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 center dot B2O3 solvent at 702 degrees 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 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.