Table of contents

The crystal structure of s-triazine at 5K has been determined by profile refinement of neutron powder data. The structure has the same symmetry as that previously determined at temperatures below the phase transition at 198K, although significant changes in the structural parameters have been observed. The results are compared with those predicted by earlier calculations of the lattice energy.

A domain-like atomic arrangement is observed in the incommensurate 2d Au₃₊Zn with two-dimensional antiphase structure of Cu₃Pd-type by high-resolution structure imaging method using 1000 kV electron microscope. Two types of domains with different structures occur by turns along (100) and (001). Inside the domains atomic arrangements are commensurate and antiphase periods are 6a and 5c, and the relative configuration of the domains produces incommensurate character.

It is shown that the re-entrant nature of the incommensurate-commensurate lock-in transition of the charge density wave in 2H TaSe₂ may be a consequence of the anharmonicity in the interlayer coupling.

Results of measurements of the thermal conductivity of crystals of ³He and ⁴He containing up to 1000 ppm Ne are presented. Rough values are given for the solubility of Ne in helium crystals formed at several molar volumes. The conductivity measurements are interpreted in terms of various scattering mechanisms including defect scattering of a resonant form.

The anisotropy of the ligand hyperfine splitting of the P_Ga⁴⁺ antisite ESR signal in GaP has been resolved and analysed. A simple molecular orbital evaluation of the hyperfine data for P_Ga⁴⁺ shows that 92 percent of the density of the second donor electron of the P_Ga double donor is localised at P_Ga and its four P ligands. The photoresponse of the P_Ga⁴⁺ ESR signal in semi-insulating and p-type material respectively indicates that the second ionisation energy of P_Ga is (1.10+or-0.10) eV.

The Ni L_2,3M_4,5M_4,5 Auger spectra, the conduction band X-ray photoemission spectra and the Ni 2p_3/2 X-ray photoemission spectra have been measured for Ni, Ni₅₀Zn₅₀, Ni₂₀Zn₈₀ and Ni₅₀Al₅₀. The shape of the L₃M_4,5M_4,5 Auger spectrum changes as the band structure of the alloy changes in a way which agrees closely with the recent calculations of Treglia et al. (1981). The data support the conclusion of Treglia et al. that the number of unoccupied d states has a larger effect on the width of the Auger lines than the width of the 3d band.

It is shown that inner displacement has a large effect on the collective coordinates of the T₂ modes in strained crystals of the zincblende structure. This has important implications for the interpretation of stress-dependent phenomena in gallium arsenide and similar materials.

The authors argue that because of the skewness of the double-minimum proton potential and the magnitude of the ground-state splitting it is not possible to construct fully localised single-particle proton states in KDP. It follows the proton-proton interactions differ according to whether each proton is in its lowest symmetrical or antisymmetrical state, and that the transverse-field Ising-model Hamiltonian must be supplemented by a further interaction term bilinear in the pseudospin operator S^x. This argument is strongly supported in the crystallographic studies by Nelmes (1980) of the temperature variation of the atom positions in KDP and DKDP.

Thermodynamic analogies have previously been invoked to interpret the temperature- and pressure-dependent electronic levels in a semiconductor as standard Gibbs free energies. In this paper it is shown by means of statistical mechanical arguments using the pressure ensemble that standard Gibbs free energies are the quantities determining equilibrium occupations of electronic levels and hence are the thermodynamic quantities that enter detailed balance relationships. Optical band gaps are identified with the 'nophonon line' gap for a transition in which the occupation numbers for the phonons and remaining electrons are unchanged. An exact expression for no phonon optical transition energies is given. It is shown rigorously that the optical no-phonon-line energy is identical to the corresponding free-energy change for any transition between band states and also for transitions involving localised states provided: (i) the effects of electronic degeneracy changes are excluded from the free energy definition; (ii) the localised state is not associated with a local lattice mode.

The recursion method is used to investigate the electronic states of ideal and reconstructed vacancy models in silicon. The matrix elements of the Hamiltonian are taken from Pandey and Phillips, but atoms coupling across the vacancy are treated as first nearest neighbours. The exponential decay factor, beta , for these elements is fitted to self-consistent calculations of the ideal vacancy. The shifts in levels of reconstructed vacancies appear weaker (by up to a factor 2) than those of self-consistent calculations.

An ion is located in a crystal at a site that is not a centre of inversion, with a Zeeman field directed at right angles to the crystal axis c. Periodic intensity variations in the optical spectrum that occur when the crystal is rotated about c are due to magnetic dipole transitions between levels of different type and to electric dipole transitions between levels having admixed opposite-parity parts of different type. Use of an antiunitary operator that commutes with the total Hamiltonian demonstrates that interference between electric and magnetic dipole radiation (or interference between any opposite-parity multipole radiation) does not occur for transitions appearing in the axial spectrum, but does occur for those appearing in the pi or sigma transverse spectrum. Depending on the site symmetry, two or four of the Zeeman components of a doublet-doublet transition have an additional periodicity as a result of this interference. Eigenstates are constructed that show the relationship between the M values of a Zeeman level, the crystal quantum number that labels it, and its type, and are used to calculate the angular dependence of certain doublet-doublet dipole transitions for ions at sites of point symmetries D_2d and C_3h. With explicit expressions for dipole matrix elements, it is discovered that two Zeeman components having the same angular dependence when interference is neglected exhibit dissimilar angular behaviour when the interference cross-term is included.

