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There is a need to design and synthesise magnetic materials that can be used as models to test our understanding of fundamental magnetic phenomena. Particularly important areas of current interest include the study of systems that model aspects of the magnetic behaviour of the high-<i>T_c</i> superconductors (low-spin Heisenberg antiferromagnets on a square lattice) and of highly frustrated magnetic systems. A family of layered copper salts, Cu(CAP)₂<i>X</i>₄, (CAP = 5 chloro-2-aminopyridine, <i>X</i> = Cl, Br) is a promising model system for the former class of magnet. The ordered magnetic structure of this system has been studied by neutron diffraction, along with (<i>H, T</i>) phase data and appears classical in character; this picture is confirmed by MuSR data, in addition, inelastic neutron scattering gives an indication of the three dimensional nature of the exchange in this system. The jarosite family of minerals, containing kagome lattices of metal ions, commonly Fe³⁺, are physical realisations of highly frustrated antiferromagnets. The excitation spectra of these materials have been studied using time of flight inelastic neutron scattering. Hydronium iron jarosite appears to behave as a spin liquid to 2 K despite strong antiferromagnetic coupling between homogeneous spin centres, whereas potassium iron jarosite exhibits conventional spin waves for a kagome lattice antiferromagnet, with contributions to the Hamiltonian of either a Dzyaloshinskii-Moriya interaction or crystal field induced single ion anisotropy. SQUID magnetometry measurements made under pressure in order to perturb the system, reveal changes in ordering behaviour in potassium iron jarosite. The mineral volborthite containing a slightly distorted kagome lattice of Cu²⁺ ions is particularly interesting as it combines both high frustration and low spin (<i>S</i> = ½). This system has been studied using various techniques including SQUID magnetometry, neutron scattering and muon spin relaxation, all of which indicate that the magnetic correlations are highly unconventional. These show that volborthite also possesses strong antiferromagnetic exchange and fluctuations remain significant down to at least 50 mK, despite the build up of short range antiferromagnetic correlations. Experiments looking at ways to minimise the hydrogen content of the various materials studied, to improve the quality of the data from neutron scattering measurements, are also reported.

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I was involved in the early research on electrical switching devices based on semiconducting chalcogenide glasses which generated such widespread interest during the later '60s and early '70s. There was great controversy at the time about the extent to which the switching mechanism is controlled by purely electronic process or is thermally induced. Papers A1, A2 and A4 summarise my contribution, the last (A4) setting out in detail the view that electrical switching in the chalcogenide glasses is basically an electrothermal process. By the late '70s it became clear that chalcogenide-based switching devices would not find major applications in mainstream electronics but the collaboration with the Dundee group on the application of amorphous silicon in similar devices has proved to be extremely fruitful. The first results are contained in paper A5, and A17 is a comprehensive review of that work, set in the general context of electronic switching in amorphous semiconductors. Concurrently, interest in chalcogenide glasses switched (!) to possible utilisation of their unique photo-induced effects. This is also proving to be a fruitful field. Potential applications include photoresists for high resolution microlithography, diffraction gratings - particularly for the infra-red, and in various non-linear electro-optic devices, also in the infra-red. Papers A8, A10 and A11 are typical original contributions; paper A16 is a general review.

The structural, vibrational, and electronic properties of anisotropic materials under compression are comprehensively investigated in this thesis. Recent developments in the techniques of high-pressure X-ray diffraction and Raman scattering, and significant advances in first principle simulations provide detailed high-pressure studies of condensed matter systems. In view of the widely disparate strength of cohesive forces, these studies consist of ionic compounds, quasi-two dimensional semiconductors, quasi-molecular solids, and end with liquid crystals. As a result of the coexistence of different hierarchical interactions in anisotropic systems, evidence of preferential pressure-induced enhancement of weak bonding is found not only in the structural response to external hydrostatic pressure but also in vibrational and electronic behaviour. Further, the understanding of pressure-induced breakdown of rigid-layer vibrations (explored in layered compounds), pressure-induced electron transfer in molecular crystals, and strong overlap of inter- and intra-molecular vibrational modes of liquid crystals provides insight into the essential physics of flexible molecular systems.

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Previous structural work has shown that strontium undergoes a sequence of phase transitions up to 46 GPa. The relatively complex structures of Sr-IV and Sr-V, stable above 35 GPa and 46 GPa respectively, could not be solved and doubts remained about the correct assignment of the Sr-III structure. It is only comparatively recently, however, that experimental techniques have been developed to allow the detailed structural characterisation of these phases. To resolve these remaining uncertainties, the author has carried out a re-examination of the high-pressure structures and transitions in strontium using angle-dispersive powder-diffraction techniques with the image-phase area detector on station 9.1 at the SRS Daresbury Laboratory. The experimental set up has been improved so that data of sufficient quality could be obtained to allow the structures of these high pressure phases to be both correctly identified and accurately refined. The previously reported structure of Sr-III was found to be wrongly identified as having an orthorhombic <i>Imma </i>symmetry (with a structure similar to that of Si-III) and this has now been correctly identified as having the archetypal <i>b</i>-tin (<i>I4₁</i> /<i>amd</i>) structure. The higher pressure phase of Sr, Sr-IV, has also been solved and the structure solution showed that it has a very complex monoclinic structure. This is the first time that this complex monoclinic structure has been reported in an element. Finally, the crystal structure of Sr-V has also been closely studied in comparison with the recently reported Ba-IV structure - based on the similarity of the diffraction patterns observed. The Sr-V structure is indeed found to be essentially the same as that of Ba-IV and it also has an intra-phase structural transition similar to those observed in Ba-IV.

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