Mossbauer effect for Fe<SUP>57</SUP> in Al<SUB>2</SUB>O<SUB>3</SUB>- effects of variety of heat treatments

The reduction of the Al<SUB>2</SUB>O<SUB>3</SUB>: Fe<SUP>57</SUP> system in a hydrogen atmosphere causes the formation of metallic iron clusters yielding hyperfine-split spectra. On firing in air, these clusters are first converted to Fe<SUP>2</SUP>O<SUB>3</SUB>, which subsequently dissolves in the lattice to give Fe<SUP>3+</SUP> at the Al<SUP>3+</SUP> site. Vacuum firing, however, merely tends to stabilize the Fe<SUP>2+</SUP> state in addition to the normal Fe<SUP>3+</SUP> state