Mossbauer diagnosis of the electronic structure and local environment of Sn in MgTiO3

Annealing of a hydroxide precursor containing equimolar amounts of Mg and Ti and small additions of Sn (0.1 at %) in air at 900DGC leads to titanate MgTiO3 with an ilmenite structure. The Sn Mossbauer spectrum of the sample (unresolved doublet with the isomer shift d = 0.10 - 0.01 mm/s and the quadrupole splitting = 0.49 - 0.02 mm/s) is evidence that the tin atoms are still in the oxidation state +4. Annealing of the precursor at the same temperature in a hydrogen atmosphere yields MgTiO3 containing Sn ions (a doublet at d = 2.82 - 0.01 mm/s and = 1.66 - 0.03 mm/s) (the Sn/MgTiO3 sample). According to the spectral parameters, the Sn ions have a low coordination number (CN < < 6) and are abnormally resistant to reduction to the metal. Analogous features of the crystal-chemical behavior of Sn were previously observed during the Mossbauer study of the samples containing tin on the surface of Cr2O3, a-Al2O3, and MgO crystallites. The conclusion drawn from analysis of the Sn Mossbauer parameters that tin in the Sn/MgTiO3 sample has surface localization was supported by X-ray photoelectron spectroscopy. Mossbauer measurements show that the tin of Sn/MgTiO3 when in contact with air is oxidized much more slowly than on the surface of Cr2O3, a-Al2O3, or MgO crystallites. The inhibition of the oxidation reaction is explained to be due to passivation of adsorbed O2 molecules caused by their interaction with mobile t 2g electrons of Ti forming in titanate during high-temperature annealing in H2. In addition to the Sn doublet, the Sn spectrum shows a spectral component with parameters (d ~ 1.6 mm/s, < = 0.2 mm/s) not fitting the known tin species that can form in MgTiO3. This component is explained by persistence in titanate of some Sn ions immobilizing the mobile t 2g electron at one of their neighboring Ti cations.