Thermochemistry of glass forming Y-substituted Sr-analogues of titanite (SrTiSiO₅)

Strontium titanium silicates are possible oxide forms for immobilization of short lived fission products in radioactive waste. Through beta decay, strontium decays to yttrium, and then to zirconium. Therefore, not only the stability of Sr-loaded waste forms, but also that of a potential decay product series with charge-balance in a naturally occurring mineral or a ceramic is of fundamental importance. Strontium titanosilicate (SrTiSiO₅) is the Sr-analogue of titanite (CaTiSiO₅). To incorporate the reaction 3Sr²⁺ = 2Y³⁺ + vacancy in the titanite composition, Y-substituted Sr-analogues of titanite, (Sr_1-xY_2/3x) TiSiO₅ (x = O, 0.25, 0.5, 0.75) were prepared by high temperature synthesis and were found to form glass upon cooling. The Y-end-member (Y _2/3TiSiO₅, x = 1) crystallized to a mixture of Y ₂Ti₂O₇, TiO₂, and SiO₂ upon quenching in air. The enthalpies of formation of Y-substituted Sr-titanite glasses were obtained from drop solution calorimetry in a molten lead borate (2PbOB₂O₃) solvent at 702 °C. The enthalpies of formation from constituent oxides are exothermic but become less so with increasing Y content. The thermodynamic stability of the Y-substituted Sr-analogue of crystalline titanite may become marginal with increasing yttrium content.