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·B₂O₃ 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 TiO₂. 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.