A new family of framework titanosilicates, A(2)TiSi(6)O(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 Cs-137, 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 center dot 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 (Delta H-f,ox(0)) and from the elements (Delta H-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(2)TiSi(6)O(15) phases with increase in the alkali cation size.