Table of contents

The IRE Standards on piezoelectric crystals (1949) recommend conventions whereby orthogonal cartesian axes are unambiguously related to the symmetry elements of all 32 crystal classes. In class 6m2 alone there appears to be a contradiction between the principles outlined in the text and the specific recommendation. To avoid confusion in the specification of tensors describing third order elasticity, piezoelectricity and internal strain effects the author recommends that the Ox₁ axis be set perpendicular to a mirror plane with Ox₃ along the hexad axis and Ox₂ completing a right handed system.

A distinction is drawn between methyl group rotation which occurs with the creation of a local strain in the host lattice due to coupling with low frequency lattice modes, and double proton pair exchange which leaves the lattice unchanged while cyclically permuting the proton space coordinates. The latter is subject to restrictions dependent on the proton spin states due to the exclusion principle but the former is not.

Using Lax's formalism for multiple scattering theory the authors show that, to take into account the effects of a cluster of order n, one has to determine a medium which does not scatter, on average, a cluster of n atoms embedded in it. The formalism is general enough to take into account both diagonal and off-diagonal disorder.

A simple relation between the rigid neutral atom density and the ion- electron-ion interaction is established. The method is applied to the alkali metals, using self-consistent Hartree-Fock-Slater neutral atom density obtained by the auxiliary neutral atom method, and yields reasonable agreement with experiment.

Some remarks about interpretation of spin waves in the spin-density wave ground state are given.

A simple model of spin-phonon interaction is studied and the possibility of an instability in the ground state of the system for the strong coupling case is investigated. The nature of the new ground state is discussed and it is suggested that strong correlations between the spin and lattice motions exist in this state.

Infrared absorption measurements on nitrogen doped gallium phosphide grown by the vapour phase epitaxial technique have revealed localized vibrational mode lines at 495.8 and 480.3 cm^-1 due to ¹⁴N(P) and ¹⁵N(P) respectively. The frequencies of these lines are compared with theoretical values.

A method is described for the measurement of high resolution spectra of dilute spins in solids. It is a pulsed nuclear double resonance technique which uses the spin system to perform an analogue Fourier transform. Results on the 1.108% abundant ¹³C resonance in solid adamantane (C₁₀H₁₆) show a resolved doublet with a splitting of (36+or-5) Hz or (9.5+or-1.3) ppm and a residual linewidth of about 20 Hz.

The superconducting transition temperature of NbSe₂ has been measured as a function of hydrostatic pressure to 20 kbar and quasihydrostatic pressure to 140 kbar. At first, T_c increases rapidly, but by 35kbar the rate of increase has fallen by an order of magnitude from its initial value and then remains constant to the maximum pressure. It is suggested that this change in slope is associated with the phase change which occurs in the vicinity of 33 kbar at room temperature and that T_c is related to the interlayer spacing.

Interference functions from liquid mercury have been obtained at 50, 100 and 150 degrees C, using a specially designed X-ray diffractometer. The important feature of the results is the rapid change of first peak height with temperature. By matching the observed interference functions to those from a hard sphere model it is found that the equivalent packing density of the liquid changes from 0.42 at 50 degrees C to 0.32 at 150 degrees C. From these values of the packing density the compressibility has been calculated at each temperature and all are higher than the published experimental values. It is concluded that there may be an increase in the inter-ionic repulsive force as the temperature increases.

It is shown that whether or not diffractive scattering is observed from solid surfaces depends not only on the elastic scattering cross section, i.e. the normalized Debye-Waller factor, but also on the surface structure or local surface potential of the particular solid.

Measurements have been made of the current noise in thin discontinuous platinum films deposited on glass substrates with surface sensitivities in the range 1.5 to 5.0*10³ k Omega sq^-1. The influence of film temperature and gaseous ambients on the variation of the spectral noise power density current flowing through the film, noise-channel frequency and surface resistance is described. The experimental data are interpreted in terms of a noise model based on quantum mechanical tunnelling through a surface layer containing adsorbed gaseous ions. The relationship between activation energies determined from conductance temperature and noise against temperature plots is investigated and it is shown that for activation energies epsilon >or=kT the noise mechanism is thermally activated.

