Table of contents

The authors discuss the influence of external dislocations on the crack tip process mainly using the superdislocation pair model. They present a detailed comparison of dislocations emitted from external sources and dislocations emitted from the crack source. The results show that external dislocations play a very significant role in the crack tip process.

Strong diffuse scattering characterised by blobs ('spots') of diffuse intensity well separated from neighbouring Bragg peaks and by diffuse streaks perpendicular to the (100)* directions have been observed in zone axis electron diffraction patterns of KCl. An ab initio lattice dynamical investigation based on the shell model has been performed and the intensity of thermal diffuse scattering calculated from the dispersion relations obtained. The calculated results indicate that the rather strong and characteristic diffuse intensity distribution of KCl can, at least qualitatively, be well understood in terms of the thermal excitation of low-energy phonon modes.

The effect of on-site anharmonicity in a one-dimensional phonon system is studied by calculating the specific heat at low temperatures. The formula for the specific heat is derived by applying the path integral method to the expression for the partition function.

Within the framework of the strong-coupling polaron theory the problem of a two-dimensional electron coupled to LO phonons under an external magnetic field is studied. When a suitably modified coherent phonon state interrelating the magnetic field and the polaron counterparts of the problem is adopted, it is seen that the theory reproduces the desirable asymptotic limits attained by the perturbation or the usual adiabatic theories. Moreover, the theory indicates that an abrupt 'stripping transition' in the internal structure of the magneto-polaron with varying magnetic field intensity is favoured as investigated by Wu, Peeters and Devreese (1985).

The electronic structures of the clusters (FeCl₆)^4- and (MnBr₆)^4- were calculated with the spin-polarised MS-X_alpha method under the actual lower-symmetry (D_3d) crystal field of CsFeCl₃ and CsMnBr₃. The calculations enable some of the splittings in the absorption spectra of the crystals to be theoretically explained.

The investigation of Er³⁺ (4f¹¹) ions in a KZnF₃ crystal is carried out by means of electron-nuclear double resonance. The parameters of the interaction of Er³⁺ with F^- ions of the nearest environment are determined. The microscopic calculation of these parameters is carried out. Also, the microscopic calculation of the pseudo-Zeeman interaction parameters is carried out for the first time.

The authors present investigations of the crystal structure the microstructure and the magnetic properties of the inter-metallic compounds XMnSb (X=Pt, Ni, Co, Au, Cu) and PtMnSn. It was found that several of these materials contain precipitates of other phases and/or large atomic disorder, which can be influenced by heat treatment. The magnetic properties show an effective paramagnetic moment which differs from the value corresponding to the saturation moment at 0 K. This effect is attributed to a decrease of the conduction electron spin polarisation at high temperature.

For pt.I see ibid., vol.1, p.2341 (1989). The electrical resistivity and the Hall effect of inter-metallic compounds XMnSb (X=Pt, Ni, Au) and PtMnSn were investigated in the temperature region 4-1000 K. The results for the anomalous Hall effect were analysed in terms of skew scattering and side-jump contributions. This analysis is possible in a half-metallic ferromagnet because the conduction electron spin polarisation at T=0 K is known.

Models for the spin systems in LiHoF₄ and the randomly diluted spin system LiHo_0.3Y_0.7F₄ are investigated by a Monte Carlo simulation. The models are described by long-range dipolar Hamiltonians. The experimentally observed ferromagnetic ground state for LiHoF₄ is reproduced in the simulations provided the 'sample' is of prolate ellipsoidal shape with an axis ratio of four or more. The transition temperature T_c(L)-where L is an effective length-is obtained from the position of the maximum of the heat capacity for a number of sample sizes. The critical temperature, T_c(L= infinity ), obtained by extrapolation of T_c(L), is in fair agreement with the experimentally observed critical temperature. The randomly diluted spin systems are obtained by removing spins from the undiluted sample. Two different dilution methods are considered. Both produce an incompletely ordered ferromagnetic ground state in agreement with experiment. Comparison with simulations on a modified long-range model indicates that the absence of a perfect ferromagnetic ground state is due to frustration effects inherent in the dipolar Hamiltonian. The estimates for the zero-temperature magnetisation and the critical-temperature depression agree with the experimental results. Some simulations that include a magnetic field along the c axis have also been performed on the diluted spin system. The results show that the ferromagnetic ground state is obtained for a sufficiently strong magnetic field. Experiments to test this prediction are suggested.

The authors have measured the inelastic neutron scattering spectra of three samples of molten ⁷LiCl with different isotopic chlorine compositions, and of one sample of molten KCl, using the time-of-flight spectrometer IN6 at the Institut Laue-Langevin. It is observed that the main peak in the total dynamical structure factor for LiCl is sharply peaked in both Q and omega directions, while that for KCl is much weaker and broader. The spectra for LiCl show evidence for two types of collective mode with characteristic energies of 10-20 and 40-50 meV. Using the differences between the samples, it is possible to show that appropriate pairs of dynamical variables for these modes are m₊, -m_- (low energies) and q₊, -q_- (high energies), whereas in molten CsCl the variables have been shown to be m₊, m_- and q₊, q_-. They propose that for the alkali chlorides (where the ionic radius ratio r₊/r_-<1) the former variables are appropriate when m₊<m_-, and the latter when m₊>m_-. There is a crossover between the variables, resulting in strong-mode coupling, when m₊=m_-. The data for KCl are consistent with this proposition.

