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.

DOI: https://doi.org/10.21954/ou.ro.0000e23b

Abstract

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 Sr_1-xM_xFe₁₂O₁₉ (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 Sr_1-xM_xFe₁₂O₁₉phase 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.