Table of contents

For original paper see J.A.D. Matthew et.al., ibid., vol.13, p.581 (1983). Matthew et. al. derived an expression for the entropy change due to disordering of an ordered one-dimensional equiatomic binary alloy at high temperatures, using the harmonic approximation and assuming a fixed-end boundary. As a first step towards the calculation of entropy changes due to disordering of ordered two- and three- dimensional binary alloys (Bakker 1982, Waegemaekers and Bakker 1983) the present author had already studied the linear chain and was able to derive in a very simple and straightforward way the exact expression for the entropy change of a binary linear chain which changes its state I of order to a state II of order.

The authors report very low-temperature specific heat measurements on a series of amorphous superconducting zirconium-based alloys. After subtracting the electron and phonon contributions, they determine from the analysis of the data in a large temperature interval the specific heat excess due to the low-energy excitations (TLS) characteristic of the amorphous state. Furthermore, at the lowest temperature (T<or approximately=0.1K) the onset of a nuclear contribution is observed, numerically correlated to that of the TLS, which suggests a coupling between the ⁹¹Zr nuclear spins and these excitations.

The systematics of the pressure derivative of the bulk modulus (B'₀) are discussed from available experimental data on the three transition series. It is shown that the variation in B'₀ across the series correlates well with the variation of the s to or from d electron transfer parameter.

The electronic densities of states in PtMn and PtCr alloys are calculated in the cluster coherent potential approximation. Numerical calculations are made for the concentration near ordering L1₂ type structure. Changes in the values and the positions of the peaks in the densities of states are observed.

The authors have developed a two-dimensional version of the KKR CPA method for the description of random alloys as a tool for studying the electronic structure at the surface of disordered systems. They present calculations of the two-dimensional band structure (i.e. the spectral function) and the local density of states in a Cu_0.95Ni_0.05 (100) monolayer. The results are similar to two-dimensional calculations for a pure Cu (100) monolayer, but show extra effects due to the disorder.

Pressure dependence of the spin susceptibility chi _spin in vanadium is calculated within the framework of the local spin density approximation. It is found that the change in chi _spin due to pressure is mainly caused by the change in the density of states at the Fermi level; the contribution from the change in the exchange-correlation integral is very small. The calculated result is compared with experiment by using a simplified model for the volume dependence of the orbital susceptibility.

A neutron diffraction investigation of Tm and its solid solutions with D, alpha TmD_0.06 and alpha TmD_0.09, has permitted the tetrahedral interstitialcy to be determined as the preferential site for D occupation at room temperature, at 60K and at 4.2K. The analysis of the magnetic lines appearing at low temperatures yielded the magnetic moment, m and the Neel temperature, T_N, of the samples indicating a regular decrease of both m and T_N with increasing D content. The results are explained in terms of a depopulation of metallic d bands, decreasing the number of conduction electrons responsible for the RKKY exchange interaction.

Integrated X-ray Bragg intensities from NbC_1-c single crystals (0.05<or=c<or=0.12) have been measured as a function of the carbon vacancy concentration c, the scattering vector and the temperature. The analysis in terms of a static Debye-Waller factor (DWF) was performed by assuming that only the displacements of the first Nb neighbours essentially contribute to the DWF. Thus displacement amplitudes of u₁=0.125+or-0.005 AA were determined. These values are in good agreement with the results of lattice statics calculations and with the results of channelling experiments on similar NbC crystals. The temperature-dependent X-ray diffraction experiments showed a decrease of the thermal vibration amplitudes (u²) of the Nb atoms in NbC_1-c with increasing vacancy concentration ( Delta (u²)/ Delta c=-0.005 AA²).

In this work the influence of the kinetics of migration of the stage III irradiation-created point defects on the peaking effect observed in internal friction measurements is analysed in the framework of a model recently proposed by Caro et al. (1982) for dislocation dynamics. The values obtained for the diffusion parameters are discussed in terms of two existing rate equation models for diffusion.

