Abstract
A study of the annealing processes and charge state of dilute Fe in rutile TiO2 single crystals was performed in the temperature range 143–662 K, utilizing online 57Fe emission Mössbauer spectroscopy following low concentrations (<10−3 at%) implantation of 57Mn (T1/2 = 1.5 min). Both Fe3+ and Fe2+ were detected throughout the temperature range. Three annealing stages were distinguished: (i) a broad annealing stage below room temperature leading to an increased Fe3+ fraction; (ii) a sharp annealing stage at ∼330 K characterized by conversion of Fe3+ to Fe2+ and changes in the hyperfine parameters of Fe2+, attributed to the annealing of Ti vacancies in the vicinity of the probe atoms; and (iii) an annealing stage in the temperature range from 550 to 600 K, where all Fe ions are transformed to Fe3+, attributed to the annealing of the nearby O vacancies. The dissociation energy of MnTi–VO pairs was estimated to be 1.60(15) eV. Fe2+ is found in an environment where it can probe the lattice structure through the nuclear quadrupole interaction evidencing the extreme radiation hardness of rutile TiO2. Fe3+ is found in a paramagnetic state with slow spin–lattice relaxation which follows a ∼Tn temperature dependence with 4.1 < n < 6.3 at T > 350 K.
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