The crystallization of Ba-substituted CsTiSi₂O_6.5 pollucite using CsTiSi₂O_6.5 seed crystals

Barium (Ba)-substituted CsTiSi₂O_6.5 materials of two types, Cs_xBa_1-xTiSi₂O_(7-x/2) and Cs_xBa_(1-x)/2TiSi₂O_6.5 were synthesized with the pollucite structure with 1≥x≥0.6. When the Ba-substituted precursor materials were heat treated to 850°C for 4 h, a mixture of amorphous and unidentifiable phases formed. However, with the addition of 10 wt% of crystalline CsTiSi₂O_6.5 to the Ba-containing precursors, nearly single-phase pollucite was obtained after 20 h at 750°C for x≥0.6. The added crystalline CsTiSi₂O _6.5 particles act as nuclei that allow the Ba-containing materials to crystallize into the pollucite phase and to avoid the formation of unwanted phases that would otherwise nucleate and grow. These new materials can be used to study the stability of CsTiSi₂O_6.5 as a durable ceramic waste form, which could accommodate with time both Cs and its decay product, Ba.