Variation of the oxygen content and point defects in tephroite, Mn2SiO4+δ
Abstract Changes in the oxygen content of Mn 2 SiO 4 +  δ in equilibrium with MnSiO 3 were measured thermogravimetrically at 1000, 1100 and 1200 °C at a total pressure of about 0.5 atm as a function of the oxygen activity, a O 2 , and were used to determine absolute values for δ . The obtained values for δ vary from about 15 · 10 − 4 at 1200 °C and about 5 · 10 − 4 at 1000 °C at high oxygen activities, to below 1 · 10 − 4 at log 10 a O 2  ≈ − 8.5 at both temperatures. Thus Mn 2 SiO 4 +  δ is non-stoichiometric over the entire oxygen activity range investigated. The deviation from stoichiometry in manganese silicate is modeled based on the formation of localized holes, (Mn Mn 2+ 3+ ) , manganese vacancies, ( V Mn 2+ )″, and Mn 4+ ions on silicon sites, (Mn Si 4+ 4+ ) x . This defect model accounts for the observed oxygen activity dependence of δ . Furthermore, to better understand oxygen activity dependencies of point defect concentrations at experimentally relevant conditions, model calculations using a more general defect model were performed to derive Kroger–Vink diagrams for a generic orthosilicate Me 2 SiO 4 +  δ for constant Si/(Me + Si)-ratios and for being in equilibrium with a second oxide of the type MeSiO 3 .