Table of contents

High-pressure X-ray diffraction studies up to 40 GPa on praseodymium (Pr) metal reveal the presence of alpha uranium above 20 GPa. The appearance of this structure is related to the participation of f electrons in bonding, in close analogy with cerium (Ce) and americium (Am).

The application of the effective medium approximation (EMA) to the muffin-tin model of a liquid metal is considered. The triple correlation function is included in the expression for the electrical resistivity. Good agreement with experienced data can be expected for liquid noble metals.

The magnetoresistance Delta R(H)/R(0), of Cu₅₀Ti₅₀ and Cu₆₅Ti₃₅ amorphous metallic alloys has been measured at temperatures between 16 mK and 4.2K. In both samples it was found that the positive Delta R(H)/R(0) is independent of the magnetic field direction, proportional to H^1/2 and 10⁴ times greater than the expected normal magnetoresistance. These results are compared with calculations of the magnetoresistance involving the effects of Coulombic interactions.

The first theoretical study of the optical absorption in three magnetic phases of an ordered FeRh alloy is reported. The transition probabilities from occupied to unoccupied states are properly taken into account. Finally, it is shown that each magnetic phase has its characteristic absorption peaks.

Measurements are presented of the nuclear quadrupolar spin relaxation rate on an inert probe atom, ¹²³Xe (using the TDPAD technique), in Ga_cTe_1-c and In_cTe_1-c alloys. Despite the apparent similarities of these systems, the relaxation rate in In-Te alloys is larger than in Ga-Te alloys. A possible connection of this difference with a difference in bond type is discussed.

The systematic critical voltage phenomena observed in high-voltage electron diffraction provide a sensitive method for determining submicroscopic alloy parameters such as Debye temperatures, the extent of long-range order and atomic scattering factors. Only a very limited number of critical voltages can be observed in the voltage range usually available, 100-1200 kV, so that quantitative interpretation of the data must be based on a few-parameter model which incorporates all the pertinent factors. A satisfactory model has been developed which can be used to interpret or compute the critical voltage of ordered b.c.c. binary alloys as functions of composition, temperature and long-range order. This model can also be used to analyse the Debye-Waller factors of these alloys measured by X-ray or neutron diffraction techniques, and thus provide supplementary information about the vibrational behaviour of these systems.

For pt.I, see ibid., vol.13, p.1581 (1983). The critical voltage effect in high-voltage electron diffraction has been used to measure the 100 low-angle structure factors of the ordered b.c.c. alloys alpha 'FeCo, beta 'CoAl and beta 'NiAl as functions of composition and temperature and the 100 low-angle structure factors of beta 'CoAl and beta 'NiAl as functions of composition at room temperature. From these measurements values of the 100 and 110 low-angle atomic scattering factors and the atomic mean-square displacements (MSD) were obtained. The results indicate that more accurate critical voltage measurements and improved measurements of the 220 and 200 structure factors are required to obtain values of the 100 and 110 atomic scattering factors accurate enough to examine the electron charge distributions. A simple model was successfully used to analyse the MSD values of alpha 'FeCo and beta 'CoAl. For beta 'NiAl, the presence of the omega phase introduced complications to the MSD analysis, but with a minor modification to the model, the static displacements due to the diffuse omega were evaluated, and good agreement with the results of other workers was obtained. Studies on Al-rich beta 'NiAl alloy indicated that both Al antistructure atoms and diffuse omega are present in these.

The ability of the model potentials to yield accurate thermodynamic properties of liquid metals is tested. The pressure and bulk moduli calculated by the homogeneous deformation (HD) method depend on the electron density derivative of the self-energy. The latter is determined predominantly by the choice and specification of the bare ion form factor for q>2k_F. Most of the empirical model potential in the literature have been tested to give a good description only in the region 0<or=q<or=2k_F. The bare ion form factors widely differ from one another for q>2k_f and are not tested to give a good description in this range. Therefore no meaningful calculation of the pressure and bulk moduli of liquid metals can be made at present using the HD method.

The authors implement the KKR CPA for systems with two atoms per unit cell. They apply this to the study of the electronic structure of nonstoichiometric Pd-H and Ag-Pd-H. They find in comparison with previous CPA calculations the effects of random hybridisation and random 'overlap integrals' to be important. They also discuss in detail the self-consistent effective medium.

The 'spherical-solid' model of Almbladh and von Barth (1976) is modified to apply to a dissolved impurity ion in a simple liquid metal. The system of impurity ion and surrounding pseudo-ions is described using Percus-Yevick hard-sphere structure factors, and the electron density is obtained in the density functional scheme. The formalism allows for the relaxation of the liquid metal ions around the impurity ion, and the liquid structure is determined by minimising the free energy of the system. Calculations have been made for Cl, F and O dissolved in liquid solution, and values for the enthalpy of solution have been obtained.

