Microporous silicotitanates can potentially beused as ion exchangers for removal of Cs~+ from radioactivewaste solutions. The enthalpies of formation from constituentoxides for two series of silicotitanates at 298 K have beendetermined by drop-solution calorimetry into molten2PbO·B_2O_3 at 974_K: the (Na_(1-x)Cs_x)_4Si_3O_(15)(OH)·nH_2O (n = 4 to 5) phases with acubic structure (P43m), and the (Na_(1-x)Cs-x)_3Ti_4Si_2O_13)(OH)·nH_2O (n = 4 to 5) phases with atetragonal structure (P4_2/mcm). The enthalpies of formationfrom the oxides for the cubic series become more exothermic asCs/(Na + Cs) increases, whereas those for the tetragonal seriesbecome less exothermic. This result indicates that theincorporation of Cs in the cubic phase is somewhatthermodynamically favorable, whereas that in the tetragonalphase is thermodynamically unfavorable and kinetically driven.In addition, the cubic phases are more stable than thecorresponding tetragonal phases with the same Cs/Na ratios.These disparities in the energetic behavior between the twoseries are attributed to their differences in both local bondingconfiguration and degree of hydration.
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