Table of contents

The authors present the results of calculations of the long-wavelength limit of the triplet concentration partial structure factor S_ccc, as a function of concentration, for liquids K-Cs at 373 K and Na-Cs at 383 K.

The X-ray scattered intensity from a liquid metal as an electron-ion mixture is described using the structure factors, which are exactly expressed in terms of the static and dynamic direct correlation functions. This intensity for a metal is shown to differ from the usual scattered intensity from a non-metal in two points: the atomic form factor and the incoherent (Compton) scattering factor. It is shown that the valence electron form factor, which constitutes the atomic form factor in a liquid metal, leads to a determination of the electron-electron and electron-ion structure factors by combining the ionic structure factor. The authors clarify some confusion which appeared in the proposal by Egelstaff et al. (1974) for extracting the electron-electron correlation function in a metal from X-ray and neutron scattering experiments. A procedure to extract the electron-electron and electron-ion structure factors in a liquid metal is proposed on the basis of the formula for the scattered intensity derived.

The density rho and the adiabatic variation of temperature T with pressure p, ( delta T/ delta p)_s, have been measured for the whole concentration range of liquid Li/Pb alloys and for a temperature range of 200 to 700 K (depending on composition) above the liquidus temperature T_L. The heat capacity C_p calculated from these data exhibits a pronounced peak at the composition Li₄Pb where at T_L it exceeds the ideal C_pⁱ by an excess value C_p^xs of about 0.8 C_pⁱ. Linear variation of C_p with T was only observed for pure liquid Pb. Strongly bent curves with more or less pronounced minima were observed for the other compositions, including pure liquid Li. The C_p^xs of liquid Li₄Pb compares well with a value calculated from neutron diffraction data by Ruppersberg and Schirmacher (1984).

The three Faber-Ziman partial structure factors as well as the Bhatia-Thornton (1970) structure factors of the Ni₃₃Y₆₇ and Cu₃₃Y₆₇ metallic glasses have been evaluated by neutron diffraction using the isotopic substitution method and X-ray diffraction. The chemical ordering is described by a function alpha (R) related to the partial coordination numbers z_ij(R). The relevant CSRO parameter alpha ₁ in the first coordination shell is chosen equal to the value of alpha (R) at the position of the first minimum of G_NN(R). Thus, the parameters alpha ₁ show a strong chemical ordering in Ni₃₃Y₆₇ and, in contrast, a tendency towards random mixing of both constituents in Cu₃₃Y₆₇. The evaluation of alpha ₁ by the same method in several Ni_xMt_1-x glasses (x approximately=0.33, Mt=Y, Ti, Zr, Dy) gives evidence of a relation between the strength of the bonding and the nature of the d transition element alloyed with Ni.

The transformation from the crystalline to the amorphous state of NiAl (B2 structure) in mixed multilayered thin films has been studied by in situ electrical resistivity measurements during ion irradiation at 77 K. Microstructural states are characterised by transmission electron microscopy observations. Kinetic behaviour shows that amorphisation is induced by an intracascade process and occurs only for heavy-ion irradiations. The influence of the cascade parameters on damage formation is investigated by varying the energy and mass of the irradiating projectile. The role of energy density, therefore, appears crucial in the collapse to the amorphous state. A reordering mechanism via enhanced radiation defect mobility is suggested. Results for irradiations with low-energy recoils with very dilute cascade structure are explained. Mechanisms are discussed in relation to previous amorphisation studies on irradiated metallic or semiconductor systems. The role of antisite defects is also emphasised for the transition towards topological disorder.

An LCAO approach to the density functional first-principles CPA theory of substitutionally disordered alloys is presented. The formal difference from the traditional tight-binding CPA consists of the explicit appearance of the overlap matrix in the CPA equations. More important, the first-principles self-consistent electronic structure theory may be directly implemented into the scheme. The theory is applied to paramagnetic CuNi alloys. Green function calculations involving Brillouin zone integrations are performed with the help of an improved complex energy k-space integration method. Details of the numerical procedures for the evaluation of the density of states (DOS) and the Fermi energy are discussed. Within the LCAO-CPA formalism an explicit formula for the integrated DOS is derived. Numerical results were obtained for the DOS, state-projected DOS and self-energies, Bloch spectral function and the Fermi surface. Good agreement is found with both previous KKR-CPA calculations and experimental data.

