Defect annealing in Mn/Fe-implanted TiO₂ (rutile)

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 (T_1/2 = 1.5 min). Both Fe³⁺ and Fe ²⁺ 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 Mn_Ti-V_O 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.