Mullens Bryce G, Marlton Frederick P, Saura-Múzquiz Matilde, Everett Michelle, Li Cheng, Manjon-Sanz Alicia M, Tucker Matthew G, Poineau Frederic, Louis-Jean James, Mukherjee Supratik, Mondal Subrata, Vaitheeswaran Ganapathy, Kennedy Brendan J
School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia.
Centre for Clean Energy Technology, School of Mathematical and Physical Sciences, Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia.
Phys Chem Chem Phys. 2024 Dec 4;26(47):29270-29282. doi: 10.1039/d4cp03707c.
A detailed variable temperature neutron total scattering study of the potential nuclear waste matrix TlTcO was conducted. The long-range average structure of TlTcO undergoes an orthorhombic to tetragonal 4/ phase transition below 600 K, consistent with previous synchrotron X-ray diffraction studies. However, several anomalies were observed in the Rietveld refinements to the neutron powder diffraction data, such as large atomic displacement parameters at low temperature and a shortening of the Tc-O bond distance upon heating. Modelling the short-range local structure of both the low- and high-temperature data required a lowering of symmetry to the monoclinic 2/ model due to the stereochemical activity of the Tl 6s lone pairs. Density functional theory calculations also verified this model to have a lower ground state energy than the corresponding long-range average structure. It is concluded that at low temperatures, the Tl 6s lone pairs are 'frozen' into the structure. Upon heating, the rigid TcO tetrahedra begin to rotate, as governed by the Γ and M modes. However, there is a disconnect between the two length scales, with the 6s lone pair electrons remaining stereochemically active on the local scale, as observed in the neutron pair distribution function fits. The orthorhombic to tetragonal 4/ phase transition is seemingly the result of a change in the correlation length of the Tl 6s lone pairs, leading to a larger unit cell volume due to their uncorrelated displacements.
对潜在核废料基体TlTcO进行了详细的变温中子全散射研究。TlTcO的长程平均结构在600 K以下经历从正交相到四方4/相的转变，这与之前的同步加速器X射线衍射研究一致。然而，在对中子粉末衍射数据的Rietveld精修中观察到了几个异常现象，例如低温下较大的原子位移参数以及加热时Tc - O键距缩短。由于Tl 6s孤对电子的立体化学活性，对低温和高温数据的短程局部结构进行建模需要将对称性降低到单斜2/模型。密度泛函理论计算也证实该模型的基态能量低于相应的长程平均结构。得出的结论是，在低温下，Tl 6s孤对电子“冻结”在结构中。加热时，刚性的TcO四面体开始旋转，由Γ和M模式控制。然而，两个长度尺度之间存在脱节，正如在中子对分布函数拟合中观察到的那样，6s孤对电子在局部尺度上保持立体化学活性。正交相到四方4/相的转变似乎是Tl 6s孤对电子相关长度变化的结果，由于它们不相关的位移导致晶胞体积增大。
