The effect of polymorphic structure on the structural and chemical stability of yttrium disilicates

Under pressure and temperature conditions like those found in deep geological repositories (DGRs), rare-earth cations may react with silicates to form rare-earth disilicates. This study establishes the stabil-ity range of yttrium disilicates in response to changes in pH. The a, ß, ?, and d polymorphs of Y2Si2O7 were synthesized by the sol-gel process at temperatures between 1100 and 1650 °C and subjected to pHstat leaching tests. By measuring the Y and Si leaching rates and monitoring the transformation of the crystalline and amorphous phases, we showed that yttrium disilicates were stable at pH > 5. At pH < 5, the pH stability sequence was consistent with the temperature-dependent stabilities of Y₂Si₂O₇ phases, with the d polymorph showing the lowest leaching rates. Because rare-earth compounds can be used as a proxy for analogous actinide hosts, the results of this study can be used to predict the performance of engineered barriers in DGR.