Table of contents

Recent experimental results on two hcp phases of barium under high pressure show interesting variation of the lattice parameters. They are here interpreted in terms of electronic structure calculations by using the linear muffin-tin orbital method and generalized pseudopotential theory with a nearly-free-electron plus tight-binding-bond approach. In phase II (5.5 - 12.6 GPa) the dramatic drop in c/a is an instability analogous to that in the group II metals Mg to Hg but with the transfer of s to d electrons playing a crucial role in Ba. Meanwhile in phase V (45 - 90 GPa), the instability decreases a lot due to the core repulsion at very high pressure. The c/a ratio in phase V is somewhat less than the expected ideal value due to some admixture of 5d and 4f components in the wave function.

High-resolution neutron powder diffractometry has been used to characterize the structural phase transition which occurs in at . The change in lattice parameters of the trigonal unit cell as a function of temperature, observed in the range from ambient to , shows that the transition from the polar space group to the non-polar space group is associated with a collapse of the a-dimension, whereas the c-dimension and the cell volume increase monotonically with small changes of slope at the transition temperature. The structure of the lower-temperature phase, previously determined at ambient temperature by single-crystal x-ray diffraction, has been refined at 20, 500 and and the structure of the higher-temperature phase determined from data taken at 535, 560 and .

The thermal evolution of the atomic coordinates shows that the deciding role in the transition is provided by a complex movement of the B - O tetrahedron, in which the main component is a rotation around the axis. The transition is continuous and results from structural changes which are displacive but also show order - disorder character. This conclusion is in accordance with the known thermal, optical and dielectric properties of this new multifunction (laser, ferroelectric and non-linear optic) material. In the high-temperature modification, the dynamically disordered double helical chain of B - O tetrahedra has no polarity in the c-direction. This is the main structural reason for the absence of ferroelectric properties above .

The lower transition temperature of in the related phase may be understood by comparing the structural aspects of its transition with those of using the Abrahams - Jamieson - Kurtz criteria. The effect of isomorphic substitution for Ge with the smaller Si atoms is equivalent to pressure being applied to the helical B chain and results in a chain which is more tilted in the silicate than in the germanate.

K-edge transmission-mode extended x-ray absorption fine structure, transmission electron microscopy, and x-ray diffraction have been used to investigate the microstructure of Mo/Si multilayers with periods ranging from 20 to 2.0 nm (the layer thickness ratios of Mo to Si were 1:2). The results confirmed that there was a Mo - Si amorphous interlayer between the Mo and Si layers, the Mo - Si coordination was about 80% in the first shell neighbouring the Mo atom in the interlayer, and the total coordination number was 7.4, approximately equal to that of bcc Mo. A thermally activated model is suggested as a basis for explaining the interlayer formation mechanism by considering the different thermal conductivities of the deposited Mo and the amorphous Si surfaces.

We have studied the densities of states of the series of alloys FeAl, CoAl, and NiAl, each in the disordered state, using the augmented-space recursion technique coupled with the tight-binding muffin-tin orbitals method. We have estimated the local moments in these alloys using the spin-polarized version of the tight-binding linearized muffin-tin orbitals method. We have also obtained the effective pair interactions for these alloys.

The one-dimensional t - J model at the supersymmetric point, J = 2t, with a magnetic impurity of arbitrary spin S is studied exactly using the Bethe ansatz technique. The impurity interacts by spin exchange with the electrons on neighbouring sites of a lattice without destroying the integrability. The discrete Bethe ansatz equations diagonalizing the model, and the thermodynamic equations are derived. The impurity free energy is obtained for arbitrary band filling as a function of temperature and external magnetic field. The impurity can localize up to one itinerant hole, and has in general mixed-valent properties.

Two different parametrizations of the semi-empirical method of the intermediate neglect of the differential overlap (INDO) are applied to the calculations of the small-radius hole polarons in the corundum crystal. The 80-atom supercell has been used for the study of the atomic and electronic structure of a free small-radius hole polaron (the self-trapped hole, STH) and a hole polaron bound by a Mg impurity (the so-called centre), respectively. Both parametrizations indicate that the two-site (quasi-molecular) configurations of both kinds of polaron have the lowest energy (which does not exclude the existence of one-site polarons also characterized by considerable relaxation energies). For centres the lower energy is calculated for the defect configuration where the Mg-ion substitution is nearest to the hole polaron. Experimental ENDOR data on defects are discussed in the light of the calculations.

