Thermochemistry and crystallization of glass-forming Y-substituted Sr-analogues of fresnoite (Sr₂TiSi₂O₈)

Strontium titanosilicate (Sr₂TiSi₂O₈) is the Sr-analogue of fresnoite (Ba₂TiSi₂O₈). It may provide useful properties due to the unusual five-coordinate titanium in its structure. Sr₂TiSi₂O₈ is also a possible oxide form for the immobilization of short-lived fission products in radioactive waste. Through β 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 ceramic is of fundamental importance. To incorporate the reaction 3Sr²⁺=2Y³⁺+vacancy in the fresnoite composition, Y-substituted Sr-analogues of fresnoite, (Sr_2-xY_23x) TiSi₂O₈ (x=0, 0.5, 0.75, 1, 1.25, and 1.5) were prepared by a high-temperature synthesis and were found to form glass upon cooling. The Y-end-member (Y_1.33TiSi₂O₈, x=2) crystallized to a mixture of Y₂TiSiO₇, TiO₂, and SiO ₂ upon quenching in air. The enthalpies of formation of Y-substituted Sr-fresnoite glasses were obtained from drop solution calorimetry in a molten lead borate (2PbO·B₂O₃) solvent at 702°C. The enthalpies of formation from constituent oxides were exothermic but become less so with increasing Y content. The enthalpy of vitrification of Sr ₂TiSi₂O₈ composition was measured to be 68.5 ± 6.2 kJmol. The thermodynamic stability of the Y-substituted Sr-analogue of crystalline fresnoite may become marginal with increasing yttrium content. Their ready glass formation, even in the absence of radiation, suggests that amorphous phases may dominate as the Sr- and Cs-containing waste forms decay.