An almost perfect single crystal of TiO2 was doped by about 50ppm of 57Co. Mössbauer spectra were measured versus sample orientation, temperature, and thermal history. It was found that Co occupied both substitutional and interstitial sites being a fast diffuser, while located interstitially. It decayed to Fe3+(S=5/2) in unperturbed lattice sites, Fe2+(S=2) in lattice sites associated with the VO2- vacancy, Fe2+(S=0) in interstitial sites having adjacent VO2-, and finally to Fe1+(S=3/2) in unperturbed interstitial sites. The thermal history of the sample could be erased by heating to about 750K. Substitutional iron following 57Co decay could be observed only in the host lattice at elevated temperatures. High-temperature data indicated two charge states of iron, i.e., Fe2+ and Fe3+. Fe2+ with S=2 existed in the vicinity of defects and converted gradually into Fe2+(S=0) with the increasing temperature, while Fe3+(S=5/2) resided in the unperturbed lattice sites. A host matrix became 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 occurred. All processes were observed to be reversible upon heating/cooling.

Emission Mössbauer Spectroscopy in TiO2 Single Crystal. Wdowik, U.D., Ruebenbauer, K.: Physical Review B, 2001, 63[12], 125101