Develops the geometrical interpretation of electron channelling patterns observed in the scanning electron microscope and derives the equations of the band edges in terms of coordinate axes located in the observation plane. A computer program has been developed to plot and index the patterns of cubic crystals. As dynamical considerations are not included, exact matching with experimental observations is not expected. Nevertheless, the computer-drawn patterns presented show good agreement with experiment and this approach is helpful in indexing unknown lines, especially those with high indices.

The coefficients of thermal expansion parallel and perpendicular to the c axis ( alpha /sub /// and alpha _{perpendicular to} ) have been measured for tellurium over the temperature range 4 to 200 K. Throughout this temperature range alpha /sub /// is confirmed as being negative and alpha _{perpendicular to} positive. The Gruneisen functions gamma /sub /// and gamma _{perpendicular to} have been obtained and analysed. Accurate values for these functions for temperatures less than about 30 K are presented here for the first time: gamma _{perpendicular to} has a broad shallow maximum centred near 30 K while gamma /sub /// is negative below about 18 K and has a sharp minimum centred near 8 K. The strain dependence of the moments of the frequency distribution gamma _{perpendicular to} (n) and gamma /sub /// (n) have been derived for -3<or=n<or=2 and the temperature dependence of gamma _{perpendicular to} (T) and gamma /sub /// (T) analysed in terms of the strain dependence of a simplified but realistic frequency distribution G( omega ). This analysis indicates that the lowest frequency peak in G( omega ), which is presumably associated with essentially transverse acoustic modes, contains modes (j) with predominantly negative gamma ^j_{perpendicular to} values, whereas acoustic modes of higher frequency have markedly positive gamma ^j/sub /// values.

Expressions are derived for the linear term in the high temperature specific heat C_v for FCC lattices using the assumption that the atoms interact with central first neighbour forces only. The harmonic contribution to the lattice free energy at high T is calculated exactly, using a trace theorem derived by Salter. In addition cubic and quartic terms are retained using an extension of the earlier work of Maradudin et al (1961) due to Feldman and Horton (1967). The present formalism is used to calculate the linear term in the expansion C_v/3Nk=1+AT+ . . . for the inert gas solids Ar, Kr, and Xe using the exp-6 potential. The calculations, which include the effects of thermal expansion, are compared with experiment and to some recent calculations by Trivedi (1971), who used a less complete form for the calculation of the A coefficient.

The electronic states of a finite crystal in a uniform electric field are studied for a linear, one-band, tight-binding chain with nearest neighbour overlaps, the most favourable case for the existence of a Stark ladder in the energy spectrum. The use of Brillouin-Wigner perturbation theory leads directly to an exact ladder for an infinite chain and strongly suggests that a fraction of the levels arising from atoms in the central part of a finite chain form a weakly perturbed 'quasi-ladder'. The properties of continued fractions enable analysis of finite size effects on the Stark levels of isolated atoms and yield accurate upper and lower bounds for the edges of the perturbed energy spectrum. Detailed numerical calculations substantiate these conclusions.

A systematic study of positron annihilation in a number of amorphous and crystalline polymers has been carried out. In most cases except polytetrafluoroethylene, the positron annihilation decay time spectrum is resolvable in only two components. The lifetime values (in ns) for the two components and the percentage relative intensities of the longer component determined in this investigation are: polystyrene (0.379+or-0.006, 1.96+or-0.01, 39.7+or-0.5); polymethylmethacrylate (0.388+or-0.005, 1.86+or-0.01, 25.7+or-0.5); polyvinylchloride (0.407+or-0.003, 1.81+or-0.03, 6.1+or-0.3); polyethylene (0.403+or-0.004, 2.03+or-0.02, 16.7+or-0.4); polyethyleneterephthalate (0.391+or-0.004, 1.67+or-0.01, 16.0+or-0.4).

