Table of contents

The authors report their high-pressure X-ray studies on quasicrystalline Al-18 at.% Fe up 100 kbar. A metal anvil clamp cell has been used during the experimental investigation with NaCl as the in situ pressure marker. While the d spacings of NaCl corresponding to various Miller indices show a similar compression under pressure, those of the quasicrystals show a large anisotropic compression.

Small-angle neutron scattering measurements on an Al-7.5 wt.% Cr-1 wt.% Fe alloy, grown by vapour deposition, have revealed a sequence of changes in cluster shape during low-temperature precipitate growth. The interpretation arose from a Monte Carlo model which allowed the shape of particular clusters to be followed as a function of time and temperature, and compared with the corresponding scattering pattern. An evolution in cluster shape from predominantly linear, through planar, to roughly spherical was found, depending on the coordination number of cluster atoms stable at each temperature.

From a first-principles local pseudopotential the authors calculated the ion-ion potential for lithium. The pseudopotential was obtained from the electron density around a lithium nucleus in an electron gas. Using the harmonic and the self-consistent harmonic approximation they found the phonon dispersion curves from the ion-ion potential. Then the phonon spectrum and the lattice specific heat were obtained. The effect of pressure was introduced by the electron gas density parameter r_s. The authors found a quadratic variation of the lattice specific heat with pressure.

Ultrasonic measurements have indicated that there are possible structural changes near 250 K, 160 K and just above T_c in the high-T_c Y-Ba-Cu-O and Y-Ba-Cu-Nb-O systems, which were confirmed to be first-order-like phase transitions by thermal analysis (DSC) results. It is suggested that the heavily suppressed pre-superconducting transition, which provides the lattice instability and the soft phonon modes, gives rise to the high T_c.

An electron spin resonance (ESR) study on Au₈₂Fe₁₈ has been carried out over a wide temperature range (4.2<T<160 K). ESR spectra have been taken after cooling the sample either in zero field or in a high field applied either parallel or perpendicular to the surface of a thin disc-like specimen. Anisotropy fields were then obtained by analysing the data for both geometries. The authors have compared their results with those of other authors, and found that changing the degree of clustering by cold working the sample causes an increase in the anisotropy and a decrease in g value. They also found that the anisotropy continues to the Curie temperature (T_C approximately=165 K), which is much higher than the canting temperature (70 K) obtained by Campbell et al. (1986) for a specimen prepared under different conditions.

Ferromagnetic resonance (FMR) measurements have been performed on amorphous Ni_81.6B_18.4 alloy, using the parallel geometry, at a fixed microwave frequency of approximately 9.3 GHz in the temperature range 100<or=T<or=300 K. Detailed lineshape analysis of the observed FMR spectra yields true values of the resonance field, linewidth, damping parameter and splitting factor. The temperature dependence of these quantities is found to be inconsistent with the notion that the observed ferromagnetic behaviour originates from the precipitated crystallites of Ni finely dispersed in the Pauli paramagnetic Ni-B amorphous matrix. By contrast, the present results substantiate their earlier claim that the weak ferromagnetism is an intrinsic property of this alloy.

To explore the behaviour of the ultrasonic attenuation in the superconducting state of a high-T_c superconductor, measurements have been made of the attenuation of 5 MHz shear waves in YBa₂Cu₃O_{7- delta} ceramic at temperatures below 100 K. The attenuation is found to increase linearly with temperature below 65 K in the superconducting state of YBa₂Cu₃O_{7- delta} . Thermal hysteresis occurs at higher temperatures. Similar regimes of behaviour are found in sound velocity and AC susceptibility. Direct immersion of this ceramic superconductor in liquid coolants can result in spurious experimental artifacts in the acoustic response.

An XPS study has been carried out on indium in both the solid and liquid states. Very small binding energy shifts (0.2 eV) have been detected for the core peaks and Auger transitions. The plasmon energy associated with the 3d_3/2 peak has been carefully measured and a shift found to a lower energy in the liquid state. These results agree with the view that the solid state local order is retained in the liquid state.

It is shown that the generalised non-local model pseudopotential theory, developed recently, can be applied to the calculation of the thermodynamic properties of liquid metallic Li and Be in a fashion similar to its application to the transition metals. The corrections to the effective ionic interaction attributed to one-body plus two-body terms are much less important than those from the three-body plus higher-body terms. In analogy to sd-mixing in the transition metals, the latter are interpreted as arising from sp-mixing effects. These corrections have relatively similar magnitudes in the calculation of the phonon spectra of Li and Be and the latter corrections are required to yield reasonable agreement with observation. Finally, the Be results vindicate the physical interpretation.

