Table of contents

A solute-vacancy binding energy of 0.36 eV has been estimated for Sc in Cu. The same value of 0.36 eV is also predicted for the case of Si in Cu.

Pd lattice parameter and ⁵⁷Fe isomer shift (IS) measurements have been correlated in PdH_x-Fe matrices (0.3 at.% Fe) over a wide range of hydrogen concentration in the alpha ' (lattice liquid), and in the beta and alpha " phases detected previously by Mossbauer measurements. The authors find that the IS is proportional to the logarithm of the unit cell volume, IS varies as 3 ln a₀, just as in IS measurements under pressure. The experimental value of the slope, +3.92 mm s^-1, is however much larger than that found by pressure measurements in similar matrices. This shows that the effect in PdH_x is of electronic origin, and gives support to the band calculations which have predicted partial localisation of hydrogen electrons in a Pd-H bond.

A new phase transition in an intermetallic compound K₂Cs has been found at about 110K. Both X-ray scattering and resistivity measurements show an enormous change in volume ( Delta V/V approximately 0.14) at the transition temperature. The samples of K₂Cs appear to be extremely soft and they recover completely after the strong deformation. In the present letter the authors suggest that these effects are connected with the electronic transition in Cs atoms discussed by R.L. Sternheimer (1950).

The K beta (3p to 1s) X-ray emission band has been recorded for liquid silicon. It provides a determination of the 3p partial density of states in the valence band. Comparisons are made with equivalent emissions from the crystalline and amorphous solid phases. It is found that the 3p distribution broadens by 1.7 eV in the liquid (full width at half maximum amplitude) compared to crystalline Si. Practically no evidence remains of the sp mixing observed in the bottom of the valence band of the crystalline and amorphous phases. The shape of the K beta emission band in the liquid closely resembles that expected of a simple metal, confirming the presence of four conduction electrons.

The electron-phonon contribution to the low-temperature electrical resistivity of copper and gold is calculated using two plane-wave states for the electrons and the eight-cone model for describing the Fermi surface. The approximate T⁴ behaviour which has been observed recently for copper and gold is examined in terms of the calculated electron-phonon contribution plus a suitable electron-electron term. The results of the calculations are presented and discussed.

The spin density induced by a magnetic solute atom in a dilute alloy is investigated in the method of the dielectric screening for transition metals by taking into account the redistribution of electrons with each spin in the d and conduction bands. Numerical calculations are carried out for dilute PdFe alloys. The calculated results are consistent with the observed results for the magnetic neutron diffraction and the magnetisation.

Neutron vibrational spectra have been measured for both crystalline and amorphous titanium copper hydride from 40-200 meV. A rather narrow distribution of optical (hydrogen) vibrations is observed for the crystalline alloy TiCuH_0.93 with maxima at 142 and 157 meV, very close to previous results for gamma -TiH₂. Amorphous TiCuH_1.3, however, exhibits a very broad band of hydrogen vibrations ( approximately 76 meV FWHM) peaked at about 145 meV. This density of states provides a probe of the local environments of the hydrogen atoms residing in holes in the metallic glass structure and suggests an 'average' occupation of tetrahedral holes, but considerable fluctuations in local symmetry around these sites.

X-ray and bulk density measurements on ordered B2 alloys in the series Co₂Ga_2-xFe_x (x=0.05-0.20) have yielded vacancy concentrations in the range 4 to 7%. Room temperature ⁵⁷Fe Mossbauer spectra of the same alloys have been obtained. A computer simulation of a lattice model in which the Fe atoms randomly occupy sites on the Ga sublattice has been used to determine major near-neighbour environments of the Fe atoms. Individual components corresponding to 0, 1 and 2 vacancies in the near-neighbour shell have then been successfully fitted to the Mossbauer spectra. Room temperature Mossbauer spectra for a second series Co_2-yGa_1.92+yFe_0.08 (y=0.0-0.20) in which even more vacancies are forced on to the Co sublattice are also presented. Marked changes in the spectra at y=0.15 appear to be due to the occupation of vacant Co sites by Fe atoms. Mossbauer parameters for the alloys CoGa₃ and FeGa₃ have been measured and provide support for this interpretation.

Anharmonic phonon frequency shifts Delta omega (T) and damping Gamma (T) in alkali metals are calculated using a previously described pseudopotential model. The theory is in good agreement with the most accurate measurements for Li. For Na, K and Rb agreement usually lies within the experimental errors. Relative shifts delta = Delta omega / omega and relative damping eta = Gamma / omega increase linearly with T when T>or approximately=(0.3-0.5) theta _D, the coefficients being close for different alkali metals. Values of 5Gd and eta vary significantly in the Brillouin zone but remain small up to the melting point for all the phonons except those in the vicinity of the soft Sigma ₄ branch. Phonon frequencies for this branch at large mod K mod increase rather than decrease with T. The sensitivity of the results to the model and calculation approximations has been studied. Comparison with other authors' calculations appears to imply that 'self-consistent phonon'-type approximations are unreliable in the description of anharmonic effects.

