Publications
Thermochemistry of framework titanosilicates A2TiSi6O15 (A = K, Rb, Cs)
A new family of framework titanosilicates, A{sub 2}TiSi{sub 6}O{sub 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 {sup 137}Cs, 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{sub 6}O{sub 15} and RbCsTiSi{sub 6}O{sub 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 {center_dot} B{sub 2}O{sub 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 and from the elements ({Delta}H{sub f,el}) become more exothermic, suggesting that this framework structure favors the cation in the sequence Cs{sup +}, Rb{sup +}, and K{sup +}. This trend is consistent with the higher melting temperatures of A{sub 2}TiSi{sub 6}O{sub 15} phases with increase in the alkali cation size.