Structural behavior of Ba_1.24Al_2.48Ti_5.52O₁₆ hollandite at high temperature: An in situ neutron diffraction study

Titanate hollandites comprise a class of potential waste form phases for immobilization of radioactive Cs (and its daughter product Ba) due to their high thermal and aqueous stability. To study their structural behavior at high temperature, we conducted in situ neutron diffraction experiments of a representative tetragonal phase, Ba_1.24Al_2.48Ti_5.52O₁₆, in the temperature range 300-1173 K. Rietveld analyses of the obtained data show that on heating, unit-cell parameters a and c increase at similar rates. This isotropic nature of thermal expansion can be explained by the increased volume and regularity of [(Ti,Al)O₆] octahedra and the widening of the Ti/Al-O2-Ti/Al angle (O2 is corner-shared by two [(Ti,Al)O₆] octahedra from neighboring [(Ti,Al)O₆] double chains) with increasing temperature. Practically, this property is advantageous, as hollandite-based ceramic or composite products would, upon heating, be less likely to form microcracks. The amplitudes of thermal vibration for Ba, Ti/Al, and O increase with increasing temperature; however, the rate of increase for Ba is much larger. This behavior is due to the occupancy of the box-shaped cavity site by Ba, which has weaker interactions with its neighboring atoms, compared with those for framework Ti/Al and O. On the other hand, the opening of the oxygen-coordinated cavity box is smaller than the size of Ba, even at high temperature, preventing evaporation of Ba from the hollandite structure. These characteristics render titanate hollandites potentially robust waste forms for Cs/Ba.