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_xCs_y(M_zTi_8-z)O₁₆, where M = Zn²⁺, Ga³⁺, and Al³⁺. 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., CsBaGa₆Ti₁₈O₄₈), 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.

中文翻译：

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Cs取代Ba1.33Zn1.33Ti6.67O16，Ba1.33Ga2.66Ti5.67O16和Ba1.33Al2.66Ti5.33O16钙铝石中阳离子有序性和相稳定性的原子尺度研究。

钛酸盐基的重钙沸石结构被认为是一种有效的固定Cs的陶瓷废料。在这项研究中，使用量子力学计算来量化A位置和B位置有序对组成为Ba _x Cs _y（M _z Ti _8-z）O ₁₆的堇青石结构稳定性的影响，其中M = Zn ²⁺，Ga ³⁺和Al ³⁺。所计算的形成焓与从熔体溶液量热法测量相关的堇青石相的实验结果一致。基态几何优化表明，对于中间成分（例如CsBaGa ₆ Ti ₁₈ O ₄₈），在A站点隧道中同时存在Cs和Ba并没有得到大力支持。但是，在储存含Cs废料的过程中产生的衰变热可能会克服Ba和Cs混合在高铁矿的隧道结构中的能量。重晶石结构适应Cs向Ba放射共生的能力对于废物的长期性能至关重要。首次，沿隧道方向（[001]区域轴）观察到Ga-锰铁矿组成以及中间的Al-钙铁矿组成的B-位有序。这些与成分有关的结构特征以及相关的形成焓对陶瓷废料形式的稳定性和辐射损伤耐受性具有重要意义。