Synthesis, charac terization and thermochemistry of Cs-, Rb- and Sr-substituted barium aluminium titanate hollandites

Titanate hollandites are of considerable interest for immobilization of radioactive Cs, its daughter product Ba and related radionuclides Rb and Sr. In this study, we synthesized three hollandites, Ba_1.18Cs_0.21Al_2.44Ti_5.53O₁₆, Ba_1.17Rb_0.19Al_2.46Ti_5.53O₁₆ and Ba_1.14Sr_0.10Al_2.38Ti_5.59O₁₆, using sol-gel methods. Rietveld analysis of synchrotron XRD data shows that they adopt the tetragonal structure (space group I4/m), and their cell parameters increase with increasing cation size (Sr²⁺ → Rb⁺ → Cs⁺). Standard enthalpies of formation of these hollandites were determined from drop solution calorimetric measurements with lead borate as the solvent at 973 K. Their formation enthalpies are similar, consistent with the occurrence of extensive cation substitutions in hollandites. Further energetic analysis with respect to BaTiO₃ and SrTiO₃ perovskites and other oxides reveals decreased thermodynamic stability from Cs- to Rb- to Sr-hollandite. This trend is consistent with the phase assemblage observed in Synroc, where Cs⁺, Rb⁺ and Ba²⁺ enter into hollandite, whereas Sr²⁺ occurs in perovskite.