Thermochemistry of Barium Hollandites

摘要

Barium hollandites, a family of framework titanates that can potentially be used for the immobilization of short-lived fission products (especially ~(137)Cs) in radioactive wastes, have been investigated by high-temperature oxide melt solution calori-metry using 2PbO·B_2O_3 solvent at 702℃. The enthalpies of formation from constituent oxides show increasing energetic stability of the hollandite phase as Ti~(4+) is substituted by Mg~(2+), Al~(3+), and Fe~(3+), 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 BaTiO_3 perovskite and Al/Fe/Ti oxides, whereas Mg-hollandite is less stable than the corresponding assemblage of BaTiO_3 perovskite, MgTiO_3 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.