The recursion method is applied to the computation of the electron densities of states (DOS) of both pure Pd and non-stoichiometric PdH_0.8. The significant influence of the local environment on the formation of a hydrogen-palladium bonding band has been observed, as well as a modification of the main band. The bonding band is not separated from the main band and charge transfer occurs from H to Pd, resulting in filling of the d-band hole and of s-p states. Only 0.34 of added hydrogen electrons are transferred to the metal states. To achieve consistency with the experimental result of 0.36 for the filling of holes in the d band at the hydrogen concentration 0.6-0.7, significant modification of the DOS is required. This means that not even a modified rigid-band model can be used.

The self-consistent symmetrised relativistic APW method is employed to calculate the band structures of Sr and Yb at normal and high pressure. The self-consistent crystal potential is constructed on the basis of the local density approximation. For 4f electrons of Yb, however, this potential does not work well. Therefore, the correlation potential is determined for them separately so as to reproduce the position of the 4f electrons expected from XPS. The results of the calculations seem to be good in Sr. The behaviour of the observed electrical resistivity under high pressure is well explained by the characteristic change in the calculated band structures. In the case of Yb, the calculated band structure at normal pressure predicts the hole-like Fermi surface around (110) and (111) directions in accord with experiment. Furthermore, the present results under pressure predict the metal-semiconductor transition in good correspondence with experiments. This picture of a 4f-localised-itinerant transition is suggested for Yb in the FCC-BCC phase transition.

Self-consistent energy band calculations of Ni₃Ga are carried out by the symmetrised augmented plane wave (SAPW) method. In the construction of a one-electron potential, the local-density approximation is used. Momentum density distributions determined by Compton scattering and positron annihilation are calculated by the SAPW method. These anisotropies are due not only to the partially occupied bands but also to the fully occupied ones. The calculated anisotropy in angular correlation curves of positron annihilation radiation is in good agreement with the recent experimental result. The corresponding calculated result in the Compton profile is also in good agreement with the recent experimental result. However, in contrast to the case of elemental transition metals, the calculated Compton profiles and angular correlation curves of positron annihilation radiation show anisotropies of different structures. It is shown that this feature originates from the localised d electron states of Ni₃Ga.

The authors calculate the electronic structure of 3d impurities in niobium and molybdenum. The calculations rely on the local density approximation of density functional theory. Assuming a muffin-tin description of the atomic potentials, the impurity potential and the ones of the nearest neighbours are calculated self-consistently while the embedding of this perturbed cluster into the host is described correctly by the KKR-Green function method. In molybdenum rather large local moments are found for Mn, Fe and Co impurities. Surprisingly, in niobium similar large moments are obtained for Cr, Mn and Fe impurities. Whereas the results for 3d impurities in Mo agree very well with the experimental data, a serious disagreement exists for the niobium case which is not understood.

The electron-excited K alpha ₁ and K alpha ₂ X-ray spectra of the metals Cr, Fe, Co, Ni and Cu have been investigated with the aid of curved-crystal spectrometry, and are compared with respect to asymmetry indices and FWHM values. Three alternative approximations have been applied in the calculations, i.e. one symmetric lorentzian per line, two symmetric lorentzians per line and two skew lorentzians per line. Fractional contributions to the extra non-line intensity from the multiplet structure, the skewness effect and from radiative Auger processes have been evaluated, indicating that the multiplet structure is of major importance for Cr and Fe, while radiative Auger processes are dominant for Ni and Cu. The three effects yield about the same magnitude of extra non-line intensity in the case of Co. The radiative Auger processes give a relative intensity of approximately the same magnitude, i.e. approximately 8% of the main line peak intensity, for all the elements in question and for both the alpha ₁ line and the alpha ₂ line.

The incremental resistivity of hydrogen in alpha -Pd has been measured in the temperature range 250-350 K at hydrogen concentrations below 1 at.%. The reliability of the values obtained by electrochemical doping with H is discussed. The strong temperature dependence of the incremental resistivity is due to the electron scattering on optical phonons having a low excitation energy in dilute Pd-H.

Measurement and analysis of the critical field and superconducting fluctuation conductivity of quench-condensed (amorphous) Ti-V alloy systems are reported. The critical field curves agree very well with WHHM theory fitted with the parameters alpha , lambda _SO. Values of the density of states obtained from the Pauli paramagnetic parameter show that it increases as Ti composition increases. The spin-orbit scattering time obtained from lambda _SO is also discussed. In zero field, including the Maki Thompson contribution, the superconducting fluctuation is shown to be essential for obtaining agreement between theory and experiment. Knowledge of the inelastic scattering time is obtained through pair-breaking parameters. The theoretical prediction that the Maki-Thompson term is relatively more important in a magnetic field agrees with measurement.

