Thermochemistry of framework titanosilicate A ₂TiSi ₆O ₁₅ (A = K, Rb, Cs)

A new family of framework titanosilicates, A ₂TiSi ₆O ₁₅ (A = K, Rb, Cs) (space group Cc), has recently been synthesized using the hydrothermal method. This group of phases can potentially be utilized for storage of radioactive elements, particularly ¹³⁷Cs, due to its high stability under electron radiation and chemical leaching. Here, we report the syntheses and structures of two intermediate members in the series: KRbTiSi ₆O ₁₅ and RbCsTiSi ₆O ₁₅. Rietveld analysis of powder synchrotron X-ray diffraction data reveals that they adopt the same framework topology as the end-members, with no apparent Rb/K or Rb/Cs ordering. To study energetics of the solid solution series, high-temperature drop-solution calorimetry using molten 2PbO · B ₂O ₃ as the solvent at 975 K has been performed for the end-members and intermediate phases. As the size of the alkali cation increases, the measured enthalpies of formation from the constituent oxides (ΔH _f,ox ⁰) and from the elements (ΔH _f,el ⁰) become more exothermic, suggesting that this framework structure favors the cation in the sequence Cs ⁺, Rb ⁺, and K ⁺. This trend is consistent with the higher melting temperatures of A ₂TiSi ₆O ₁₅ phases with increase in the alkali cation size.