Emission Mössbauer spectroscopy inTiO2single crystal

Abstract

An almost perfect single crystal of ${\mathrm{TiO}}_{2}$ was doped by about 50 ppm of ${}^{57}\mathrm{Co}.$ M\"ossbauer 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 ${\mathrm{Fe}}^{3+}(S=\frac{5}{2})$ in unperturbed lattice sites, ${\mathrm{Fe}}^{2+}(S=2)$ in lattice sites associated with the ${\mathrm{VO}}^{2\ensuremath{-}}$ vacancy, ${\mathrm{Fe}}^{2+}(S=0)$ in interstitial sites having adjacent ${\mathrm{VO}}^{2\ensuremath{-}},$ and finally to ${\mathrm{Fe}}^{1+}(S=\frac{3}{2})$ in unperturbed interstitial sites. Thermal history of the sample could be erased by heating to about 750 K. Substitutional iron following ${}^{57}\mathrm{Co}$ decay could be observed solely in the host lattice at elevated temperatures. High-temperature data indicate two charge states of iron, i.e., ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}.$ ${\mathrm{Fe}}^{2+}$ with $S=2$ exists in the vicinity of defects and converts gradually to ${\mathrm{Fe}}^{2+}(S=0)$ with the increasing temperature, while ${\mathrm{Fe}}^{3+}(S=\frac{5}{2})$ 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.