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A co-precipitated hydroxide precursor containing equimolar quantities of Mg 2+  and Ti 4+ , doped by impregnation with ca. 0.1 at-% Sn 4+ , after annealing in flowing H 2  at 600 °C, yields MgTiO 3  microcrystals containing Sn 2+  ions. As attested by in situ  119 Sn Mössbauer spectroscopic measurements (at 295 K, isomer shift δ = 2.80±0.01 mms −1  and quadrupole splitting Δ = 1.80±0.02 mms −1 ) the Sn 2+  ions possess a low coordination number (CN &lt; 6) and exhibit anomalously high resistance to be transformed to metallic β -Sn. Upon contact with air, at r. t., fast oxidation of Sn 2+  to Sn 4+  (δ = 0.03±0.01 mms −1  and Δ ≤ 0.3 mms −1 ) occurs. Quite a similar behavior was previously observed for the tin dopant located on the surface of Cr 2 O 3 , α-Al 2 O 3  or MgO crystallites. Independent evidence for the presence of tin on surface sites of the MgTiO 3  substrate also is provided by XPS measurements. Whereas the Sn 2+  Mössbauer spectrometric parameters are virtually unaffected upon further annealing in H 2  at higher temperature (900 °C), this treatment prevents the tin from reacting with ambient O 2 . Such a passivation effect is imputed to itinerant t 2g  electrons which inactivate absorbed oxygen. The high-temperature annealing is also responsible for the appearance of a minor single-line spectral component with δ = 1.6±0.1 mms −1 . This isomer shift value cannot be attributed to any known compound of tin that could be formed under the experimental conditions used. The puzzling spectral component is accounted for by the presence of residual Sn 4+  ions immobilizing itinerant t 2g  electrons on one of the neighboring Ti 4+  cations in the bulk of the MgTiO 3  crystallites.

Tin-doped MgTiO 3 : A New Material for Studying the Solid-Gas Interface Making Use of the 119 Sn Mössbauer Spectroscopic Probe