find a T² temperature dependence for the electron scattering rate which is in complete agreement with the available experimental data on Cu and Al.

Recent experimental data on positron annihilation in alkali metals are compared with the results of different theoretical approaches to this problem within the Kahana formalism. The necessity of abandoning the biparabolic formula of Kahana (1963) in order to make progress in the investigation of momentum-dependent enhancement factors is emphasised.

This review deals with spin fluctuation theory (SFT) in the regime where quantum-mechanical correlations are important (usually at low T). A detailed description of different sorts of SFT is given, including paramagnon theory, local spin-fluctuation (LSF) theory, the Fermi liquid theoretic (FLT) approach and functional integration techniques. It is shown how each of these theories can be applied to various physical systems, including ³He (normal and superfluid), metals, alloys and impurity systems, with the emphasis on the structure of the theories rather than how they have actually been applied. Briefer discussions are given of 'polarisation potential' theories, Gutzwiller theory and renormalisation group methods. A critique of the relative strengths and weaknesses of the various SFTs, and of the information they actually provide about spin fluctuations, is then given. Finally, some areas of current excitement are reviewed, namely the theory of two-dimensional Fermi systems, spin-polarised Fermi systems and 'heavy fermions'.

A model potential for metal-salt solutions in the metal-rich region, based on the linear screening theory, was examined by molecular dynamics (MD) simulations for molten K-KCl mixtures. The effective potentials between ions are characteristic of their nonadditive repulsive cores. The computed structure factors are in semi-quantitative agreement with neutron diffraction data. At a metal concentration of 0.8 the simulated system showed a tendency to local ordering of chlorine ions.

Pseudopotential calculations of the valence electron densities in the simple metal Laves phases KNa₂, MgZn₂, CaLi₂, CaMg₂, and CaAl₂ are presented. The results show an increased electron density due to chemical bonding effects at the centre of the tetrahedra formed by the small atoms, and between the large minority atoms, in analogy to recent experimental charge-density studies. It is shown that the variation of these characteristic features parallels the observed trends in the stability of these compounds.

The phonon dispersion relations for the FCC cubic alloys Fe_0.7Ni_0.3, Fe_0.5Ni_0.5, Fe_0.25Ni_0.75 and Fe_0.72Ni_0.28 have been investigated using an angular force model. The theoretical results are found to be in good agreement with the experiment. An inference is drawn that in these alloys there is no disorder or clustering effect.

The authors have measured the diffusion of hydrogen in Pd and Nb as a function of magnetic field by means of a dilatometric method (Pd) and the Gorsky effect (Nb). For Pd at T=570K, (H)/(Pd)<or approximately=10^-2 and B=10 T no influence of the magnetic field was detected within an experimental accuracy of <or approximately=3%. The same conclusion was reached for the experiments on NbH_x (T=455K, (H)/(Nb) approximately=0.04 and B=10 T) within an accuracy of approximately=1.5%. The results are discussed on the basis of the theory of irreversible processes and compared with earlier work on systems for which significant magnetic field effects on the diffusion in metals have been reported.

A simple tight-binding model is for a quantitative study of hybridisation effects in transition-metal-polyvalent-metal (T-P) alloys. The model is not an ab initio calculation but it requires only the data on elemental properties of pure metal constituents which are readily available in the literature. The Cluster-Bethe lattice method is used to calculate the electronic density of states and thermodynamic properties of (Fe, Co, Ni)_xAl_1-x and Co_xGa_1-x alloys. It is shown that the electronic density of states around equiatomic composition is characterised by the occurrence of a pseudogap near the top of the d band. The authors use the model to discuss the essential features of some anomalous electronic and thermodynamic properties of T-P alloys.

For pt.I see ibid., vol.14, p.1673 (1984). A simple method is used to calculate the dynamic local field corrections for the dielectric function of an electron liquid. Compared with other dynamical approaches, the calculations yield the best results for the correlation energy and the pair correlation function.

In many dilute transition-metal alloys impurity atoms produce virtual bound d states which lie above the host d band. The factors governing the widths of these virtual bound states are discussed and a numerical calculation of these widths due to other authors is analysed and interpreted in the light of this discussion. The results are then used to interpret qualitatively experimental data on the magnetic susceptibility of some concentrated Zr alloys. Photoemission studies of some alloys are also discussed.

The theory of the anomalous skin effect is generalised to include a general elastic boundary scattering law. The results pertaining to the conventional diffuse or specular boundary scattering are obtained as special cases. An exact result is derived for the high-frequency limit of the surface impedance. It is suggested that this limiting value can be used to discern different boundary scattering mechanisms. Explicit numerical calculations are presented for model boundary scattering laws.