The stability of local fourfold coordination for divalent and trivalent metal ions in liquid mixtures of polyvalent metal halides and alkali halides is classified by means of structural coordinates obtained from properties of the elements. In parallel with earlier classifications of compound crystal structures and molecular shapes, the elemental properties are taken from first-principles calculations of valence electron orbitals in atoms in the form of (i) the nodal radii of Andreoni, Baldereschi and Guizzetti or (ii) the pseudopotential radii of Zunger and Cohen (1979). As a third alternative a classification based on Pettifor's phenomenological chemical scale of the elements is also considered. The alternative structural classification schemes that are developed from these elemental properties are generally successful in distinguishing molten mixtures in which the available experimental evidence indicates long-lived fourfold coordination of polyvalent metal ions. In addition, Pettifor's chemical scale scheme (1986) is useful in sorting out finer details of local coordination in the liquid state.

Ab initio SCF molecular orbital calculations have been performed on the free Cu(OH₂)₆²⁺ complex in D_2h symmetry. Two extrema in the adiabatic electron energy potential surface were obtained corresponding to a tetragonal distortion from the regular T_h symmetry of the octahedral complex, as expected from the Jahn-Teller theorem in a case with strong E-e type vibronic coupling. The tetragonally elongated octahedral structure with Cu-20_ax at 2.25 AA and Cu-40_eq at 2.06 AA gave a slightly lower energy (72 cm^-1) than the compressed geometry with Cu-20_ax at 2.02 AA and Cu-40_eq at 2.17 AA. The calculated Jahn-Teller energy is 650 cm^-1 from that of the regular T_h symmetry with Cu-60 at 2.115 AA. EXAFS measurements were performed on aqueous Cu²⁺ solutions and showed a greater distortion with Cu-40_eq at 1.99 AA and Cu-20_ax at about 2.29 AA. Infrared absorption spectroscopic measurements were made on the O-D stretching vibrations of HDO molecules in aqueous Cu²⁺ and Ni²⁺ solutions with added D₂O in order to study the hydrogen bonds from the hydrated ions. A distorted Cu(OH₂)₆²⁺ ion is expected to form hydrogen bonds of unequal strength because of the stronger polarisation of the more strongly bonded equatorial water molecules. Two bands ascribed to the hydration of the Cu²⁺ ion were obtained, the more intense corresponding to the strongest hydrogen bonds at approximately=2400 cm^-1, and the weaker at approximately=2530 cm^-1, whereas for Ni²⁺ only one band at approximately=2420 cm^-1 was found. With the use of a correlation between R_O...O versus nu _OD from crystal structure data the mean hydrogen-bonded O...O distances to the second hydration shell are estimated to about 2.74 AA from the equatorial water molecules and 2.88 AA from the axial. The IR data show that the distortions induced by the Jahn-Teller effect on the second hydration sphere are visible on a vibrational timescale, despite the fast intramolecular inversion of the distortion axis between the three principal octahedral directions. The splitting of the nu _OD bands in the Tutton salts Cs₂(M(OH₂)₆)(SO₄)₂, M=Cu or Ni, with some added D₂O has also been measured and compared to crystal structure data of the hydrogen bonded distances. The separation of about 107 cm^-1 between the two groups of O-D stretching vibrations found in the Cu salt can be ascribed to a static Jahn-Teller effect.

Aluminium is very reactive with the quartz glass vessel generally used for electrical resistivity measurement cells. In the paper the authors describe a new geometrical arrangement to measure accurately the electrical resistivity of liquid alloys, using an alumina vessel. They give experimental results for the Al_1-xGe_x liquid alloys. This system presents an anomalous concentration-dependent temperature coefficient of the resistivity, with respect to the calculated curves, when using the Faber-Ziman formula with pseudopotential form factors and hard-sphere partial structure factors. The authors have measured total structure factors of four Al-Ge alloys by neutron scattering on the 'Orphee' reactor at Saclay, using sapphire monocrystal cells. The temperature dependence of the total experimental structure factor explains qualitatively the anomalous resistivity temperature coefficient.

The shell model used in lattice dynamics is applied to construct an interionic potential for use in molecular dynamics (MD) simulations of molten NaI. The ion shell-core displacements generated by the shell model potentials are used as an approximation to the instantaneous dipole moments of the ions. Time correlation functions of aggregates of these instantaneous ionic dipoles are related to the polarised and depolarised Raman spectra of the melt; fair agreement between simulated and experimentally observed spectra is obtained, indicating that the dipole-induced-dipole mechanism is an important component but not the exclusive contributor to the Raman polarisability in ionic melts.

The authors report the first measurement of the pressure dependence of methyl tunnelling by NMR techniques. 4-methyl-2, 6-ditertiary-butyl-phenol has been studied using a field-cycling technique to explore the anomalous proton magnetisation in magnetic field regions where the electron Larmor frequency of free radicals matches the methyl tunnelling frequency. The pressure dependence has been analysed to extract information about the inter-atomic potentials that contribute to the hindering potential barrier.

Raman scattering from acoustic phonons in Ge_mSi_n ultra-thin superlattices grown on (001) Si substrates is reported. The authors identify unexpectedly intense broad peaks in the low-frequency spectrum with resonant phonon modes arising from substrate-superlattice interactions. A linear chain model is used to simulate the spectrum and it reveals the importance of substrate-superlattice interactions and surface boundary conditions for intensity calculations in these superlattices.