Intercalation of bromine into single crystalline and well oriented pyrolytic graphite leads to the formation of several distinct, coexisting phases having compositions in the range C₄₆₀₀Br to C₁₄₀Br. These phases are best regarded as graphitic regions of characteristic Fermi energy and dimensions, separated by condensed monolayers of Br^- ions which have accepted their ionic charge from the pi electron system of the graphite matrix. Such acceptors do not markedly increase the collision broadening of the graphite Landau levels. Thus, using the transverse Shubnikov-de Haas effect, one can observe the dominant hole period which characterizes each phase of the graphite. Typically, the new hole periods are 5.27, 1.63, 1.24 and 0.21*10^-6 G^-1, which, using the Sloncewski-Weiss energy band model, give thicknesses for the individual tabular phases ranging from 700 to 21 AA, with corresponding Fermi energy depressions of 0.035 to 0.49 eV. In order of decreasing thickness, the Fermi surfaces of these phases evolve from the triple ellipsoids of pristine graphite through an elongated dumb-bell to an ovoid with a single extremum. The periods vary as the cosine of the angle of rotation of H away from c, confirming that in the present composition range the local ellipticity of the Fermi-surfaces remains of order 10. Landau state broadening and the relative stability of the new phases are briefly discussed.

The phonon excitation spectra of Pb and Sn have been obtained from an analysis of the electron tunnel characteristics of junctions with the electrodes in the normal and the superconducting state. Metallic junctions with two different types of insulating barrier Al₂O₃ and Formvar, are used. The large resonant peak due to the Formvar long molecular chains has been resolved for lead. Self energy effects for the quasi-electrons in tin have been revealed and are interpreted in terms of coulombic contributions since tin is a weakly coupled electron-phonon system. Similar effects for the quasi-holes have been observed for tin in the presence of Formvar. It is also found that proper vibrations of the insulators and the metals may be reciprocally transferred.

The author studies the kinetic secondary emission of two component targets submitted to an ionic bombardment. He examines in particular the compounds (Al, O) and (Al, Cu). The problem of electronic excitations in the collision of two particles in a solid is strongly connected to the problem of the collision of the same particles in the gas phase. Thus he has used, in this paper, the new ideas about the correlation diagrams in asymmetric systems which have been recently discovered in inelastic gaseous collisions. His results show that the kinetic emissions (kinetic positive aluminium ions and Auger electrons) of the compounds under consideration are much lower than the kinetic emission of the pure metals. These conclusions are in agreement with recent experimental data.

Linear dichroism measurements are used to study a specimen of composition Y₃Fe_5-xSi_xO₁₂ with x=0.08. After cooling the specimen to 1.9 K in an applied saturating magnetic field directed along the (111) crystallographic direction, a spontaneous dichroism of 9.6% was observed using radiation of wavelength 1250 nm; the dichroic axis was along the field direction. Removal of the applied field caused little change in the dichroism. However by irradiating the specimen with plane polarized near infrared radiation whose E vector was along the (111) direction, the dichroic axis was caused to rotate and to lie along this direction. The authors interpret this to mean that the magnetization of the specimen was also switched between these axes. However the rotation of the magnetization did not occur coherently throughout the whole irradiated volume of the specimen.

For pt. II see abstr. A6958 of 1972. The magnon spectrum and thermodynamic properties of GdCl₃, a ferromagnet in which the dipolar interactions are comparable in magnitude with the exchange interactions, are investigated both analytically and numerically. The calculations make use of formal results derived previously by Cottam for a general ferromagnetic insulator with dipole-dipole interactions. The magnon energies at low temperatures are calculated, including the leading order effects of spin wave interactions. The dispersion relation is found to be very anisotropic, and in particular it has a minimum energy gap of approximately 0.36 K at k=(0, 0, 0.28)AA^-1. By numerical integration over the whole Brillouin zone, results are obtained for the spontaneous magnetization over a wide range of temperatures, and the Curie temperature is estimated. A comparison is made with experimental data. Calculations are also carried out for the static susceptibility and the magnetic specific heat.