The vibrational spectrum is investigated for the realistic models of amorphous metallic alloy Fe_100-xP_x. It is shown that the vibrational spectrum has two peaks. The high-frequency and low-frequency peaks are caused by the Fe-P and the Fe-Fe interaction, respectively; the former is interpreted as being due to the localised optical vibrational mode and the latter to the acoustic one. The first moment and the sound velocity in amorphous systems are reduced by about 10% in comparison with those in crystal. This is understood in terms of larger atomic volume in amorphous systems.

The authors derive a general thermodynamic mean field theory that yields the coherent potential approximation and the usual Bragg-Williams approximation directly. Short-range order effects are directly related to effective pair interactions identical to those obtained in previous works.

Heats of absorption and desorption of FeTi in equilibrium with hydrogen were determined in a heat flow calorimeter together with pressure-composition isotherms at 274, 298, 314 and 344K in the composition range of Fe_0.5Ti_0.5H_x for 0<or=x<or=1. The hydrogen concentration in the dilute alpha -phase is determined by the Sieverts' law relation, ln x=¹/₂ ln (p(H₂))-2-554 K/T (endothermic solution) where p(H₂) is measured in bar. The plateau pressure for the coexistence of the gas phase wth the two hydride phases alpha and beta in the range 0<x<0.5 is given by ln (p_{alpha beta} )=4.8-1257 K/T; the plateau pressure for the coexistence region ( beta + gamma ) in the range 0.5<x<1 is given by ln (p_{beta gamma} )=6.5-1571 K/T. Very high exothermal heats of reaction observed at low concentrations (Fe_0.5Ti_0.5H_x<1%) are ascribed to adsorption effects at the surface of the powder (<10 mu m) used for the samples.

Flory's formula (1942) for the entropy of mixing is applied to a binary mixture of hard spheres. The results from this approach are compared with those obtained from hard-sphere model calculations and the validity of Flory's formula is discussed.

Calculations of the orbital and spin susceptibilities of conduction electrons in a metal film of arbitrary thickness and under a magnetic field applied perpendicular and parallel to the surface of the film are made using the infinite-square-well potential model. The position dependence of these susceptibilities in the film is also calculated. The orbital susceptibility in the direction perpendicular to the surface is -1/3 of the spin susceptibility at any position in the film. It is shown that both the orbital and spin susceptibilities have an oscillating dependence on the thickness of the film.

On the basis of self-consistent APW bandstructure calculations for the stoichiometric alloys FeAl, CoAl and NiAl, densities of states and partial densities of states for these alloys are calculated using a hybridised NFE-TB interpolation scheme. The calculated density of states of NiAl is compared with its XPS and UPS spectra. Whereas the calculated joint density of occupied and unoccupied states of NiAl agrees rather well with the experimental imaginary part of the complex dielectric function epsilon ₂( omega ), an approximative calculation of transition moments was necessary to reproduce also the main structure of the epsilon ₂( omega ) curve of CoAl.

The de Haas-van Alphen effect has been measured in the intermetallic compound NiSi, and the bandstructure has been calculated by the LMTO method. Most of the frequency branches observed experimentally can be obtained on the basis of the calculated Fermi surface.

The energy bandstructure for the cubic Laves-phase intermetallic compound YAl₂, which has recently attracted interest as a proper reference material for the study of anomalous physical properties of rare-earth dialuminides, is calculated self-consistently by a non-relativistic APW method with the Kohn-Sham exchange potential. The Fermi surface consists of various sheets, for which the extremal cross section and the cyclotron mass is carefully calculated. The eighth-band Fermi surface consists of small hole pockets around the Delta axes. The ninth-band Fermi surface is a multiply connected hole sheet, the topology of which is a junglegym running from an X point to a nearest-neighbour X point. Open orbits exist in (100) directions. The tenth-band Fermi surface is an electron sphere centred at the Gamma point, but has bumps in (100) directions. The calculated extremal cross sectional area of the tenth-band Fermi surface agrees within 10% with an experimental result of the recent de Haas-van Alphen effect measurement.

