Mössbauer effect in inorganic glasses

Abstract

⁵⁷Fe- and¹¹⁹Sn-Mössbauer 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 Mössbauer 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 Mössbauer 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 “T_g-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.