The effect of vacancy and barium substitution on the stability of the cesium titanium silicate pollucite

Cesium titanium silicate (CsTiSi₂O_6.5) is a titanium analogue of pollucite CsAlSi₂O₆ and a possible ceramic form for immobilization of short-lived fission products in radioactive waste. Through β decay, cesium (Cs) decays to barium. Therefore, not only the stability of Cs-loaded waste forms, but also that of a potential decay product series is of fundamental importance. Ba-substituted CsTiSi₂O _6.5 is a potential β decay product with the pollucite structure. Here, we report the effects of the reaction synthesis condition and the study of the thermodynamic stability of potential intermediates in the decay product series (1) with charge-balance in pollucite as two Cs ions are replaced by one Ba and a vacancy and (2) with one-to-one replacement of Cs by Ba. The enthalpies of formation of Ba-substituted CsTiSi₂O_6.5 were obtained from drop solution calorimetry in a molten lead borate solvent at 702°C. The enthalpies of formation, from constituent oxides, are exothermic and it decreases with increasing Ba content. The effect of vacancies in the pollucite structure is a more dominant factor in the energetics than that of Ba replacement. The thermodynamic effects of acetate and/or nitrate precursors and of adding acetic acid during synthesis of single phase Ba-substituted CsTiSi₂O_6.5 pollucite are insignificant except for the sample prepared from acetate precursors without acid treatment.