Table of contents

The techniques of neutron diffraction have been combined with the method of isotopic enrichment to obtain, for the first time, an ion-ion partial structure factor in an aqueous solution. The data provide clear evidence that in the solution studied-NiCl₂ in D₂O-the Ni ions are arranged on a quasi-lattice and possess a substantial degree of long-range order.

From a neutron scattering study it is concluded that the space group F43m(T_d²) recently suggested for several spinels does not apply to magnetite, Fe₃O₄.

Field emission characteristics in He II at temperatures 0.3<T<0.8 K and under pressures 10<P<25 bar have been used to deduce values of the vortex ring nucleation rate nu in electric fields epsilon approximately 3 kV cm^-1. The values of nu obtained are an order of magnitude larger than those recently determined by Zoll and Schwarz (see abstr. A10022 of 1974), who employed a completely different technique at the same T and P but with epsilon approximately 20 V cm^-1.

This letter contains some calculations of the thermodynamic properties of lattice model interfaces. The work is compared with that reported recently by Hunt and Gale (see abstr. A16653 of 1974). Their results are shown to be greatly influenced by their restrictive limitation of the interface width.

The conducting 'skin' formed on the surface of chalcogenide glasses as a result of chemical interaction with O₂ or H₂O at elevated temperatures is shown by Raman scattering to be crystalline Te. The DC conductivity and thermoelectric power of the skin are consistent with the interpretation of the Raman data.

A phenomenological theory has been developed to study nonlinear acoustoelectric interaction in nondegenerate semiconductors exhibiting both piezoelectric and deformation potential interactions (e.g. InSb). The theory is then applied to semiconductors in which the deformation potential interaction is the dominant scattering mechanism (e.g. Ge). Numerical results show that the harmonic yield is a maximum at lower acoustic-wave frequencies for deformation potential coupling than for piezoelectric coupling. Moreover, a lower value of the magnetic field is required to achieve optimum harmonic yield for deformation potential coupling than for piezoelectric coupling.

A three-parameter model pseudopotential is proposed for semiconductors. A calculation is carried out to obtain the form factor, the binding energy, the monovacancy resistivity, the liquid-state resistivity, and the coefficient of thermoelectric power for silicon and germanium. Satisfactory agreement with the experimental data is obtained in most cases.

Two association regions have been found in the conductivity plots of log sigma against 1/T for NaCl crystals doped with trivalent chromium. These regions can be ascribed to two kinds of complexes, (Cr³⁺ Square Operator _k^-)⁺ and (Cr³⁺ Square Operator _k^- Square Operator _k^-), the association energies of which are found to be 0.63 and 0.51 eV respectively.

The effect of spin ordering on phonon frequencies is investigated for the Ising model. A shift in the phonon frequencies, proportional to the nearest-neighbour spin-spin correlation function, is found. The frequencies will thus have a sharp kink at the transition temperature.

Magnetic susceptibility and magnetization measurements on Ni_xMn_3-xO₄ compounds (x=0.1, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) indicate that Mn₃O₄ is a good example of Neel's model, whereas all other compositions exhibit complex behaviour. The ferrimagnetic Neel temperatures for NiMn₂O₄ and Ni_0.8Mn_2.2O₄ are 100 K and 90 K respectively.

Measurements of the polarization of protons in the glycerol:Cr(V) complex at 1.4 K and 9 kG are presented. Higher proton signal enhancements were observed than in a similar complex of ethyleneglycol.

The isotropic EPR spectrum of Sc²⁺ ions (3d¹ configuration) in ZnS crystals has been measured at 9.3 GHz and 4.2 K during optical excitation. The parameters of the spin hamiltonian were determined to a first approximation.

The infrared transmission and reflection spectra of samples of amorphous and crystalline GeSe have been studied. Features are observed in the spectra of the amorphous phase which are attributed to the change in atomic coordination numbers on vitrification. High- and low-frequency dielectric constants are also obtained for both phases.

The results of Monte Carlo computations for the Stockmayer potential are used to assess the numerical accuracy of the perturbation scheme developed by Stell, Rasaiah and Narang (see abstr. A39024 of 1972) for the calculation of thermodynamic properties of polar fluids. The Stockmayer model consists of a Lennard-Jones potential plus a dipole-dipole interaction and the perturbation theory is based on an expansion of the Helmholtz free energy of the Stockmayer system about that of the Lennard-Jones fluid in powers of mu ², where mu is the dipole moment.