Electrical resistivities due to dislocations, rho _d(T), in pure aluminium and the dilute alloys containing Mg, Ag and Zn solute atoms were measured, and the resistivity minimum of rho _d(T) was found to be approximately 20K. The resistivity minimum was not found for highly deformed samples or samples annealed at fairly low temperatures after the deformation. The temperature dependence of rho _d(T) was -log T-type in the range below the temperature at which the resistivity minimum was found. The characteristic temperature dependence is suggestive of the spin-dependent Kondo effect or the two-level system. Experimental results were analysed by the expression rho _d(T)=A-B log (T²+T_Delta ²), and the best-fit parameters A, B and T_Delta varied with the dislocation density, the species of solute atom and the concentration.

The authors report measurements on the lattice parameter, electrical resistivity, thermal conductivity, thermoelectric power, susceptibility, magnetisation and specific heat as well as preliminary neutron diffraction results for the orthorhombic compound CeCu₂. The Ce valence is close to three and the overall crystal-field splitting seems to be approximately=200K. CeCu₂ behaves as a Kondo lattice system with a 'Kondo temperature' in the crystal-field ground state being of the order of 10-20K. Below 3.5K CeCu₂ shows magnetic order of an anisotropic antiferromagnetic type.

Mossbauer and DC susceptibility studies are reported on the mixed spinel Mg_1+tFe_2-2tTi_tO₄ at various temperatures and in an external magnetic field. Results are obtained indicating that on increasing dilution t, the collinear ferrimagnetic phase breaks down before reaching the ferrimagnetic percolation limit as a result of the presence of competing exchange interactions. In this dilution region there is a second transition, at T_f, to a spin-glass-like (SGL) state, well below the ferrimagnetic ordering temperature T_N. The lower limit of re-entrant ferrimagnetism is determined by observing the spontaneous spin canting which occurs in the presence of the external magnetic field on going into the SGL state. At higher dilution a transition to a pure spin-glass state occurs. The results are discussed in terms of the model proposed by Villain (1979) based on the competition between dilution and frustration in the spinel lattice. The re-entrant ferrimagnetism found in this insulating system with near-neighbour interactions is similar to the re-entrant ferromagnetism found in some metallic alloys, and the authors propose a canting order parameter for the low-temperature state as in the metallic case.

The susceptibility, magnetisation (ZFC and FC) and time dependence of the thermoremanent magnetisation are measured on ordered pseudobinary alloys Pt₃(Mn_1-xFe_x) with 0<or=x<or=0.75. A detailed magnetic phase diagram for the system is given, which can be understood on the basis of a cluster model. In the range 0.47<x< 0.62 a re-entrant spin-glass phase is identified. Comparison with results as achieved on cold-worked disordered alloys shows that for the understanding of the re-entrance phenomenon, a finite magnetic correlation length has to be assumed.

For pt.I see ibid., vol.15, p.681 (1985). Magnetic and structural studies using X-ray, neutron diffraction, magnetisation and small-angle neutron scattering (SANS) techniques are reported for FeAl_1-xT_x alloys in which partial substitution of Al by Mn or Co has been effected. The results of the neutron diffraction studies are consistent with a B2 model in which the Co atoms exercise a site preference for Fe sites, whereas Mn atoms appear to enter both Fe and Al sites. Consequently, for both systems displacement of Fe atoms into Al sites creates 'antistructure' (AS) Fe atoms. The susceptibilities of both alloy systems were observed to increase with the concentration of the transition metal solute. An apparent onset of spontaneous magnetisation was observed beyond certain 'critical' concentrations: x=0.3 and x=0.2 for Mn and Co substitutions respectively. However, it was not until a Co concentration with x=0.3 was reached that a Curie temperature could be established in the Arrott plots. No T_C could be established at all in similar plots for the Mn series having x<or=0.4. Furthermore, the SANS measurements revealed no sharp peaking in the intensity contours of the critical magnetic scattering at temperatures where Arrott plots (based on high-field magnetic data only) had indicated a Curie temperature. Possible mechanisms for these observations are discussed in terms of current theories of the onset of magnetic order in magnetic transition intermetallic compounds.

X-ray emission spectra of 3d metal impurities (Cr, Mn, Fe, Co, Ni: 0.5-4 at.%) in Cu are presented. It is shown that the width of L_alpha emission spectra depends on the presence of local magnetic moments on emitting atoms. For Cu-Ni and Cu-Fe alloys, L alpha spectra become broader as Ni or Fe concentration grows; for Cu-Mn and Cu-Cr alloys, L_alpha emission spectra of isolated impurities carrying local magnetic moments are broader than spectra of pure Mn and Cr. These results are discussed on the basis of cluster calculations of electronic structure.