Emission Mössbauer spectroscopy in TiO2 single crystal

An almost perfect single crystal of TiO₂ was doped by about 50 ppm of ⁵⁷Co. Mössbauer spectra were measured versus sample orientation, temperature, and thermal history. It was found that Co occupies both substitutional and interstitial sites being a fast diffuser, while located interstitially. It decays to Fe³⁺ (S=52) in unperturbed lattice sites, Fe²⁺ (S=2) in lattice sites associated with the VO^2- vacancy, Fe²⁺ (S=0) in interstitial sites having adjacent VO^2-, and finally to Fe¹⁺ (S=32) in unperturbed interstitial sites. Thermal history of the sample could be erased by heating to about 750 K. Substitutional iron following ⁵⁷Co decay could be observed solely in the host lattice at elevated temperatures. High-temperature data indicate two charge states of iron, i.e., Fe²⁺ and Fe³⁺. Fe²⁺ with S=2 exists in the vicinity of defects and converts gradually to Fe²⁺ (S=0) with the increasing temperature, while Fe³⁺ (S=52) resides in the unperturbed lattice sites. A host matrix becomes more covalent at very high temperatures as well as slightly anharmonic. No significant diffusivity of the substitutional iron could be seen. The total area under the spectrum follows unusual pattern due to the gradual disappearance of the signal coming from iron located interstitially, i.e., a transfer of iron atoms into fast diffusing interstitials with increasing temperature occurs. All processes were observed to be reversible upon heating/cooling.