Thermochemistry of barium hollandites

Barium hollandites, a family of framework titanates that can potentially be used for the immobilization of short-lived fission products (especially ¹³⁷Cs) in radioactive wastes, have been investigated by high-temperature oxide melt solution calorimetry using 2PbO·B ₂O₃ solvent at 702°C. The enthalpies of formation from constituent oxides show increasing energetic stability of the hollandite phase as Ti⁴⁺ is substituted by Mg²⁺, Al³⁺, and Fe³⁺, in that order. In general, the thermodynamic stability increases with decreasing average cation radius in the β sites, and when the tolerance factor approaches one. The Al- and Fe-hollandites are more stable than phase assemblages containing BaTiO₃ perovskite and Al/Fe/Ti oxides, whereas Mg-hollandite is less stable than the corresponding assemblage of BaTiO₃ perovskite, MgTiO₃ ilmenite, and TiO ₂. This instability makes Mg-hollandite a less suitable host for fission products. Hollandite phase formation during metal citrate combustion synthesis depends more on thermodynamic stability and phase chemistry than on the annealing temperature.