On the basis of the method recently used in the study of the trivalent impurity state in (M: In, Ga, Sc and Y), the structure of the electron trapped at an centre in is determined. Although a bistable system is obtained, due to the locally soft lattice around the ion the localized compact state has higher energy than the delocalized state. The excited p-like states and the oscillator strengths for the optical transition between the s- and p-like states in the three bistable systems (with Al, Ga, In) are evaluated. The changes in the refractive index as the systems change from localized to delocalized states are estimated, and the results are discussed in the context of the recently reported data on the photorefractive properties of In and Ga centres in .

A first-principles theoretical investigation of positron annihilation in alkali halide crystals is carried out using a simplified cluster-embedding scheme. The system is represented as a halide-centred cluster with basis functions only at the centre. The rest of the crystal is modelled in two ways: (i) point ions located at lattice positions; and (ii) frozen-orbital ions derived from an energy band calculation for the pure crystal. Calculations for both models are carried out within the self-interaction-corrected local spin-density approximation and by incorporating an electron - positron correlation functional. The effect of the model assumed on the calculated positron lifetimes is analysed by demonstrating the sensitivity of the results to the inclusion of the Madelung potential. A comparison of positron lifetimes of the ground state of the positron to lifetime components identified in experimental work on lithium and sodium halide systems is made.

The effect of pressure (up to 15 kbar) and magnetic field (up to 8 T) on the low-temperature (down to 1.3 K) DC conductivity and magnetoconductance (MC) of ion-irradiated ( and ions) polyimide films with a room-temperature conductivity of 200 - 400 has been investigated. It was shown that for samples on the metal side of the metal - insulator transition with the resistivity ratio the application of pressure increases (300 K) by a factor of 2 - 3 with respect to that at ambient pressure and the value of is slightly reduced. The low-temperature conductivity shows a dependence. The sign of the MC at ambient pressure is negative at all fields, thus implying that the contribution for the electron - electron interaction is dominant. However, at 15 kbar a positive MC at low fields, in both the transverse and the longitudinal directions to the field, has been observed. This implies that under pressure the system is more metallic with respect to that at ambient pressure, and the positive MC due to the dominance of the weak-localization contribution is enhanced by making the system more metallic under pressure.

Impedance and phase measurements on a sandwiched-structure thin-film device thick were carried out at room temperature as a function of frequency in the range 0.1 Hz - 1.0 MHz under dark and visible-light environments . A thorough impedance analysis was carried out using two types of equivalent-circuit model to explain the observed AC behaviour. One of these models utilizes the classical concept of interfacial space-charge polarization based on the Maxwell - Wagner two-layer capacitor system, whereas the other takes into consideration the physical meaning of a optoelectronic device. The latter model has been invoked to calculate the individual electrical, photoelectric and dielectric parameters of the . The results of such an analysis enable us to deduce a value for the room-temperature volume dark resistivity of the material equal to . Furthermore, these results confirm that the photoconduction phenomenon in is due to semiconductivity with the photoconductivity varying with light intensity F according to the generally accepted experimental power law of the form , with . The relative dielectric constant of this semi-insulating material has been found, for the first time, to have a value of 14.9, irrespective of the illumination level. On the other hand, the electrical, photoelectric and dielectric properties associated with the contact, which is probably responsible for the low-frequency dispersion in the device, were found to have a large influence on its overall AC characteristics and this has been attributed to interfacial space-charge effects which seem to be enhanced by illumination.

Spin fluctuations manifest themselves in the temperature dependence of the spontaneous magnetization M(T,H=0) or the `in-field' magnetization M(T,H), through a contribution that varies with T as in the intermediate-temperature range and as for temperatures in the vicinity of (Curie point) but still away from criticality. Suppression of spin fluctuations by either the Co concentration in alloys or the field H for a given composition in the and alloy series is, for the first time, monitored through the decrease in the coefficients A and with increasing x for H = 0 or with H for fixed x or y. While A(0) and scale with and , respectively, A(H) and follow the relation where or , or , or and is the external field corrected for demagnetization. In contrast with the non-monotonic variation in B and n with x, varies with x as and is independent of x in the range . The spin fluctuation model explains many of, but not all, these features.

We present the first ab initio calculations of the internal magnetic field of a molecular magnet using a modified Ewald method. We apply this to both the -phase and -phase forms of the organic radical para-nitrophenyl nitronyl nitroxide (p-NPNN). The -phase is known to exhibit molecular ferromagnetism, whilst the -phase is antiferromagnetic. These results are of particular relevance to the interpretation of muon spin-rotation experiments, where the muon acts as a local probe of the internal magnetic field. As further guidance to the interpretation of such experiments, we also calculate the isotropic hyperfine constant for implanted muons.