Mössbauer and isotope enchange studies of internally oxidized Ag-Sn alloys

Abstract

119Sn Mössbauer spectroscopy and oxygen isotope exchange experiments have been used to study internally oxidized Ag-Sn alloys containing 0.33 and 6.1 at.% Sn. The main results for the dilute alloy are as follows: (1) Approximately 4 O atoms per Sn atom could be absorbed, only 3 of which were exchangeable. (2) The internally oxidized Sn ions are all in an electronic charge state close to +4, but with approximately 0.20 electrons in the 5 s and 5 p states; the Mössbauer spectra showed no evidence for any intermediate charge states analogous, for example, to SnO in either totally or partially oxidized samples of this alloy. (3) 2 H atoms per Sn atom could be absorbed by the oxidized alloy, and the resulting isomer shifts revealed that the Sn ions had been driven even closer to a pure +4 charge state. (4) A large fraction of the oxygen diffuses through the oxidized alloy as undissociated atom pairs. For the 6.1 at. % Sn alloy, only 2 O atoms per Sn atom could be absorbed and the Mossbauer spectra showed that most of the Sn had precipitated as SnO 2, while a small fraction (approximately 4 per cent) remained unoxidized; the Sn atoms which remain unoxidized are believed to be those which have several Sn nearest neighbors. An atomistic model is given for the Sn-O-Ag molecular complex which is consistent with the experimental observations.