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This paper reports a thorough description of the phase transformations taking place in the Al₆₅Cu₂₀Fe₁₅ ingot and melt-spun samples during heating and cooling runs as recorded by neutron diffraction. Upon cooling, the icosahedral phase is found to form in a peritectic reaction involving theta -Al₁₃Fe₄, a BCC phase of approximate composition Al₇Cu₂Fe and liquid. Its isothermal growth mechanism is discussed. There are some indications that the so-called stable and perfect icosahedral phase is not a truly equilibrium phase at room temperature. In the case of the melt-spun sample, upon heating, the T-Al₇Cu₂Fe phase participates in the transformations concerning the icosahedral phase.

For pt.III see Phys. Chem. Minerals, vol.16, p.365 (1988). The time evolution of Al-Si ordering and ferroelastic distortion in isothermally annealed Mg cordierite (Mg₂Al₄Si₅O₁₈) is studied using Raman spectroscopy. The transition from the initial hexagonal to the modulated structure is found to be discontinuous, with the quenched modulated structure being locally orthorhombic. The bilinear coupling between Al-Si ordering and lattice distortion proposed previously to account for observed behaviour in the macroscopically orthorhombic phase obtained after prolonged annealing is shown to be also valid on a local scale in the modulated phase. Modes of the hexagonal phase, which split on passing into the modulated phase, are found to be inhomogeneously broadened. This is related quantitatively to sub-unit cell distortions brought about by Al-Si ordering. The same ordering is found to be related linearly to the decrease in widths of modes of other symmetries with annealing.

The observed zero-field splitting parameter D of the triplet self-trapped excitons in alkali fluorides is negative, while in alkaline-earth fluorides of the fluorite structure it is of comparable magnitude but positive. D in these materials is almost completely due to spin-spin interaction. Analysis of D allows an estimate of the separation between the hole and electron charges. These are found to be in satisfactory agreement with the results of recent calculation which predicted the off-centre character of the self-trapped exciton in alkali halide crystals. As for the alkaline-earth fluorides with the fluorite structure, the analysis explains the positive sign of D in this structure and also identifies which of the proposed nearest-neighbour F-H pair configurations corresponds to the observed self-trapped exciton.

A Landau-Ginzburg formulation is presented to describe a series of superconducting islands interacting via Josephson weak links in the presence of a magnetic field. It is demonstrated that the structure of superconducting islands (and the lattice of magnetic vortices), describing by the order parameter envelope, may be commensurate or incommensurate with a superimposed array of weak links via the order parameter phase for each island. A discussion is provided in terms of a Kosterlitz-Thouless transition in the latter structure. A description of space modulation of these phases is given through sine-Gordon equations.

The localisation properties of spin waves for Heisenberg ferromagnets with random uniaxial anisotropy energy and random exchange interaction is studied in the framework of the weak localisation theory. The correlation effect is found to influence the phase diagrams near the band edge region strongly.

The authors present X-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility and specific heat measurements on a series of Ce(Cu_1-xAu_x)₆ compounds with x<or=0.25. These measurements indicate that alloying produces an increase in cell volume and changes in the sd density of states. Thus, while CeCu₆ has a non-magnetic ground state, addition of Au leads to the onset of magnetic order (e.g. T_N approximately=2 K for the Au alloy with x=0.15). The results also reveal alloying with Au produces modifications in the crystalline electric field scheme.

The authors present here the determination of the distribution P(q) of the electric field gradient in Fe-doped Al-Si-Mn icosahedral and decagonal quasi-crystals and in the related crystalline phases, using in-field Mossbauer effect spectroscopy. The distribution P(q) for the quasi-crystalline compounds shows a bimodal, nearly symmetric form with P(0) approximately=0 and equal averages over positive and negative q=¹/₂eQV_zz, -(q_-) approximately=(q₊) and equal standard deviations: sigma _- approximately= sigma ₊. The results for the majority sign of the distribution P(q) will be compared to the results for the cubic, hexagonal and orthorhombic compounds. The results show that there are similarities in the local order between the icosahedral quasi-crystalline phase and the hexagonal phase. No evidence is found for a two-site model, which has recently been proposed by other authors.

The dependence of the photoluminescence of red amorphous phosphorus upon the energy of excitation photons is discussed in the framework of models of recombination at defects. The CW luminescence of this material at 4 K is found to show a shift in the emission energy proportional to the excitation energy below a critical energy near the bandgap. Near the same energy the width of the luminescence band switches from being linearly related to the excitation energy (low energies) to being independent of excitation energy (higher energies). The authors discuss the role of the processes involved in absorption in determining the excitation dependence of the luminescence, and conclude that the sharpness of the transition from linear variation with excitation energy to independence of energy indicates the influence of the mobility edge upon the initial distribution of optically injected carriers.

Compton scattering of a circularly polarised photon beam is shown to provide direct information on orbital and spin magnetisation densities. Experiments are reported which demonstrate the feasibility of the method by correctly predicting the ratio of spin to orbital magnetisation components in iron and cobalt. A partially polarised beam of 45 keV photons from the Daresbury Synchrotron Radiation Source produces charge-magnetic interference scattering which is measured by a field-difference method. Theory shows that the interference cross section contains the Compton profile of polarised electrons modulated by a structure factor which is a weighted sum of spin and orbital magnetisations. In particular, the scattering geometry for which the structure factor vanishes yields a unique value for the ratio of the magnetisation densities. Compton scattering, being an incoherent process, provides data on total unit-cell magnetisations which can be directly compared with bulk data. In this respect, Compton scattering complements magnetic neutron and photon Bragg diffraction.

The ion-electron correlation function in liquid sodium, estimated from the difference in measured structure factors determined by X-ray and neutron diffraction methods, has been extended to describe the distribution of conduction electrons around an ion. The present results imply that the conduction electrons in liquid sodium would most likely behave as the atomic wave function at the closed vicinity of the ionic radius, corresponding to the orthogonalised plane wave.

A theoretical analysis on the structural stability of HF under pressure, using the linear muffin-tin orbital method in conjunction with the Andersen force theorem, predicts a new phase transition to a BCC phase at high pressures, in agreement with recent diamond cell experiments.

The dynamic exchange-correlation (XC) kernel of Gross, Kohn and Iwamoto (1985-87) is analytically continued into the lower-half frequency plane and incorporated in a microscopic calculation of multipole surface plasmon dispersion on a jellium aluminium surface. The multipole mode found in earlier calculations using a static XC kernel is still present, the change to dynamic XC causing a small frequency shift and a more substantial percentage increase in damping.

A series of interstitial ternary nitrides R₂Fe₁₇N_y with 2.3<y<2.9 has been prepared for R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y. The nitrides have structures related to those of Th₂Zn₁₇ or Th₂Ni₁₇, but the unit cell volumes are 6-7% greater, and the magnetic ordering temperatures some 400 K higher than for the 2:17 parent compounds. The Fe-Fe exchange interactions are more than doubled by nitrogenation, whereas the R-Fe exchange interactions are little changed. All compounds exhibit easy-plane anisotropy at room temperature, except for Sm₂Fe₁₇N_y, which is easy-axis.