Helicon damping in indium is investigated in magnetic fields extending to 8 T. A fine structure, observed for field directions within about 1 degrees of (100), is associated with magnetic breakdown. A critical breakdown field B₀<or approximately=7 T is indicated.

An analysis is presented of the partial quenching of phonon drag in the temperature-dependent part of the electrical resistivity rho (T) of strained samples of the alkali metals. The analysis is based on the anisotropy of electron-dislocation scattering, which is present in strained samples. The calculation of the dependence of rho (T) on strain is carried out for all the alkali metals. The calculated results are in good agreement with the low-temperature resistivity data for strained samples of potassium. A discussion is presented of the important role played by the Umklapp-to-normal scattering ratio and of the effect on the analysis of phonon-dislocation scattering.

The influence of the measuring temperature T_M on the residual resistivity Delta rho has been investigated in four alloys (Al-Ag, Al-Cu, Al-Zn and Cu-Be) where Guinier-Preston zone information leads to the time dependence of the resistivity showing a maximum during ageing. In the two alloys where spherical precipitates form (Al-Ag and Al-Zn), the influence of T_m on Delta rho seems to be negligible while the presence of monolayer plate-like particles (in Al-Cu and Cu-Be) induces a higher peak occurring at a later ageing time when the measuring temperature is raised. These experimental results are analysed by using the most recent theoretical models and the effects on Delta rho of the deformations induced by the precipitates are investigated.

The coupling between magnetism and chemical short-range order (SRO) is investigated in ferromagnetic alloys through an experimental investigation of the temperature dependence of the local order in both ferromagnetic and paramagnetic phases around the Curie temperature. Experiments were carried out in quenched dilute FeCo alloys through zero-field NMR at ⁵⁹Co nuclei. The conditions which allow the equilibrium SRO parameters to be obtained and their temperature variation to be measured from low-temperature NMR spectra are carefully discussed. An abrupt change in the behaviour of the local order is observed at the Curie temperature. The authors use a very simple theoretical model based on an Ising-Heisenberg Hamiltonian to account qualitatively for the experimental results. They point out the importance of the magnetic coupling on the chemical SRO so that measurement of local order in equilibrium states allows only a combination of chemical and magnetic pair interactions to be determined.

The authors present ESR data concerning manganese impurities dissolved in pure palladium and in the beta phase of palladium hydride. In the pure Pd matrix, the behaviour of the resonance of the Mn impurities in low-concentration alloys shows that: (i) the resonance is bottlenecked, (ii) the g factors of the conduction electrons and Mn²⁺ ions are different, (iii) the conduction electron-manganese coupling is strongly positive with a Korringa shift triangle g_K/g_i approximately=0.5 which agrees with the presence of a giant moment on the manganese ions. In the hydrides they observe a smaller but negative g shift, in agreement with the familiar covalent mixing interaction and some indication of a bottlenecked resonance. The spatial dependence of the conduction electron-manganese interaction is analysed using the ESR data and the residual resistivity. The influence of indirect and direct Mn-Mn interactions on the ESR is observed and discussed for the more concentrated alloys.

The seven-line quadrupole-split NMR spectrum of ¹⁶⁵Ho in the ferromagnetic alloy Ho_0.01Gd_0.99 has been studied by the spin-echo method at temperatures between 0.5 and 4.2K. The temperature dependence of the echo amplitudes from the two outermost lines shows that the total quadrupole interaction of the nucleus with its environment is positive. Re-analysis of earlier NMR measurements on ¹⁶⁵Ho in terbium- and thulium-doped gadolinium alloys, using more recent data on the nuclear quadrupole moment gives V_cc=+(5.3+or-0.2)*10¹⁹Vm^-2 for the contribution of the host medium to the axial component of the electric field gradient (EFG) at the holmium site. This figure indicates that the conduction electron contribution to the EFG is nearly three times larger than the contribution of the tripositive ions. Similar conclusions are derived from a re-analysis of NMR data on ¹⁶⁵Ho in orthorhombic terbium- and dysprosium-rich alloys at liquid helium temperatures.

The screened electromagnetic field is calculated for light incident on a jellium sphere; this is then used to calculate the photoemission from spheres of radius 20-30 au, using initial and final state wavefunctions appropriate to a square-well potential. The probability of photoemission is enhanced by a factor of about 100 around the Mie plasmon frequency compared with emission from a plane surface, an important contribution coming from resonances in the final state wavefunctions at low energies. The photoemission from an Ag sphere of radius 25 au is also calculated, using the bulk dielectric properties to find the screened electromagnetic field, and free-electron-like initial and final states. As the Mie frequency is well below the threshold for Ag, the photoyield is enhanced by a factor of only 5, and much smaller than the experimentally observed enhancement.