The equation of motion of the density fluctuation Green function is cast into a form from which an integral equation for the full Hartree-Fock proper polarisation function Pi (q omega ) is extracted. An iterative solution procedure which can be used to write down approximate 'all order' solutions is presented. It is shown that even the lowest order iteration satisfies the full Hartree-Fock compressibility sum rule exactly. An approximate procedure to calculate the fully iterated all order dynamic polarisability Pi (q omega ) is presented. The small r_s behaviour of Pi (q,0) yields a local field which is in excellent agreement with that of the first-order HF result, and provides estimates of the second-order HF function Pi ₂(q,0) which has not been calculated directly, as yet. Further, it is shown that if the Coulomb potential is replaced by a certain type of Fermi surface average (FSA), the resulting very simple polarisation function Pi ^FSA(q omega ) provides a surprisingly good approximation to Pi (q omega ).

The interaction between positrons and dislocations is considered from a new point of view, which leads to a new interpretation of positron results obtained for metal samples containing dislocations. The positron binding energy in a perfect dislocation line is assumed to be small (<or approximately=0.1 eV). The observed changes in the positron annihilation parameters due to the presence of dislocations then originate from annihilation in point-like defects (e.g. jogs) associated with the dislocation line. The positron-dislocation line trapping rate due to phonon emission is calculated for various binding energies and temperatures and is found to be unexpectedly large. It is shown that this interpretation predicts strong temperature dependences for the positron trapping rate in annealed samples at temperatures below 300K due to positron detrapping.

The true low field Hall coefficients of various high purity copper samples and samples containing low density defects have been examined in a field of 0.0085 T over the temperature range 4.2-77K. The results on pure copper are compared with measurements made in a field of 0.5145 T. The Hall effect in copper and its dilute alloys can be analysed satisfactorily in terms of a two-group model and the band parameters obtained are able to account for the extremely large deviations from Matthiessen's rule observed in copper containing dislocations. The temperature dependence of the electron-phonon scattering anisotropy is deduced from the Hall results and found to be in good agreement with an earlier theoretical calculation.

Anisotropic cluster model electron lifetimes are calculated for impurity scattering in the dilute potassium alloys K(Na) and K(Rb). The system is modelled by a finite cluster in free space. The largest cluster contains 35 scatterers corresponding to 4 BCC shells. An electron described with respect to a host cluster is scattered by an impurity cluster. An impurity cluster consists of a substitutional impurity surrounded by a number of possibly relaxed shells of host atoms. Only s and p scattering are accounted for. The phaseshifts used for the individual scatterers are taken from published APW fits as well as from potentials constructed in a standard way. It is found that the average lifetime for both alloys has the right order of magnitude. Further p scattering contributes significantly for the case of K(Rb). The anisotropy of the inverse lifetimes is very dependent on cluster size and does not converge when the cluster size increases.

Tunnel junctions have been formed under high vacuum, by the evaporation of ytterbium on to a GaAs single crystal surface. The ytterbium metal had been purified by evaporation-condensation cycles under vacuum. At normal pressure, the differential resistance of the junctions shows a very strong dependence on bias voltage and temperature. The application of a hydrostatic pressure of 25 kbar destroyed this anomaly. It was then possible to observe the semimetal-semiconductor (SM-SC) transition near 10 kbar and measure the variation of gap parameter with pressure.

Spin waves with intrinsic linewidth have been observed in disordered Ni_1-xFe_x (x=0.5, 0.6, 0.68) and Ni_1-xMn_x (x=0.05, 0.075, 0.13) alloys. The lineshapes of the neutron groups have been analysed in order to extract the linewidth Gamma in a simple Lorentzian approximation of the cross section. The variations of Gamma with the spin wave energy omega obtained for each sample show a different behaviour for the NiFe alloys from the NiMn ones. It is suggested that in the NiFe alloys, spin waves are broadened by strong magnetic inhomogeneities (clusters) while in the NiMn alloys the broadening reflects a competition with the Stoner excitations.

Electron spin resonance measurements for the pseudobinary system Gd(Co_xAl_1-x)₂ with x<or=0.6 are reported. The results indicate a change of sign of the Gd-conduction-electron interaction parameter J at x approximately=0.3. This is shown to be consistent with a variation of the Curie temperature with x.

The ⁷Li Knight shift has been measured in liquid Li-Pb and Li-Sn alloys through the entire concentration range and at several temperatures. The Knight shifts in liquid Li-Pb are discussed together with the electrical conductivity and the spin-lattice relaxation time T₁, which also exhibit anomalous behaviour near the composition Li₄Pb. The results support the assumption of a partially salt-like mixture around the composition Li₄Pb.

Measurements are reported for the K emission and self-absorption edges of metallic beryllium. The self-absorption effect is small in this material and both edges are dominated by large core level widths.