Table of contents

It is commonly supposed that hindered methyl groups in solids undergo phonon induced rotations through +or-120. This may be symbolized (1) to (2) or (1) to (3) where (1), (2) and (3) indicate equivalent orientations of the group. The authors show both theoretically and experimentally that a second type of motion may be more important. In this motion a transition to a mixed state occurs: (1) to ²/₃((2)+(3)-¹/₂(1)). The operator which converts (1) to the mixed state converts the latter back to (1) so the motion involves two states only and may be described as flip-flop motion. It may be distinguished experimentally from rotation because it does not modulate matrix elements between pairs of states of E symmetry. Both the rotation and flip-flop rates are measured for the 4-methyl group in 4-methyl-2,6-ditertiarybutylphenol (MDBP) between 10K and 50K. The flip-flop rate is six times greater than the rotation rate. The measurement technique combines proton spin-lattice relaxation measurements with a novel way of determining the rate of conversion between proton spin symmetry species.

A formula is derived for the velocity of ions in the B phase of superfluid ³He as a function of electric field and temperature. Comparison between theory and experimental results gives values of the energy gap which are within 3% of that predicted by BCS theory.

Neutron diffraction experiments and magnetization measurements were performed on DyMg and ErMg (CsCl structure: a=3.759 AA and a=3.737 AA, respectively. DyMg is antiferromagnetic in zero applied field ( mu _Dv=6 mu _B). A strong ferromagnetic component develops in moderate applied fields. ErMg is ferromagnetic ( mu _Er=2.0 mu _B).

Matrix elements describing one-photon and two-photon magneto-optical transitions in InSb have been calculated by treating the cubic nonsphericity and inversion asymmetry of the crystal with perturbation theory. The results obtained explain most of the intraband (one-photon) and intraband-interband (two-photon) resonant excitations observed by various authors with light polarizations epsilon perpendicular to H and epsilon //H for a magnetic field H parallel to the (001) direction.

Derives an expression for the lineshape of the Brillouin spectrum of an acoustic phonon in an absorbing material taking account of phonon reflection at the sample boundary. The shape is asymmetric and differs appreciably from that previously used to interpret experimental spectra.

The far-infrared reflectivity of K₂MnF₄ and Rb₂MnCl₄ single crystal was studied for E perpendicular to c and E approximately=c at several temperatures. By means of a Kramers-Kronig analysis, the frequencies of the long-wavelength infrared-active phonons were determined. In addition, Raman spectra of Rb₂MnCl₄ were measured. Two peaks were found in both of the x(zz)y and x(zy)z spectra. From these maxima the frequencies were deduced for phonons with the Gamma ₁⁺(A_1g) and Gamma ₅⁺(E_g) symmetries, respectively. The frequency dependence in K₂MnF₄ and Rb₂MnCl₄ was studied between 13K and room temperature. This investigation was performed with the intention of obtaining a complete set of experimental data for lattice dynamical considerations especially with respect to the intralayer and interlayer forces in K₂MnF₄ and Rb₂MnCl₄.

For pt.I see ibid., vol.9, p.4213 (1976). A rigid-ion model with axially symmetric, short-range interactions originally developed for perovskite materials has been adapted to perovskite type layer structures by introducing new short-range force constants for the interlayer interactions. This model has been applied to KMnF₃, K₂MnF₄ and Rb₂MnCl₄. The adjustable parameters were fitted to the phonon frequencies at q=0 from infrared and Raman studies. It is shown that for KMnF₃ and the intralayer forces in K₂MnF₄ the same values of the parameters can be used. This lattice dynamical comparison indicates that the short-range interactions are the same in the cubic perovskite and the related layer structure. The dominant interlayer interaction between alkali and halogen ions yields parameter values close to those in the corresponding alkali halides. This indicates that the interlayer bonding is more of the ionic type than of the van der Waals type.

In an interacting spin-phonon system, coherences in the phonon scattering from different ions lead to coupled spin-phonon modes. The lifetime of these excitations in the phonon regime is calculated for a crystal containing ions with S>¹/₂. The poles of the phonon Green function are related directly to those of a generalized cumulant spin Green function for which a diagrammatic expansion in terms of a polarization matrix is presented. Averages of time-ordered products of spin operators occur and these are expressed in terms of semi-invariants. To evaluate the latter, a generalized Wick's theorem is developed that deals directly with operators that are of higher order than linear in the spin components. A detailed analysis of the attenuation is made in the case when paramagnetic ions are in sites of octahedral symmetry in a cubic crystal. An explicit expression is given in a situation of experimental interest, namely, for transverse wave propagation along a cubic axis with the propagation and polarization vectors respectively perpendicular and parallel to the axis of quantization.

