Yttrium substitution in MTiO₃ (M=Ca, Sr, Ba and Ca+Sr+Ba) perovskites and implication for incorporation of fission products into ceramic waste forms

CaTiO₃ has been proposed as a ceramic waste form for immobilization of radioactive waste that might include short-lived fission products ¹³⁷Cs and ⁹⁰Sr and consequently their decay products ¹³⁷Ba (stable), ⁹⁰Y (intermediate), and ⁹⁰Zr (stable). In order to characterize substitution relations of Y³⁺ for M²⁺ (M=Ca, Sr, Ba singly or in combination) in MTiO₃, precursor mixtures with nominal atomic ratios of M:Y:Ti=0.75:0.25:1 were synthesized by solid-state reactions. To ensure homogeneous starting material and to explore low temperature behavior of the Ca-Y-Ti-O system, coprecipitated xerogel powder was synthesized and heated to various temperatures for different periods of time. All M-Y-Ti-O systems formed two major phase mixtures of perovskite MTiO₃ and pyrochlore Y ₂Ti₂O₇ with low substitution of M²⁺ by Y³⁺ in MTiO₃ and Y³⁺ by M²⁺ in Y₂Ti₂O₇. The study of xerogel confirmed the tendency of the formation of mixtures of CaTiO₃ and Y ₂Ti₂O₇ phases, even at lower temperature. The temperature dependence of the substitution of Ca²⁺ by Y³⁺ in CaTiO₃ appears to be small. Despite the minor substitution of M by Y in the perovskite no significant ⁹⁰Y³⁺ build-up is expected to occur in the waste form due to the rapid decay of ⁹⁰Y³⁺ relative to that of ⁹⁰Sr²⁺.