The problem of incorporating crystal field ideas into the description of a system with a regular lattice is considered using a recently developed method, and applied to NiO. The basic idea is to define an unperturbed hamiltonian which is symmetric with respect to interchanges of electrons, which has the lattice symmetries, and which is conceptually simple. The difference between it and the full hamiltonian can then be treated by perturbation theory. The unperturbed hamiltonian has energies and states close to those expected for the full hamiltonian, with one major difference, that no propagation properties are contained in it. These are found in the perturbation, which has matrix elements between unperturbed states in which excitations have moved. A rather surprising result is that the crystal field splittings, at least in the unperturbed hamiltonian, can be calculated by conventional methods and no account need be taken of overlap, covalency, etc. The states of the electrons must, however, be orthogonal functions.

Linear dichroism has been measured in both the absorption and emission bands of the F⁺ centre in CaO under uniaxial stress. In order to analyse the emission results the method of moments is generalized to transitions taking place from an orbitally degenerate level. It is shown that this analysis allows a direct determination of the Ham reduction factors. It is verified that absorption and emission measurements lead to the same values for these factors K(E)=0.40, K(T₂)=0.39. These values when compared with K(T₁)=0.02 clearly show that the excited level is subject to the special kind of Jahn-Teller effect described by O'Brien (1971). Detailed analysis of the polarized spectra allows the determination of the symmetry of the modes of vibration giving rise to the vibronic structure and clearly shows that modes in a large range of energies are involved in the Jahn-Teller coupling.

Neutron inelastic scattering experiments have been carried out to determine the energies and widths of the crystalline electric field levels in the neodymium monopnictides NdP, NdAs, and NdSb. The energy level sequence is derived from the observed crystal field transition peak intensities, which are in good agreement with calculations based on elementary crystal field theory. The energy level widths are qualitatively discussed. It is found that the point-charge model cannot reproduce the crystal field levels satisfactorily.

The K beta ₂ emission bands have been studied for Ge and the 'cation' in the isostructural series of semiconducting compounds GaAs, ZnSe, CuBr. The experimental results are found to be in agreement with available predictions of the energy band calculations. The normalized integral intensity of the x-ray emission band decreases with increasing ionicity of the bond and reflects the valence electron shift toward the 'anion'. This is accompanied by the localization of the valence electrons on the 4s level of the corresponding 'anion'.

Analysis of the Mossbauer hyperfine interaction data for the octahedral 3d(t_2g⁵) Fe^III complex in K₃Co_1-xFe_x(CN)₆ results in a magnetic contact term of (H_C/2S)=+255+or-5 kOe and an (r^-3)_m approximately 1.0 au for the 3d(t_2g) wavefunction. A comparison with the corresponding theoretical free-ion values for Fe³⁺, -126 kOe and 5.7 au, discloses an extensive overall expansion of the 3d(t_2g) wavefunction accompanied by a significant 4s participation in the bonding of the central transition metal ion. Similar results are obtained from the analysis of EPR data for the 3d(t_2g⁵) electronic configuration in K₄Fe_1-xMn_x(CN)₆.3H₂O. These results are in accord with the available EPR data for several octahedral covalent complexes of V⁴⁺, Cr⁵⁺ and Mo⁵⁺, which have the complementary electronic configurations 3d(t_2g¹) and 4d(t_2g¹). Supporting evidence is derived from the analysis of the quadrupole splitting, isomer shift and molecular vibrations in K₃Fe(CN)₆.

Measurements have been made at temperatures between 300 K and 1160 K of the multiphonon infrared absorption by samples of crystalline and molten KBr, NaCl and LiF 1-2 mm thick. The relatively small change in the spectra which accompanies the melting transition is pertinent to the organisation of molten salt dilution studies and implies that at least the high frequency limits of the respective vibrational spectra are approximately the same in the solid and liquid phases close to the melting point.

A study of the cathodoluminescence edge emission spectrum of zinc telluride at liquid helium temperatures after bombardment with monoenergetic electrons in the 100 to 500 keV energy range has been made. Production of a sharp emission at 2.362 eV after electron radiation damage of p type zinc telluride at 77 K allowed the determination of an energy threshold at 300 keV, which is assigned to the displacement of tellurium atoms. An electron of 300 keV is capable of transferring a maximum of 6.7 eV to a tellurium atom.