For pt.II see ibid., vol.16, p.1905 (1986). The influence of electron irradiation on the decomposition morphology of Cu-Ni-Fe alloys containing 46-48 at.% Ni and 2,4 and 8 at.% Fe has been investigated by means of small-angle neutron scattering. Null-matrix samples (enriched in ⁶²Ni) were used to achieve the required sensitivity. Besides an accelerated kinetics, the measurements reveal a significant modification of the structure functions after irradiation in comparison with those after thermal treatment at comparable temperatures, and in some cases a reduction in the volume fraction and/or the scattering contrast of the two phases. The irradiation influence is attributed to an interstitialcy contribution to the interdiffusion, where the interstitialcy migration causes a reduction in the thermodynamic force driving towards decomposition.

The defect recovery of a nickel electrodeposit was studied in the 45-240 degrees C temperature range by positron lifetime spectroscopy. The initial recovery resulted in a large increase in the trapping rates of various lattice defects and was associated with the release of trapped hydrogen from vacancies. The observed lifetime changes were analysed in terms of a four-state positron trapping model. Employing a simple model of hydrogen thermal detrapping from vacancies the authors found E_B=0.44+or-0.04 eV for the hydrogen-vacancy binding energy in nickel.

Measurements have been made of the changes in X-ray lattice parameter which result when a PdH_x alloy is taken on a path in the PdH_x phase diagram which traverses the mixed ( alpha + beta )-phase region to reach beta -phase concentrations (0.70<x<0.92). The changes were followed by comparing the lattice parameter values for alloys which had been taken along such a path with lattice parameter values of alloys taken along paths avoiding the mixed-phase region in reaching beta -phase concentrations of comparable value. From previous experience with the mixed ( alpha + beta )-phase region and the observed diffraction line broadening and magnitude and direction of the lattice parameter shifts, it can be inferred that the lattice parameter changes are due to internal stresses resulting from the progression of the alpha to beta phase change as the alloy is moved through the mixed-phase region. A discussion is given of some implications of the observations for the use of lattice parameter measurements in locating coexistence curves for M-H alloys.

The temperature dependence of the phonon frequencies in ferromagnetic BCC iron is discussed using nearest- and second-nearest-neighbour force constants, which are evaluated from elastic moduli obtained previously by the spin-fluctuation theory with the tight-binding model. The simple calculation accounts semi-quantitatively for the essential features in the observed phonon spectrum. In particular, the marked softening of the transverse (110) T₁ mode is shown to be due to the softening in the second-nearest-neighbour force constant arising from the magnetic band Jahn-Teller effect.

Praseodymium (Pr) like cerium (Ce) has a number of phase transitions under pressure. In this work, the authors present a fully self-consistent band structure of Pr at four different pressures, using the linearised augmented plane wave method. For this purpose a potential of the muffin-tin form has been assumed. The exchange and correlation parts of the potential have been calculated by the local density formalism. The results show that between Pr III and Pr IV a band broadening occurs that looks similar to the band broadening between the gamma and alpha phases of Ce, in the sense of similar f-band widths. From the band-structure results, the density of states, coefficient of linear specific heat and superconducting transition temperature are calculated and are compared with experimental data wherever possible.

A new method to calculate the electronic density of states in pseudo-binary intermetallic compounds of (A_1-xB_x)_mC_n type is developed by combining the coherent potential approximation and the recursion method. Numerical examples are given for the density of states of the tight-binding s-band model with a simple atomic structure. An application to the tight-binding d-band model for Y(Fe-Co)₂ and Y(Mn-Co)₂ compounds with the cubic Laves structure is also given.

Measurements of the electrical resistance of an ultrahigh-purity single crystal of palladium have been made to a temperature within 100 degrees C of the melting point. These are reported along with measurements of the molten palladium resistivity. The results are discussed and compared with previously reported values.

From the study of the electronic and phonon contribution to the specific heat and the temperature and field dependence of the electrical resistivity of the metallic glass Fe₂₀Zr₈₀ the main relevant parameters for the characterisation of the superconducting behaviour were derived, i.e. bare density of states N_b(E_F), electron-phonon interaction constant lambda _ep and spin-fluctuation mass enhancement lambda _sf. The low value of lambda _sf ( approximately=0.03) means the electron interaction with spin fluctuations has little influence on the critical temperature T_c of Fe₂₀Zr₈₀.