The preparation and superconducting properties of a disordered 3D superconductor-normal metal composite are described. The composite consists of spherical micrometre-sized grains of Pb with a typical separation of 10 mu m embedded in a very pure Al matrix. The composite exhibits bulk diamagnetism and zero resistance well above the transition temperature of the Al matrix, and has a well defined critical current which varies exponentially with temperature. The effect of a magnetic field on the electrical transport properties is examined and discussed, as is the nature of the superconducting and dissipative states.

A study of low-temperature specific heat in the Zr-Co crystalline system is presented, the results being analysed with the magnetic properties which have been investigated for ZrCo and ZrCo₄. Two regions can be distinguished in the Zr-Co system: a Zr-rich concentration range of compounds which are superconductors and a Co-rich concentration range of compounds which are magnetic. Two slight bumps of the electronic specific heat coefficient gamma are observed. One corresponds to Zr₂Co which has a high superconductivity temperature T_cs and a low Debye temperature theta _D, and the other corresponds to ZrCo₂, whose concentration corresponds nearly to the appearance of ferromagnetism. The ZrCo compound shows an unexplained high gamma value.

The electronic structures of T₂P (T=Mn, Fe, Ni) which have two types of T atoms T_I and T_II, were calculated by the KKR and the LMTO methods in the framework of the LSD approximation. From the position of the Fermi level in the paramagnetic DOS, one can roughly guess that Mn₂P exhibits antiferromagnetism, Fe₂P ferromagnetism and Ni₂P paramagnetism and that the magnetic moment is smaller on T_I than on T_II in T₂P. The existence of the very sharp and high peak of DOS at the Fermi level suggests that the magnetic properties of Fe₂P are sensitive to the experimental conditions. Electrons transfer from the P and T_II atoms to T_I atoms.

The authors present results obtained by ⁵⁷Fe Mossbauer spectroscopy on R₂Fe₁₄B, where R represents any of the rare earth elements except Pm, Eu and Yb. Additional measurements have been performed on oriented powder for R=Y, Pr and Ho. For the case of Y the measurements at 4.2 K were performed in applied magnetic fields up to 4 T. From these measurements it is deduced that the only compounds with an easy c axis at room temperature which show a spin reorientation of the iron sublattice at a lower temperature are the Nd and Ho alloys. Iron hyperfine fields are anisotropic and the anisotropy has different signs on different sites. An analysis of the non-4f contribution to the electric field gradient at the ¹⁶¹Dy or ¹⁶⁶Er nuclei in Dy₂Fe₁₄B and Er₂Fe₁₄B is consistent with published results on ¹⁵⁵Gd in Gd₂Fe₁₄B. In order to explain that a spin rotation is found only for R=Nd, Ho the authors invoke the relative small quadrupole moment of the 4f shell for these two ions and crystal field terms of higher than second order.

The authors present a first calculation of the 2p to 5d X-ray absorption (L_III-XAS) in light rare earth metallic compounds by using the Anderson impurity model in relation with recent L_III-XAS experiments. The results are exact in the limit of large degeneracies N_f and Coulomb interaction U_ff to infinity . Hybridisation V between localised 4f and extended 5d states is shown to be determinant in both the initial and the final states of the photo-absorption process and a close relationship through a convolution is established between the core XPS and the L_III-XAS spectra from symmetry requirements. The authors discuss the absorption spectra with respect to the valence of the considered compound as well as to the parameters entering the model (f position, V, U_fc, density of states etc.). They obtain a good agreement with the experimental data (XPS, XAS) especially for LaPd₃ compounds and for the gamma alpha transition in cerium, and show that the L_III-XAS spectra allow one in general to obtain a reliable value for the ground state valence.

The authors describe a procedure to calculate the X-ray spectra of the intermetallic compounds FeAl and FeRh with the aid of the ab initio self-consistent band structures obtained by the method of linear muffin-tin orbitals (LMTO). The oscillator strengths are properly calculated by using the LMTO. The calculated X-ray spectra of FeAl compare well with other theoretical and experimental data. The X-ray spectra of Fe-Rh in three magnetic phases have been obtained for the first time. The particular features of these spectra in each phase have been laid out to distinguish one magnetic phase from another.