$^{57}$Fe emission Mössbauer spectroscopy following dilute implantation of $^{57} Mn$ into In $_2O_3$

Emission Mössbauer spectroscopy has been utilised to characterize dilute $^57$Fe impurities in In $_2O_3$ following implantation of $^57$Mn ($T_{1/2}$ = 1.5 min.) at the ISOLDE facility at CERN. From stoichiometry considerations, one would expect Fe to adopt the valence state 3 + , substituting In $^{3+}$, however the spectra are dominated by spectral lines due to paramagnetic Fe$^{2+}$. Using first principle calculations in the framework of density functional theory (DFT), the density of states of dilute Fe and the hyperfine parameters have been determined. The hybridization between the 3d-band of Fe and the 2p band of oxygen induces a spin-polarized hole on the O site close to the Fe site, which is found to be the cause of the Fe$^{2+}$ state in In $_2O_3$. Comparison of experimental data to calculated hyperfine parameters suggests that Fe predominantly enters the 8b site rather than the 24d site of the cation site in the Bixbyite structure of In $_2O_3$. A gradual transition from an amorphous to a crystalline state is observed with increasing implantation/annealing temperature.