Thermochemistry of framework titanosilicates A2TiSi6O15 (A = K, Rb, Cs)

A new family of framework titanosilicates, A2TiSi6O15 (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: KRbTiSi6O15 and RbCsTiSi6O15. 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 · B2O3 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 and from the elements (ΔHf,el) 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 A2TiSi6O15 phases with increase in the alkali cation size