Electron paramagnetic resonance and Mössbauer spectroscopy of transition metal ion doped mullite

Appreciable difference in the properties of undoped and oxide-doped mullite are observed. The oxidation state of cation, its concentration and the position of the mullite lattice occupied by it appear to be the responsible factors. Mullite has, therefore, been doped with four transition metal ions, Mn, Fe, Cr and Ti. With the help of EPR and Mossbauer spectroscopy (supplimented by X-ray diffractometry) the oxidation states of these ions and the mullite lattice sites where they enter has been investigated. It was observed that Mn ion was present in Mn2+ and Mn3+ states, the former remained as clusters and the latter occupied the octahedral sites in the mullite lattice. Only Fe3+ ion was detected and conclusive evidence was obtained for the entry of Fe3+ in the octahedral lattice position of mullite from the analysis of Mossbauer spectra with the help of a specially written computer programme. The Cr ion entered the mullite structure only in the Cr3+ state. The change in lattice parameters of Cr doped mullite were measured by the XRD technique. The results showed that the expansion of b-axis was more than that of the a-axis which supported the presence of Cr3+ ion in the octahedral site of mullite lattice. The absence of signal in the EPR spectra of Ti doped mullite suggested the presence of only Ti4+ (3d0) ion. Very low electrical resistivity of Ti doped mullite and close similarity between mullite and Al2TiO5 structures stood as evidence for incorporation of Ti4+ ion in the octahedral site of mullite lattice by replacing Al3+ ion.