The theory is proposed of an amplitude-independent dislocation damping which takes into account a point-defect atmosphere. It is shown that the effective viscous-drag coefficient B* increases as C¹/2 at low frequencies, and as C at high frequencies (C is a point-defect concentration). For high concentrations, B*>>B. For a strong renormalisation (B*>>B) there is an initial increase and subsequent decrease in internal friction. The results are in good agreement with the experimental features of the Simpson-Sosin peak in irradiated copper. It is suggested that the determination of the atmosphere formation time.

Electronic properties of TiC_x are investigated using a chemically disordered cluster-Bethe lattice approximation to model the main features of chemical bonding. The assertion that hybridisation is between titanium d and carbon p states leads to an interpretation of experimental X-ray emission results that is more consistent with a band model than with arguments based upon finite cluster calculations.

A statistical-mechanical model is developed to account for the complex melting curve of caesium. The model assumes the existence of three different species of caesium defined by three different electronic states. On the basis of this model, the free energy of melting and the melting curve are computed up to 60 kbar, using the solid-state data and the initial slope of the fusion curve as input parameters. The calculated phase diagram agrees with experiment to within the experimental error. Other thermodynamic properties including the entropy and volume of melting were also computed, and they agree with experiment. Since the theory requires only one adjustable constant, this is taken as strong evidence that the three-species model is satisfactory for caesium.

The diffusion of hydrogen in aluminium has been measured by the permeation method using a quadrupole mass spectrometer. Hydrogen was introduced by gas phase charging in the temperature range of 300-400 degrees C, and by electrochemical charging at room temperature. It was found that the diffusivity of hydrogen depends on the relative concentration of vacancies to the dissolved hydrogen concentration. The activation energy for the diffusion of hydrogen was found to be 0.61-0.62 eV at high temperatures where the concentration of vacancies was much higher than that of dissolved hydrogen. The value is near the migration energy of a vacancy in aluminium. On the other hand, at room temperature where the hydrogen showed a diffusivity considerably larger than that extrapolated from high temperatures. These facts suggest that hydrogen in aluminium migrates together with a vacancy at high temperatures due to a large binding energy between them.

For pt.I see ibid., vol.10, p.2609 (1980). In a series of metals many physical quantities oscillate in sign as a function of the filling of the bands. A power-counting method to determine the number of such sign changes has been developed previously. This is extended here to disordered alloys and non-zero temperatures. The minimum number of sign changes is independent of disorder and temperature, and is only weakly affected by changes in the one-electron Hamiltonian along the series. The theory shows that the Friedel-RKKY oscillations in polarisation around an impurity, although damped, persist in disordered alloys and at all temperatures.

The tight-binding orthogonalised plane-wave (TB OPW) method is used to reproduce the band structures obtained by the SAPW method. The spin-orbit interaction is treated by perturbation theory to calculate the orbital angular momentum of Heusler alloys Co₂YZ on the basis of the band structures. The contributions of the orbital angular momentum to the magnetic moment on Co in Cp₂YZ are 0.1, 0.07 and 0.025 mu _B for Co₂MnSn, Co₂TiSn and Co₂TiAl respectively.

The formulae for calculating the whole spectrum of cyclotron masses of charge carriers in a crystal are obtained on the basis of new numerical methods in the pseudopotential scheme. To avoid the numerical differentiation of approximately calculated dispersion dependences, E=E(k), the required values are represented in the form of integrals dependent on parameters. Since the values appearing under the integral sign after differentiation are calculated directly, the accuracy of the calculation of cyclotron masses can be greatly increased. The relations obtained are illustrated by calculations of cyclotron masses in Sb, which are compared with experimental data. The existence is predicted of additional cyclotron frequencies in Sb due to the peculiarities of its Fermi surface.

Inelastic neutron scattering spectra from Dy, Ho, Er or Tm ions dissolved in Mg show transitions that allow an accurate determination of the crystal-field strength, even at the low concentrations used (around 0.1 at.%). The values of the crystal-field parameters are comparable with those found for heavy rare-earth ions in other hexagonal metals. In most cases the transitions have a finite intrinsic line width which ranges from less than 0.04 to 0.34 MeV (FWHM) and is possibly due to exchange broadening.