Percolation-theoretic considerations have been used by various workers to evaluate the DC hopping conductivity in disordered systems in the low-temperature single-phonon-assisted hopping regime. The authors apply percolation theory to the high-temperature multiphonon-assisted small-polaron hopping regime. A general expression for the conductivity is obtained and comparison with the low-temperature case is made.

Resistance, DC Hall effect and transverse magnetoresistance (TMR) measurements were attempted on a synthetic monocrystal of magnetite over the range 60-280K in fields up to 47*10⁵ A m^-1. The conductivity below T_v is described by a model which incorporates the temperature-dependent overlap of the wavefunctions. Systematic tendencies in the Hall and TMR effects above T_v suggest two components: one that is sensitive to the relative orientation of current flow and the domain walls and the other that arises from an interaction at the Fe ions. The former makes the current distribution very sensitive to the magnetic state and so accounts for some improbable results reported previously for the range below T_v.

Raman measurements on the quasi-1D Ising antiferromagnets CsCoCl₃ and CsCoBr₃ are presented. The magnons in both systems may be described by a spin-¹/₂ model and exhibit most interesting quantum features. These features are discussed in the 3D ordered and in the 1D short-range order regimes. Due to the hexagonal structure, 1D phenomena may be studied even in the presence of short-range order. Most of the effects may be described very satisfactorily by a recently developed theory, apart from strong magnon-phonon coupling in CsCoCl₃. A set of parameters describing the magnons in these quantum systems is extracted from the data. It is suggested that 3D cooperative phenomena are still present in the 1D regime.

Results of Raman scattering by magnons in Cd_1-xMn_xTe are presented. The modes of magnetic origin for x=0.50, 0.62, 0.64 and 0.68 are studied in the temperature range from 2 to 40K. The spectra are interpreted as a two-magnon scattering process, and interpreted in Ising model approximation. Exchange integrals with the nearest neighbours are evaluated for all investigated samples.

A mean-field theory of spin-density-wave wavevector and polarisation vector rotation induced by the application of a magnetic field to MnSi in its spin-density-wave phase, and a prediction of the associated spin reorientation phase transitions, is presented. The dependences of the Neel temperature and the homogeneous magnetisation on applied magnetic field strength and orientation are studied, as is the staggered susceptibility which diverges at the spin orientation phase transition.

Fluorine second moments and spin-lattice relaxation times in the laboratory and rotating frames were measured as a function of temperature. The results show that the low-temperature orthorhombic structure of K₃ZrF₇ is disordered. Each of the four non-equivalent ZrF₇^3- molecules has four possible orientations which can be described as twofold reorientations about the crystallographic axes. The activation energies associated with these motions range from 6.3 to 10.0 kcal mol^-1. In the high-temperature cubic phase the ZrF₇^3- molecules are equivalent. The molecule has sixteen possible orientations but the results show that an additional reorientational mechanism results in an isotropic reorientation in which each F-F vector occupies all other F-F positions in the course of time. Due to the cross-relaxation between ⁹¹Zr and ¹⁹F nuclei, the quadrupolar relaxation time of ⁹¹Zr could be obtained from the T₁ results. Although the TaF₇^2- molecules has no symmetry axis, the relaxation results show clearly that it is involved in a two-stage reorientation mechanism. These reorientations are described as 'floppy' reorientations in which the shape of the molecule changes during each jump.

Aspects of the copper-green emission in zinc sulphide crystals have been studied by means of optically detected magnetic resonance (ODMR). The observation of ODMR signals that correspond to decreases in emission intensity at resonance is taken as evidence for the presence of recombination paths that compete with the copper-green and other emission processes. One centre that can provide such a path has a g value of 2.0038+or-0.0007, which corresponds to that reported for the F⁺ centre in ZnS.

Optical absorption and magnetic circular dichroism (MCD) spectra of CaF₂:Sm²⁺ in the visible (700-400 nm) region have been measured over the temperature range 20-300 K. The so-called A-term MCD lineshape has been obtained for three bands at 509.2, 506.0 and 499.2 nm. The g-value of the 509.2 nm band has been estimated to be 1.60 using the moment method. An energy level diagram of 4f⁵5d configuration of Sm²⁺ has been calculated and compared with the absorption spectrum. A satisfactory agreement has been obtained between the calculation and experiment concerning the band positions and the g-value.

L₃ and L₁ X-ray absorption spectra of single crystals of TmSe and Tm metal were measured at the synchrotron radiation facility PULS using the Frascati storage ring Adone. Valence fluctuations in TmSe appear from the analysis of the X-ray absorption near edge structures (XANES). The XANES of TmSe shows two well resolved 'giant resonances' at the L₃ threshold due to 2p to 5, epsilon d* transitions localised on the Tm³⁺ and Tm²⁺ ions. The joint analysis of the L₁ and L₃ edges gives information on the symmetry of the conduction states near the Fermi level. The effects of the core-hole-induced relaxation on the measured intermediate valence are not important in Tm compounds. The values of the intermediate valence of Tm in TmSe can be extracted from XANES in contrast to what seems to be the case for Ce in intermediate valence Ce compounds.