Redox and mixed valence in some solid state systems

Whittle, Karl R. (1998). Redox and mixed valence in some solid state systems. PhD thesis The Open University.



The silver amalgamation process whereby silver sulphide ore is converted to silver metal using copper chlorides and mercury proceeds through a redox process. The process involves the transformation of Ag2S to AgCl with a corresponding reduction of copper from Cu2+ to Cu+ and subsequent oxidation of Cu+ to Cu2+ by atmospheric oxygen. The AgCl is then reduced to Ag by mercury to form an amalgam. It has been found that the process proceeds similarly when iron replaces copper and involves a reduction of Fe3+ to Fe2+.

The doping of SrFe12O19 by europium and lanthanum to form compounds of the type Sr1-xMxFe12O19 (M=Eu,La) has been achieved by three different methods, the calcination of oxides at 1250°C, calcination of a gel at 925°C, and hydrothermal processing of metal nitrates at 220°C. X-ray powder diffraction (XRD) indicates the formation of similar materials by each method. X-ray absorption fine structure (XAFS) has shown that the La3+ ions substitute for Sr2+ ions. The particle sizes of the Sr1-xMxFe12O19phase have been found by Scherrer analysis of the X-ray powder diffraction data to increase with the temperature involved in the synthesis method. The results show that the materials formed by the calcination of oxides have a size ~800 nm, while those made by hydrothermal processing are ~300 nm.

57Fe Mössbauer spectra collected at various temperatures have shown that the dopant has negligible effect on the magnetic properties of the system. The results show a gradual coalescence of hyper fine fields as the temperature is decreased from 298K to 25K. Magnetic susceptibility measurements have shown that Eu- and La-doped SrFe12O19 prepared by the co-precipitation of a gel has a greater magnetic coercivity than those prepared by other methods.

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