Thermochemistry of microporous silicotitanate phases in the Na₂O–Cs₂O–SiO₂–TiO₂–H₂O system

Microporous silicotitanates can potentially be used as ion exchangers for removal of Cs⁺ from radioactive waste solutions. The enthalpies of formation from constituent oxides for two series of silicotitanates at 298 K have been determined by drop-solution calorimetry into molten 2PbO · B₂O₃ at 974 K: the (Na_1−xCs_x)₃Ti₄Si₃O₁₅(OH) · nH₂O (n = 4 to 5) phases with a cubic structure (P43m), and the (Na_1−xCs_x)₃Ti₄Si₂O₁₃(OH) · nH₂O (n = 4 to 5) phases with a tetragonal structure (P4₂/mcm). The enthalpies of formation from the oxides for the cubic series become more exothermic as Cs/(Na + Cs) increases, whereas those for the tetragonal series become less exothermic. This result indicates that the incorporation of Cs in the cubic phase is somewhat thermodynamically favorable, whereas that in the tetragonal phase is thermodynamically unfavorable and kinetically driven. In addition, the cubic phases are more stable than the corresponding tetragonal phases with the same Cs/Na ratios. These disparities in the energetic behavior between the two series are attributed to their differences in both local bonding configuration and degree of hydration.