Tin-doped MgTiO3: A New Material for Studying the Solid-Gas Interface Making Use of the 119Sn Mössbauer Spectroscopic Probe

Abstract

A co-precipitated hydroxide precursor containing equimolar quantities of Mg2+ and Ti4+, doped by impregnation with ca. 0.1 at-% Sn4+, after annealing in flowing H2 at 600 °C, yields MgTiO3 microcrystals containing Sn2+ ions. As attested by in situ 119Sn 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 Sn2+ ions possess a low coordination number (CN < 6) and exhibit anomalously high resistance to be transformed to metallic β -Sn. Upon contact with air, at r. t., fast oxidation of Sn2+ to Sn4+ (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 Cr2O3, α-Al2O3 or MgO crystallites. Independent evidence for the presence of tin on surface sites of the MgTiO3 substrate also is provided by XPS measurements. Whereas the Sn2+ Mössbauer spectrometric parameters are virtually unaffected upon further annealing in H2 at higher temperature (900 °C), this treatment prevents the tin from reacting with ambient O2. Such a passivation effect is imputed to itinerant t2g 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 Sn4+ ions immobilizing itinerant t2g electrons on one of the neighboring Ti4+ cations in the bulk of the MgTiO3 crystallites.