The application of the Kubo-Greenwood formula and the long mean free path approximation are considered in a calculation of the electrical resistivity and thermoelectric power parameter of liquid noble metals. These calculations take into account up to third-order terms with respect to the t matrix. It has been shown that there is a negative third-order contribution to the resistivity. Good agreement with experimental data has been obtained.

The theory of the strain dependence of the electron-electron scattering resistivity is extended to Cu. Good agreement is obtained with the recent data of Steenwyk et al. (1981).

Electrical resistivity measurements for Na-Pb alloys are presented over the whole composition range and in the temperature interval 600-823K. The variations of the electrical resistivity and its temperature dependence with alloy composition exhibit interesting features, i.e. a maximum and a negative minimum respectively, around the composition corresponding to that of Na₄Pb. The results are discussed in terms of the Faber-Ziman model and the Mott intuitive approach. It is suggested that a tendency towards a metal-nonmetal transition exists which leads to a partial charge transfer from one component of the alloy to the other to form non-metallic species.

In order to study the origin of the minimum at a few at.% solute concentration in the isotherm of the thermoelectric power Q versus composition of liquid mercury alloys (the 'minimum in Q'), the electrical resistivity rho of liquid Hg-In alloys and Hg-Sn alloys has been measured precisely as a detailed function of composition. The present measurements indicate that the temperature dependence of rho , delta rho / delta T, shows a hump at about 5 at.% In for liquid Hg-In alloys in contrast to a smooth decrease observed for liquid Hg-Sn alloys; the former system shows the 'minimum in Q' and the latter does not. These experimental facts suggest that the higher order correlation effect stressed already by the present authors is important in understanding the origin of the 'minimum in Q'.

A microscopic justification is given of MacDonald's suggestion (1980, 1981) that an important source of the low-temperature resistivity of simple metals is an effective inter-electron interaction mediated by virtual phonons. It is shown that Migdal's theorem (1951) require restatement to deal with the low-temperature regime correctly.

High-resolution magnetoresistance measurements, Delta rho (T,H), have been performed on a single crystal of Gd, with the electric current and magnetic field (H) directed along the c axis. The data cover the immediate vicinity of the Curie temperature T_c(1.4*10^-3<or= epsilon <or=8.8*10^-3, T>T_c; 8*10^-4<or= epsilon <or=1.7*10^-2, T<T_c; epsilon identical to (T-T_c)/T_c) and refer to H from 0 to 660 Oe. A direct analysis of the critical behaviour of the low-field magnetoresistance was made, and the results compared with the available theoretical models. The magnetoresistance in the critical region was always negative. Above T_C, the results confirm the low-field theoretical prediction ( Delta rho varies as h²; h>> epsilon , where h is reduced field) for h/ epsilon <or approximately=6*10^-2; the temperature dependence of Delta rho then satisfied the theoretical prediction Delta rho varies as epsilon ^{- gamma -1}with gamma =1.18+or-0.15; for h/ epsilon >or approximately=6*10^-2, but with h/ epsilon still much smaller than 1, a new regime appears, characterised by a strong departure from the previous dependence. Below T_c the low-field theoretical prediction Delta rho varies as h was found to be valid over a wide range of the ratio h/ mod epsilon mod , at least up to h/ mod epsilon mod approximately 0.9; the temperature dependence of Delta rho was in disagreement with the theoretical expectations ( Delta rho varies as mod epsilon mod ^{1- alpha - beta delta} ), the results showing a much weaker temperature dependence just below T_C. A brief discussion is also presented for experimental situations where the low-field condition h>> mod epsilon mod begins to fail.

Superconductivity has been studied through resistivity measurements on the A15 systems (V_1-xM_x)₃Ge with M=Ti, Cr, Mn, Zr and Mo. An increase of concentration x in all the systems causes a monotonic decrease of the transition temperature T_c, while the residual resistivity rho ₀ rises. An increase of the V site disorder due to alloying, which is indicated by the rise of rho ₀, is accompanied by a monotonic decrease of the electron-phonon resistivity rho _ep(300K). The electron-phonon parameters, lambda and lambda _tr, deduced from the experimental result are expressed as universal functions of the electron-atom ratio e/a. A deviation of e/a from 4.75 due to alloying lowers both lambda and lambda _tr, which is in contrast with the recent prediction based on the rigid-band model.