Supercooling of microsize droplets of water dispersed within emulsions has been studied by means of a differential scanning calorimeter Perkin-Elmer DSC 2. In an emulsion cooled steadily at 2.5K min^-1, the breakdown of supercooling in individual droplets is distributed in temperature over the range -37.5 degrees C to -40.5 degrees C, the most probable temperature of freezing being T*=-(39.0+or-0.5) degrees C. When an emulsion is maintained at T_C, 0 degrees C>T_C>T*, the freezing of individual droplets is distributed in time, the time taken to achieve total freezing of the emulsion becoming shorter as T_C becomes closer to T*. If an emulsion which has been partially frozen by being maintained at T_C and then melted is cooled steadily at 2.5K min^-1, then the distribution of freezing temperatures shows two most probable values of T*, -(34.0+or-0.5) degrees C and -(39.0+or-0.5) degrees C. This phenomenon is briefly discussed.

Presents an extension of the cluster-Bethe-lattice method to study the electronic properties of binary alloys with varying coordination numbers. Expressions for transfer functions which account for neighbour correlations are obtained. The method is applied to study effects of short-range order in the CsAu liquid alloy and in a hypothetical solid with one s-state per atom, and the coordination properties of the Si-O system.

The coherent potential approximation is applied to a study of the DC electrical conductivity at zero temperature of disordered ternary alloys. In the model a semielliptical density of states is used for numerical purposes. The comparison between the conductivities of binary alloys and the corresponding, in some sense, ternary alloy is given as well as the discussion of the influence of the parameters describing ternary alloys on the DC electrical conductivity.

The optical absorption of CdS was measured in a high vacuum after desorption of chemisorbed species. The absorption edge obtained under these conditions was found to be significantly different from the absorption edge obtained in air under atmospheric pressure.

Recent work has shown it possible to derive single-ligand contributions from the observed spin-Hamiltonian parameters for the ⁸S_7/2 ground state of f⁷ lanthanide ions using the superposition model. Here the authors apply this model to the ⁶S_5/2 ground state of Mn²⁺ and Fe³⁺ and show it to give a consistent description of experimentally determined parameters in several systems. The significance of this result is understanding the splitting of the ⁶S_5/2 ground state is discussed.

The high-temperature series (HTS) of seven terms for the susceptibility for the ABX₃-type Heisenberg cubic antiferromagnets have been analyzed. A formula, relating the NN exchange parameter to the spin and temperature at which susceptibility maximum occurs, is given. Another formula relating the exchange parameter to spin and maximum susceptibility is also given. These relations give exchange parameters that are in fair agreement with the experimental results for a number of compounds. This shows the wide applicability of the given relations. The measured paramagnetic susceptibilities of two compounds have also been analyzed. In addition, the presence of zero-point spin reduction has been supported by comparing experimental susceptibilities with those obtained from the spin-wave theory.

EPR measurements of the ground state g-values of Co²⁺ ions at sites of trigonal symmetry in CdCl₂, CdBr₂, CsCdCl₃, MgCl₂ and K₄CdCl₆ are reported. Two distinct axial EPR spectra are observed in CsCdCl₃, corresponding to the two crystallographically inequivalent Cd²⁺ sites. In the case of K₄CdCl₆ one axial spectrum and a second spectrum of lower symmetry are observed. All of the other crystals display single axial spectra. A superposition model analysis of the crystal-field parameters for CdCl₂:Co²⁺ gives intrinsic parameter values of B²=-11000 cm^-1 and B⁴=-4060 cm^-1. These values are employed in a calculation of the crystal fields for Co²⁺ ions in CsCdCl₃, K₄CdCl₆ and CsMgCl₃, using the empirical superposition model for nearest-neighbour interaction and the point-charge model for contributions from more distant ions.

For pt.II see ibid., vol.8, p.1737 (1975). The theory of nuclear spin relaxation by translational diffusion in solids previously developed by Sholl is extended to the case of single crystals. Numerical results are presented over the entire temperature range for FCC, BCC and SC crystal structures enabling the relaxation rates to be determined for polycrystals and for any orientation of magnetic field in single crystals. Some errors in previous work are corrected. The magnitude of the field dependence is quite different for the three crystal structures considered. The validity of the approximation of averaging the relaxation rate rather than the magnetization is examined.

Superconvergent sum rules are written for the Voigt effect for isotropic nonmagnetic materials.

A detailed study has been made of Tomlin's method of determining the optical constants of an absorbing specimen by measuring the normal-incidence reflectances from the specimen itself and when coated with a thin transparent layer. It has been applied successfully to the measurement of the optical properties of amorphous and crystalline germanium in the spectral range from 700 to 300 nm where specimens are highly absorbing. For thin films it has thus been possible to extend the range previously covered by the reflectance and transmittance measurements of Denton and Tomlin (1972). The absorption of the amorphous thin films near the absorption edge has been re-examined and it is shown that four band gaps may be recognized, for which an interpretation in terms of a Mott-Davis (1971) model is offered.

The X-ray K-absorption edge of copper in the metal and some of its compounds involving metal-metal exchange interaction has been investigated using a 40 cm bent crystal spectrograph. The observations lead to the conclusion that the relative energy of the principal absorption maximum is a more reliable criterion for fixing the oxidation state of the central metal ion rather than the shift of the main edge, which has hitherto been relied on. Furthermore, these observations do not support the hypothesis of boosting in edge shift as a consequence of the metal-metal exchange interaction. These edge shifts, however, may be better understood in terms of ionic/covalent character of the metal-ligand bond.