The connection between the anisotropic Ising model and the second-neighbour Ising model is discussed. Series results are consistent with the prediction that the second-neighbour Ising model on a simple quadratic lattice has critical behaviour described by scaling with respect to the nearest-neighbour coupling strength.

It has been shown that the anomalous experimental results of the negative isotope effect in the lattice resonant-mode absorption of KCl-Li⁺ systems can be understood in terms of the static quartic anharmonic effects and the dynamical coupling of the impurity motion to the phonons of the host crystal matrix.

In the layer structure MoS₂, the oscillatory inter-band magneto-optic absorption has revealed a mass ratio of the order of 0.01. Theory is presented to show that this mass may be associated with the H-point of the Brillouin zone. Moreover, it is proposed that the origin of the mass is an instability of the equally-spaced layer arrangement giving rise to a distorted form with alternate layers displaced in the axial direction.

Positron annihilation in a single crystal of rutile (TiO₂) is investigated by analysing the energies of the emitted gamma -rays. The anisotropy in the energy distribution for emission parallel and perpendicular to the crystal c axis is measured and found to be 2.3+or-0.3% for k=0, and it vanishes at k=3.7+or-0.3 mrad, where k is the resultant positron-electron momentum. The anisotropy is attributed to covalency of the oxygen 2p shell.

Positron lifetime spectra have been determined for both uncoloured and additively coloured KCl. Spectra were obtained during light irradiation and at different temperatures. For the coloured crystals the intensity of the shortest lifetime component ( approximately=240 ps) increases markedly at the cost of the longest lifetime component ( approximately=1040 ps) in the temperature region 150 to 190 K. This is attributed to positron capture in F^- centres. In both coloured and uncoloured crystals the light irradiation causes a decrease in the intensity of the medium component ( approximately=550 ps). This is tentatively interpreted in terms of the competition between trapping of positrons and of V_k centres in defects.

The transient current in a dielectric material or a semiconductor is calculated. Mathematical expressions taking into account the generation and recombination phenomena at a single level are given. The transient behaviour is governed by two time constants which are connected in a simple manner to fundamental parameters of the sample studied, such as density, mobility, diffusivity of the carriers, and generation and recombination coefficients. The expression for the complex dielectric constant is also derived; it shows that two Debye-type dispersion regions exist.

Detailed measurements of the conductance characteristic of a Ti-doped Al/Al oxide/Ag tunnel junction in zero magnetic field are described. The sample system was carefully chosen so that it corresponded to the theoretical model considered by Appelbaum (1967). The results are compared with the theoretical expressions and it is found that they describe the functional form of the behaviour in detail. The asymptotic expression is shown to be inadequate.

For pt.I see ibid., vol.7, 1279-92 (1974). The conductance characteristics in a magnetic field are reported for the same Ti-doped Al/Al oxide/Ag tunnel junction which was used for the zero-field comparisons in a previous paper. The results are compared with Appelbaum's theory (1967) and the agreement is found to be poor, as indeed all other comparisons have been. However the points of disagreement between our measurements and theory can be removed by the one assumption that the Ti ions have a spectrum of g values. Such a spectrum could arise from the distorted crystalline environments of the Ti ions in the barrier oxide.

The author reports the effect of doping with 3d transition elements on the conductance of Al/Al oxide/Al and Al/Al oxide/Ag tunnel junctions. The conductance-voltage characteristics have been measured as a function of concentration of dopant, temperature and magnetic field. The doping lowers the conductance G(V=0) approximately linearly with concentration, and G(V) becomes 'vee' shaped over a wide voltage range and the sides of the 'vee' are steeper at higher concentrations. These effects are greater with the Al than the Ag functions.

For pt.I see abstr. A38891 of 1973. Following the study of the influence of the impurities on the tunnelling in metal-semiconductor contacts, the theory set up by Caroli et al. (1971) is applied to the effects of electron-phonon interaction. At first, intervalley processes leading to real phonon effects are studied. Excellent agreement with experiment is found regarding the magnitude of this effect in p-n diodes, M/n-Ge as well as M/n-GaSb contacts. Then situations where both real and virtual phonon emission occur are investigated. Theoretical phonon lineshapes are found in excellent agreement with experiment. Intravalley processes in M/p-Ge and M/p-Si contacts are also considered and the great variety of phonon lineshapes found in both these experiments is described by the theory in a very satisfactory way.