The scattering of neutrons from mictomagnetic CuMn alloys containing between 10 and 20 at.% Mn has been examined using both unpolarised and polarised neutrons. Apart from the extensive atomic and magnetic short-range order producing a diffuse peak at the 1¹/₂0 position, longer range magnetic correlations cause large increases in the magnetic scattering at small angles. There is considerable difficulty in fitting the scattering intensity including both the short and longer range effects.

A general matrix equation for the local paramagnetic susceptibility of a surface impurity is derived in the random phase approximation. The system impurity plus substrate is described by a tight-binding Hubbard Hamiltonian and the static impurity susceptibility is first evaluated for an unenhanced substrate using a model density of states. The local susceptibility of a surface impurity is shown to be higher than that of the same bulk impurity provided the condition of local charge neutrality is approximately satisfied. The same model is used to evaluate the local susceptibilities of surface and bulk impurities on (in) an exchange-enhanced matrix. It is shown that the surface of an exchange-enhanced matrix is much more favourable than the bulk for the formation of local moments even if the substrate surface enhancement is reduced. It is predicted that even non-magnetic impurities might become 'live' on the surface of an exchange-enhanced substrate. The present model is applied to discuss qualitatively recent measurements of the susceptibility of iron impurities on the surfaces of Pd, Pb, In and Sn.

In PdFe, PdCo, PdNi, PdMn or PdGd alloy systems long-range ferromagnetic order always sets in whenever the concentration of magnetic units is about 0.11 at.%, irrespective of the type of solute or the total magnetic moment of the magnetic unit, the latter being a polarisation cloud seeded either by a single solute atom or by a statistically formed cluster of solute atoms. This observation, in conjunction with the facts (i) that in PdGd the d band polarisation is negative in zero or weak external fields with (ii) that for all the weakly ferromagnetic alloys the ferromagnetic Curie temperature, T_C, is proportional to the solute concentration, c, is interpreted to mean that the spatial extent of the polarisation cloud cannot be as large as is currently assumed and that the onset of ferromagnetic order is through a percolation process involving ferromagnetic correlations between the otherwise isolated or independent magnetic units, rather than through any direct overlap of such units. Also, in appropriate cases, the concentration dependence of the bulk magnetisation, mu _b, is approximately as predicted by the Landau theory of phase transitions for the weakly ferromagnetic alloys whereas for higher solute concentrations mu _b is proportional to T_c, ironically a characteristic of very weak itinerant ferromagnetism. The average moment on a Pd atom, mu _Pd, increases linearly with T_c up to T_c approximately=340K, above which temperature it assumes a constant value equal to the maximum possible Pd moment. This abrupt saturation of the Pd moment, indicative of some sort of phase transition, is discussed in relation to the theoretically predicted metamagnetic transition in Pd and more generally to the concept of a characteristic temperature for an exchange-enhanced but non-magnetic transition metal.

The authors find from NMR (spin-echo) and X-ray diffraction that plastic deformations during cold work introduce many dislocations and associated lattice deformations in a number of Heusler alloys: Pd₂MnSn, Pd₂MnSb, Cu₂MnIn, Cu₂MnAl, Ni₂MnIn, Pd₂MnIn and NiMnSb. In Pd₂MnSn these effects are very important and result in atomic disorder. In other alloys cold work produces distortions but apparently no atomic disorder. The strong effects of cold work on Heusler alloys (L2₁ structure) contrast with almost unobservable effects in the Cl_b structure alloy NiMnSb.

The results presented cover a series of alloys, namely Li dissolved in HCP Mg up to about 15 at.% Li. Concentration-dependent changes in the edges of both components are observed along with changes in the gross form of the bands. At all concentrations studied the emission from Li dissolved in Mg exhibits none of the effects due to incomplete phonon relaxation which are believed to be responsible for the apparently anomalous shape of the high-energy edge of the K emission from pure Li metal.

An improved method is given for extracting absorption information from soft X-ray emission spectra using the Gaussian expression recently proposed by Packwood and Brown (1981) for the ionisation as a function of depth for an electron beam in an X-ray target. Quantitative agreement is found with attenuation coefficients measured directly.