Mössbauer diagnosis of the electronic structure and local environment of 119Sn in MgTiO3

Abstract

Annealing of a hydroxide precursor containing equimolar amounts of Mg2+ and Ti4+ and small additions of Sn4+ (0.1 at %) in air at 900°C leads to titanate MgTiO3 with an ilmenite structure. The 119Sn Mossbauer spectrum of the sample (unresolved doublet with the isomer shift δ = 0.10 ± 0.01 mm/s and the quadrupole splitting Δ = 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 Sn2+ ions (a doublet at δ = 2.82 ± 0.01 mm/s and Δ = 1.66 ± 0.03 mm/s) (the Sn2+/MgTiO3 sample). According to the spectral parameters, the 119Sn2+ 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 119Sn2+ were previously observed during the Mossbauer study of the samples containing tin on the surface of Cr2O3, α-Al2O3, and MgO crystallites. The conclusion drawn from analysis of the 119Sn2+ Mossbauer parameters that tin in the Sn2+/MgTiO3 sample has surface localization was supported by X-ray photoelectron spectroscopy. Mossbauer measurements show that the tin of Sn2+/MgTiO3 when in contact with air is oxidized much more slowly than on the surface of Cr2O3, α-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 t2g electrons of Ti3+ forming in titanate during high-temperature annealing in H2. In addition to the Sn2+ doublet, the 119Sn spectrum shows a spectral component with parameters (δ ∼ 1.6 mm/s, Δ ≤ 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 Sn4+ ions immobilizing the mobile t2g electron at one of their neighboring Ti4+ cations.