The temperature dependence of the electrical resistivity of monocrystalline alpha -ErH_x has been measured for x=0, 0.01, 0.02, 0.04, 0.06, 0.08 and 0.11 at. H/at. Er parallel to the c axis and for x=0 and 0.06 parallel to the b axis, between 1.6 K and room temperature. The maximum solubility of H in the low-temperature alpha phase was determined to X_max=0.065+or-0.01 at. H/at. Er. The absorbed hydrogen leads to an increase of the Curie temperatures T_C and T_C1 and to a decrease of the antiferromagnetic transition temperatures T_H and T_N, increasing strongly the resistivity jump at T_C, Delta rho (T_C), as well as the difference Delta rho _HN= rho (T_H)- rho (T_N) in the c axis crystals. It also modifies the superzone effects associated with the sinusoidal and helicoidal spin structures. A quench from room temperature leads to a further increase of T_C and T_C1, but also to an increase of T_H and T_N in the hydrogenated crystals. The results are explained by magnetoelastic and electronic interactions of hydrogen in various (ordered and disordered) configurations with the magnetic structures. The thermal variation of rho in the cone-structure region gives evidence for the existence of gap-like behaviour in the spin-wave spectrum; this behaviour can be understood through the overall softness of the whole spectrum and the presence of relatively flat portions in the latter.

Intending to describe band-structure effects on the low-frequency and long-range quantum fluctuations in heavy-fermion metals, especially in UPt₃, analytical results for the wavevector- and frequency-dependent spin susceptibility chi ₀(q, omega ) of non-interacting electrons at zero temperature on the basis of an uniaxially anisotropic and nonparabolic single-band model are derived. The Fermi surface (FS) anisotropy and topology results in a qualitatively different behaviour of chi ₀(q, omega ) compared with the spherical FS model. The static susceptibility chi ₀(q) for the closed FS reveals a maximum at either q=0 or at the Brillouin zone boundary depending on the band filling. For the open FS various Kohn anomalies in chi ₀(q) are found. The spectral density Im chi ₀(q, omega ) for small omega , mod q mod , and omega / mod q mod shows pronounced structures and, for the open FS, a logarithmic divergence (Taylor anomaly).

Self-consistent spin-polarised calculations are used to study the electronic structure and magnetic behaviour of transition-metal impurities of the 3d and 4d series in Ni. Electronic exchange and correlation are treated in the local spin-density approximation of density functional theory. The single-particle equations are solved for a muffin-tin model of the electronic potential. The potentials in the impurity muffin-tin sphere and in the surrounding muffin-tin spheres of the first-shell neighbour host atoms are assumed to be perturbed and calculated self-consistently. A correct embedding of these potentials in the infinite array of unperturbed host potentials is achieved by a Green function technique. The calculations yield local impurity moments which are antiparallel to the Ni host moments for the early elements Sc to Cr, Y to Mo, parallel for the late elements Mn to Co, Tc to Pd and nearly zero for Cu, Zn, Ag and Cd. For most impurities the results agree rather well with the information from neutron scattering and magnetisation measurements; however, there are unexplained discrepancies for Cr and Rh impurities.

An earlier publication by the author presented a self-consistent set of soft X-ray emission line wavelengths from pure metal targets in the range 40-165 AA. Using the same spectrometer, now equipped with a grating giving a fourfold increase in dispersion, this set has been extended down to 13 AA with new measurements in several orders of the M zeta lines of the pure metals ₃₈Sn through ₅₀Sr, the Ll lines of ₁₉K through ₂₂Ti and the L alpha lines of ₁₉K through ₂₂Ti and of ₂₆Fe through ₂₉Cu. The measured line peak wavelengths were adjusted, as in the earlier work, by a least squares iteration which yielded a self-consistent set with standard deviations typically of the order of 0.05 AA. Wavelengths for the M zeta line and two other M series lines of ₄₉In are reported for the first time; the wavelengths of the unresolved doublet L_i-L_ii,iii of ₁₄Si and of the line N xi (N_v-O_iii) of ₅₆Ba are also reported.

The lattice-strain problem around an interstitial C or N atom in martensitic iron was treated by an electronic theory. By using the DV-X alpha cluster method, the local electronic structure near an interstitial atom was calculated self-consistently. The strong bond between Fe and C (or N) atoms was attributable to the Fe 3d-C (or N) 2s, 2p covalent interactions. The bond strength increased remarkably when the first neighbour Fe atoms are displaced away from a C (or N) atom along the c axis of martensite. A series of analyses on the bond order, orbital populations of atoms, ionicities and electron density distributions show that an interstitial atom tends to exist stably in a central site of the regular octahedron, six corners of which are occupied by iron atoms. This is approximately in agreement with Jack's classical model for the structural component of martensite.