Table of contents

From X-ray measurements of the attenuation of integrated Bragg intensities due to H in Nb, Ta and V single crystals the displacement of the lattice atoms closest to H are determined. The experimental values are compared with model calculations using lattice statics.

For original paper see ibid., vol.12, p.L67 (1982). A recent letter claimed that the three-jump frequency ratios W₂/W₁, W₃/W₁ and W₄/W₀ for the standard five-frequency model for dilute FCC alloys contained all the thermodynamic information on these alloys, namely the value of the binding enthalpy of an impurity-vacancy pair. The authors show that this conclusion is erroneous since it rests on an incorrect sampling procedure along the chain of the microscopic configurations through which a given alloy evolves with time.

For original paper see ibid., vol.13, p.183 (1983). It is shown that, contrary to the supposition of Takacs and Hargitai, there is no conclusive experimental evidence for a substantially reduced atomic magnetic moment in amorphous iron.

The compound LaNi₄Co is isostructural with LaNi₅. The distribution of the Co atoms in the 2c and 3g special positions of the space group D_6h¹-P6/mmm is not random. Deuterisation yields LaNi₄CoD₄ with structure that belongs to the orthorhombic space group D_2h¹⁹-Cmmm. The deuterium atoms occupy the 4e and 4h special position of this symmetry. The reduced deuterium content in LaNi₄CoD₄ as compared with that in LaNi₅D₆ is consistent with geometrical considerations and the different symmetries.

The dynamic scattering function for liquid bismuth at 580K has been determined by neutron inelastic scattering for wavevector transfers, Q, in the range 0.5<or=Q<or=1.6 AA^-1. The zeroth energy moment is satisfied within some ten per cent for all Q. No clear evidence of collective excitations is seen. Thus it is concluded that the molecular dynamics in liquid bismuth is different as compared to the classical liquid metals rubidium and lead.

Amorphous alloys of the type Ti_1-xNi_x were prepared by means of melt spinning in the concentration range 0.25<or=x<or=0.64. The crystallisation behaviour was studied by means of differential scanning calorimetry and X-ray diffraction. The heat of crystallisation and the activation energy for crystallisation were determined for several alloys. Electrical transport properties were studied in the temperature range 4.2-300K. The concentration dependence of the resistivity at 4.2K has a maximum near x=0.45. The temperature coefficient of the resistivity exhibits a minimum at the same concentration. From these results it would appear that in the middle concentration range in Ti_1-xNi_x the Nagel and Tauc (1975) stability criterion (Q_p=2k_F) is satisfied. It is shown that this criterion is not suited for describing the thermal stability of amorphous alloys.

The temperature dependence of the positron lifetime spectra in Mo single crystals irradiated with fast neutrons at 570K and 370K has been measured. In the specimen irradiated at 570K the intensity of the long lifetime component, attributed to trapping at voids about 6-10 AA in radius, increases linearly from 40K to 460K. This result can be understood if it is supposed that the trapping is diffusion limited and that the diffusion constant varies linearly with temperature as it might if scattering is primarily from extended defects. The specimen irradiated at 370K shows very little temperature dependence of positron trapping into small voids-clusters of perhaps a dozen vacancies. It is presumed that the trapping is transition-rate limited in this specimen.

The dependence of the high-temperature vibrational entropy Delta S of equiatomic one-dimensional chains on the short-range order parameter r is obtained analytically. It is found that Delta S/Nk depends linearly on (1-r) as found in numerical simulation, in contrast to the non-linear dependence on r predicted by the Einstein approximation.

The EMF of Na in the liquid Na-Pb system has been measured as a function of temperature using a concentration cell. The associated thermodynamic quantities of mixing Delta G, Delta S and Delta H are obtained, and the deviation from the additivity of specific heat, Delta C_p, is also obtained. The curve of the concentration-concentration fluctuation in the long-wavelength limit, S_CC(0), obtained from Delta G, shows three kinds of atomic associations at the compositions of Na₄Pb, NaPb and NaPb, respectively.

Low-temperature heat capacity data are presented for a series of AuCu₃ type MPd₃ compounds (M=Ce, La, Lu, Y and Sc). The coefficient gamma of the linear term of the electronic specific heat of the non-magnetic compounds ranges from 0.28 for LaPd₃ to 8.2 mJ mol^-1 K^-2 for ScPd₃ and reflects a lower density of states at the Fermi surface than that of M or Pd. The high gamma value of intermediate valence CePd₃ is confirmed and shown to vary with Ce content near stoichiometry: a maximum value of 38.6 mJ mol^-1 K^-2 is obtained while the electronic contribution to the T³ term of the specific heat is found to be very small. The electrical resistivity of CePd_3+x shows the usual behaviour of intermediate valence compounds for Ce concentrations near or above stoichiometry.

The bonding nature of hydrogenated palladium, nickel and FCC iron-nickel alloys was investigated by means of differential scanning calorimetry (DSC). A progressive endothermic or exothermic reaction process was observed as the temperature was raised. From the DSC measurements and their analysis, the differential heats of reaction for Pd-H, Ni-H, and Fe-33 at.% Ni-H alloys were obtained and interpreted semi-quantitatively in terms of the energy of elastic dipole interactions and electronic metal-hydrogen interactions. The nature of bonding between metal and hydrogen atoms was discussed accordingly.

