Atomistic scale investigation of cation ordering and phase stability in Cs-substituted Ba(1.33)Zn(1.33)Ti(6.67)O(16), Ba(1.33)Ga(2.66)Ti(5.67)O(16) and Ba(1.33)Al(2.66)Ti(5.33)O(16) hollandite

The titanate-based hollandite structure is proposed as an effective ceramic waste form for Cs-immobilization. In this study, quantum-mechanical calculations were used to quantify the impact of A-site and B-site ordering on the structural stability of hollandite with compositions Ba(x)Cs(y)(M(z)Ti(8-z))O(16), where M = Zn(2+), Ga(3+), and Al(3+). The calculated enthalpy of formation agrees with experimental measurements of related hollandite phases from melt solution calorimetry. Ground state geometry optimizations show that, for intermediate compositions (e.g., CsBaGa(6)Ti(18)O(48)), the presence of both Cs and Ba in the A-site tunnels is not energetically favored. However, the decay heat generated during storage of the Cs-containing waste form may overcome the energetics of Ba and Cs mixing in the tunnel structure of hollandite. The ability of the hollandite structure to accommodate the radioparagenesis of Cs to Ba is critical for long term performance of the waste. For the first time, B-site ordering was observed along the tunnel direction ([001] zone axis) for the Ga-hollandite compositions, as well as the intermediate Al-hollandite composition. These compositionally dependent structural features, and associated formation enthalpies, are of importance to the stability and radiation damage tolerance of ceramic waste forms.