title: Synthesis and characterisation of metal oxides and metal doped variants
Creators: name: Skinner, Stephen John
Publisher: name: The Open University
Year: 1997

description: The <sup>57</sup>Fe Mössbauer spectra recorded from inverse spinel related Fe<sub>3</sub>O<sub>4</sub> below the Verwey transition temperature are best interpreted in terms of tetrahedral A sites and two non-equivalent octahedral B sites. The broadening of the <sup>57</sup>Fe Mössbauer spectra recorded from Fe<sub>3</sub>O<sub>4</sub> at 298K is associated with the superposition of the two components arising from the non-equivalent sites as opposed to an electron hopping process. <br><br>The unit cell size of the metal-doped ferrite, Fe<sub>3-x</sub>Sn<sub>x</sub>O<sub>4</sub>, 0 &lt; x &lt; 0.4, has been shown by X-ray powder diffraction(XRD) to increase linearly as tin content increases. Extended X-ray Absorption Fine Structure (EXAFS) show that the SN<sup>4+</sup> ions substitute exclusively on the octahedral B sites of the inverse spinel related Fe<sub>3</sub>O<sub>4</sub> structure. The amount of Fe<sup>2+</sup> present in the Fe<sub>3</sub>O<sub>4</sub> increases as the tin content increases in a fashion consistent with charge balance being maintained by reduction of Fe<sup>3+</sup> on either the A or B sites. <br><br>The Curie temperature of Fe<sub>3-x</sub>Sn<sub>x</sub>O<sub>4</sub> decreases with increasing tin content. Exsolution of tin from the Fe<sub>3</sub>O<sub>4</sub> structure after prolonged heating at elevated temperatures <i>in vacuo</i> has also been observed. Oxidation of tin-doped magnetite involves a transformation to structurally related tin-doped maghemite before undergoing a structural transformation to tin-doped hematite. A lowering of the Curie temperature for both the tin-doped maghemite and and tin-doped hematite has been identified. <br><br>Chromium- and gallium- doped magnetite have also been found to have considerably lower Curie temperatures than pure magnetite. <br><br>Cubic-, tetragonal- and monoclinic- zirconium(IV) oxides have been produced by boiling zirconium (IV) acetate under reflux and calcining at increasing temperatures. Raman spectroscopy has been used to distinguish between tetragonal- and cubic- zirconia. Addition of HCl to the initial solution suppresses the formation of the tetragonal and cubic phases. Addition of ammonia to the initial solution produces tetragonal and cubic phases. Addition of ammonia to the initial solution produces tetragonal zirconia which remains stable to 900°C. <br><br>Hydrothermal synthesis of zirconium(IV) oxide from a solution of ziroconium(IV) acetate produces a mixture of tetragonal- and monoclinic- zirconia. Increasing the calcination temperature induces a transformation to the monoclinic polymorph. Addition of HCl to the initial solution gives monoclinic zirconia via an oxychloride phase. Addition of aqueous ammonia to the initial solution stabilises the tetragonal phase at temperatures up to 900°C. <br><br>Tetragonal iron-doped zirconia has been synthesised from iron(II) acetate and zirconium(IV) acetate by boiling under reflux. Increasing calcination temperature induces a transformation to the monoclinic phase. Increasing pH stabilises the tetragonal phase against transformation to monoclinic zirconia. Iron segregates from the zirconia structure at temperatures exceeding 900°C. <br><br>Hydrothermal processing of iron(II) acetate and zirconium(IV) acetate produces a mixture of tetragonal- and monoclinic- iron-doped zirconia. Calcination at 900°C induces the segregation of iron from the zirconia structure. Tetragonal iron-doped zirconia is characterised by a large quadrupole splitting in the <sup>57</sup>Fe Mössbauer spectrum.