The vibrational density of states of hydrogen in aluminium was calculated by the recursion method using central pair potentials determined from pseudopotential theory. The coupling between hydrogen and aluminium atoms is fairly weak, and the low-frequency resonance appears in the spectral density due to weak transverse coupling.

The electronic structure of Au_xNi_1-x, where x=0.9, 0.8, 0.7, 0.6 and 0.5, is calculated using the fully relativistic KKR CPA method. Spectral functions such as the density of states, component densities of states, partial local-angular-momentum-dependent densities of states and Bloch spectral functions are used to characterise the electronic structure in Au_xNi_1-x. For diffuse scattering experiments the separation between the (000) and (200) scattering spots as a function of concentration is predicted from a study of parallel sheets of the Fermi surface. The calculated values for the linear coefficient of the specific heat agree rather well with corresponding experimental data.

Electronic energy bands of LaH₂ and NdH₂ have been calculated by the composite-wave variational version of the augmented plane wave method. Crystal potentials for both the hydrides were constructed by the superposition of the atomic potentials of Herman and Skillman (1963). From the band structure data, density of states, joint density of states, optical absorption spectra and Fermi surfaces of these two hydrides have been calculated. The results of the optical studies have been successfully compared with the experimental results of Weaver et al. (1977, 1978). From the theoretical point of view the present results for LaH₂ have been satisfactorily compared with the results of Gupta et al. (1978, 1980). Since no theoretical calculation exists for NdH₂, theoretical comparison, in this case, was not possible for NdH₂.

The LMTO method has been applied to calculate band structures for the heavier rare-earth metals. The calculations are relativistic. Thulium in particular has been considered, where a frozen core approximation is used, and the outer electrons are treated self-consistently. Problems associated with the localisation and interactions of the 4f electrons are discussed. The comparisons between experimental data and calculated quantities are encouraging, but more data on high-purity single crystals would be helpful.

Electrical resistivity data on slow-cooled Co_xGa_1-x alloys with x=0.63, 0.60, 0.585 and 0.50 are reported. The negative temperature coefficient of resistivity obtained in the case of x=0.50 is discussed. The resistivity behaviour of alloys with x=0.63, 0.60 and 0.585 through their ferromagnetic-to-paramagnetic transition temperatures are analysed. It is proposed that the unusually large rho values are due to a contribution from the scattering of electrons by the highly anisotropic small magnetic clusters.

The Boltzmann equation for the flow of current through a small metallic contact has been solved numerically for a series of values of a/l, where l is the mean-free-path for elastic scattering, and a is the radius of the circular contact. Results are presented for the current, the electrostatic potential, the current distribution in the contact and the electron-distribution function in the neighbourhood of the contact.

It is shown that the presence of asymmetric Mossbauer spectra the distribution of the hyperfine field can be evaluated. The calculation of Billiard and Chamberod (1975) is extended to cover spin glasses and metallic glasses, and the results are applied to the series Pd_77.5-xCu₆Si_16.5Fe_x. Coupling of the hyperfine field to both the isomer shift and the quadrupole effect is discussed. The results can be used to study short-range order changes in alloy glasses.

A study has been made of the residual resistivity and magnetoresistivity at 4.2K, the Hall coefficient between 4.2 and 77K and the deviation from Matthiessen's rule at 77K in Al-210 PPM Ti and Al-522 PPM Ti in both the slow-cooled condition and when quenched from the melt. It is shown that in the slow-cooled state, clustering/precipitation of solute atoms occurs creating electron scattering entities which, while appearing to behave like point defects, nevertheless possess a scattering anisotropy entirely different from conventional point defects. As a result these clusters produce considerably enhanced deviations from Matthiessen's rule, Hall and magnetoresistance coefficients. Values of the anisotropy in the mean free paths are calculated using a three-group model and a simple clustering model is invoked which demonstrates why the scattering becomes more 'interstitial' in the clustered alloys. This leads to a scattering anisotropy for which the mean free path of electrons in the third zone becomes greater than that for holes in the corners of the second zone.

Ferromagnetic resonance studies have been carried out in thin films of Mn_1-xFe_xSb (0<or=x<or=0.3) and Mn_1-xCo_xSb (0<or=x<or=0.3) to study the variation of the strain-anisotropy field and the g value. The composition dependence of the strain-anisotropy field showed that the spins reorient from the basal plane in hexagonal MnSb to a direction parallel to the c axis at x=0.21 in Mn_1-xFe_xSb and at x=0.16 in Mn_1-xCo_xSb, at 300K. The spin-wave resonance spectra were analysed and the variation of the exchange coupling constant with composition was obtained.

The complex multiplet structure of the partially filled 4f shell in the rare-earth metals is calculated using intermediate coupling. The eigenfunctions obtained are used for a calculation of photoemission intensities in the fractional parentage scheme including all final states. The result is in excellent agreement with recent X-ray photoemission spectra and demonstrates the importance of intermediate coupling in the rare earths.