Mössbauer spectroscopy as a probe of silicate glasses

Abstract

Mössbauer spectroscopy of tin and iron has been used to probe silicate glasses. The researchwas motivated by the need to understand the structure and behaviour of float glass whentin diffuses into the surface during manufacture. Iron is also present in small quantitiesas it is an impurity in the sand from which the glass is made, and it is knownto affect the tin uptake. Because float glass is a complex mixture of silica andseveral modifier oxides, Mössbauer spectroscopy on simpler systems containingsilica and a single alkali or alkaline earth modifier has been carried out first tofacilitate the interpretation of the data. The spectra have broad lines, reflectingthe large number of different sites in an atomically disordered material. Severalconclusions were reached from a review of the literature on silicate glasses. The isomershifts and quadrupole splittings identify the ionic states of the Mössbauer atoms,and the changes when the composition is varied by adding modifier atoms aredue to small changes in the local structure and atomic volume. It appears thatSn2+ and Fe3+ act as conditional glass formers in silicate glasses, whileSn4+ and Fe2+ are modifiers. From measurements of the relative areas ofSn2+ and Sn4+ in the Mössbauer spectrum of sectioned specimens, the depth profiles of eachoxidation state were determined separately. By monitoring the oxidation ofSn2+ toSn4+ produced by heat treatment in air, the diffusion coefficient of oxygenwas determined. Measurements on tinted float glass containing severalper cent of iron show that the iron and tin react with each other, theFe3+ oxidizingthe Sn2+ to Sn4+. By combining these data with the depth profiles, a flow diagram of the chemical reactionsin the float-glass process has been proposed.