The crystal field in a LiYF₄ lattice is analysed in the framework of a superposition model and the results are compared with the findings in alkaline-earth fluorides and scheelites. Comments on the selection of coordinate systems (with respect to crystal axes) such that either of the imaginary crystal-field parameters could be ignored, or of approximating the S₄ symmetry by D_2d, are included.

Magnetization, Mossbauer and neutron diffraction measurements have been made on powdered and single crystal samples of Fe₃P. The average moments on the iron sites are 2.12 mu _B, 1.25 mu _B and 1.83 mu _B. The Mossbauer spectra indicate a splitting of the moments of approximately 0.1 mu _B for the atoms on each iron site. The observed magnetic hyperfine fields are 28.4 T and 27.3 T for Fe_I, 17.3 T and 17.5 T for Fe_II and 23.0 T and 25.0 T for Fe_III at room temperature.

The electrical susceptibility for the two-magnon Raman scattering in Heisenberg antiferromagnets with single-ion anisotropy and exchange integrals up to the third neighbours is derived by means of an equation of motion method and decoupling procedure. The calculated Raman lineshapes for CoF₂ and FeF₂ in different geometries at T=0 are compared with the experimental findings when available. In the case of CoF ₂ the two distinct peaks at the 112 cm^-1 and 115 cm^-1 respectively observed in the xy and xz geometries by Dugautier et al. are reproduced. For FeF₂ a good agreement is found with the experimentally observed width (6.0 cm^-1) and peak position (153.5 cm^-1) provided due account is taken of the instrumental resolution function. The possibility of deriving the exchange parameters in the hamiltonian by means of light-scattering experiments is briefly discussed.

It is shown that the most complete way of treating the antisymmetric contributions to the photoelastic tensor of shear elastic waves is by considering the photoelastic tensor as a mode property of the wave which must conform to the symmetry of the wave in the particular medium. This means that one must take the gradients of the elastic-wave coordinates consistently with the spatial symmetry of the crystal. Neumann's principle of crystal physics is thus extended to include dynamical crystal phenomena. The present approach applies to all mode properties of elastic shear waves.

Comparative powder Raman studies on zircon (ZrSiO₄) and hafnon (HfSiO₄) are reported. It is shown that rotational and translational modes of the same symmetry can be identified by comparing the spectra of these two crystals in a similar way as could be done by means of isotopic substitution. The authors also show that changes in the potential, to which the internal motion of the SiO₄^-4 radical is subject, can be used for mode assignment in powder samples.

Isolated cation vacancies are created by neutron irradiation of MgO crystals maintained at approximately 32 degrees C; these vacancies trap one positive hole to form stable V^- centres (Mg O⁺(=)O Mg). Subsequent X-irradiation at 77K causes some V^- centres to trap an additional hole to form V⁰ centres (Mg O⁺(=)O⁺ Mg). In these representations, only derivations from normal site charge are shown. Independently, some unstable V_M centres (Mg O⁺(=)O M⁺) are formed. Here M⁺ represents a substitutional trivalent ion. ESR spectra of V^- and V_M centres are indistinguishable at 77K; however, at room temperature, the V^- centres give an isotropic line which permits the unambiguous determination of their number. Decay of V⁰ and V_M centres at room temperature regenerates the number of V^- centres originally present before X-irradiation. Analysis of the optical absorption of V⁰ centres shows that at 77K their absorption band in MgO is centred at 2.37 eV and has a half-width of 0.95 eV. Measurements of the oscillator strength of the V⁰ centre give f=0.24, approximately 1.7 times that of the V^- centre.

The authors show from a comparison of optical absorption and emission spectra of high-quality single crystals that the characteristic narrow doublet in the edge luminescence is produced by the decay of excitons bound to neutral acceptors from the J=⁵/₂ and J=³/₂ exciton states, with the former lying lowest. The further J=¹/₂ bound exciton state expected on a j-j coupling model is now firmly identified with a weak higher-energy line previously reported for both materials. These assignments are consistent with the more positive of the conclusions obtained from Zeeman analysis of these transitions, although some less certain features conflict.

The energy distribution curves of photoelectrons emitted by crystalline antimony excited with photons in the energy interval between 5.25 and 9.75 eV are presented. The comparison with the distribution curves of SnTe makes it possible to point out the photoelectric structures basically due to the structural similarity of the two compounds and the features typical of the semimetallic nature of antimony.