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Title: Nuclear magnetic resonance in some binary liquid systems showing a metal non-metal transition
Author: Kirby, David John
ISNI:       0000 0001 3600 1783
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 1981
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The results are reported of a pulsed nuclear magnetic resonance investigation at frequencies up to 59 MHz of three binary liquid alloy systems which demonstrate a wide range of electrical properties as a function of composition and temperature. The systems studied are caesium-gold and caesium-antimony, which both show marked deviations from metallic properties at the stoichiometric compositions CsAu and CsSb,-and selenium-tellurium, where increasing temperature or tellurium cOTitent give rise to more metallic properties. The aim of the work was to investigate the mechanism of the metal non-metal transition and to determine the nature of the non-metallic states which form. In Cs-Au and Cs-Sb the major emphasis is placed on the concentration dependence of properties at constant temperature, and in Se-Te two concentrations Se0 .5Te0. 5 and Se 0.4 Te 0.6 are studied as a function of temperature. Nuclear resonances shifts, spin-lattice T1, spin-spin T2, and phase memory T2, relaxation times are measured. For the system Cs-Au, the addition of gold to caesium has only a small effect on the resonance properties, until for excess mole fraction of caesium = 0.07 a sharp peak is observed in the 133Cs relaxation rate and the shift changes from a nearly free electron behaviour to linear dependence on concentration with only a small shift at stoichiometry. The metal non-metal transition gives rise to strongly localised electron states in the form of F-centres in liquid alloys near the composition CsAu. A thermally generated background concentration of paramagnetic centres is observed near stoichiometry. Addition of large amounts of gold (0.50 mole fraction) to stoichiometric CsAu has negligible effect on the small 133cs shift. An ionic, molten salt description of liquid CsAu is appropriate. In contrast to Cs-Au, the addition of small amounts of antimony to caesium causes a rapid decrease in the 133Cs Knight shift. Near stoichio­metric Cs3Sb, the density of states is low and the resonance shift small, (although larger than in caesium salts implying a larger chemical shift contribution). Covalent bonding is much more important in Cs3Sb than in CsAu. In Cs3Sb at room temperature a quadrupolar splitting of 133cs resonance was observed. Neither 121gb or 123gb were observed in any of the alloy samples investigated due to the strong quadrupolar inter­action which gave rise to extremely rapid spin lattice relaxation times < 3 μs. For Se-Te near the melting point T1 > T2 and values of the hyperfine field correlation time are deduced for each nucleus. We find TTe> TSe. The hyperfine field data suggests non-random bonding in these liquids. Shift and relaxation rates for both 77Se and 125Te in both alloys demonstrate similar behaviour and are intrepreted in terms of interaction of the nucleus with a thermally activated concentration of unpaired electrons which localise as chain end defects. 77Se in Se0.4.Te0.6 demonstrates a change of hyperfine field strength with increasing temperature which is interpreted in terms of a structural transition from a Se-like to a Te-like liquid. This was not observed for other nuclei due to loss of signal with increasing temperature.
Supervisor: Not available Sponsor: Science Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics