Mössbauer effect in inorganic glasses

Abstract

57Fe- and119Sn-Mossbauer effects become a very effective tool for the local structural study of inorganic glasses. Fe3+ and Sn4+ occupy network former (NWF) sites in several oxide glasses, while they are present at interstitial sites as network modifier (NWM) in phosphate, germanate and sulfate glasses. Characteristicγ-ray or thermal neutron irradiation effect indicates the structural role of Mossbauer ions; an increase in the 4s electron density or a reduction of Fe3+ to Fe2+ is observed due to the charge transfer from oxygen to Fe3+ when iron occupies NWF sites. By contrast, oxidation of Fe2+ to Fe3+ takes place when iron occupies NWM sites. Debye temperatures (Θ D) obtained from low-temperature Mossbauer measurements are higher than 280 K when Fe3+ and Sn4+ are covalently bonded to oxygen at NWF sites, while theΘ D is lower than 270 K when these ions are ionically bonded to oxygen or halogen at NWM sites. A linear relationship between the glass transition temperatureT g and the quadrupole splitting (Δ) of Fe3+, named “Tg-Δ rule”, also indicates the structural role of Fe3+, i.e.,T g versusΔ plot yields a large “slope” of 680°C/mm s−1 when Fe3+ occupies NWF sites, whereas it is only 35°C/mm s−1 when the iron is present at NWM sites.