Department of Nuclear Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA.
Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996, USA.
Nat Mater. 2016 May;15(5):507-11. doi: 10.1038/nmat4581. Epub 2016 Feb 29.
There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales). In particular, compounds with the isometric pyrochlore structure, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization, fast ion conduction, and thermal barrier coatings.
人们越来越关注理解具有非常规有序结构的能量学（例如，在不同长度尺度上具有不均匀相关性的无序）。特别是，具有等轴钙钛矿结构 A2B2O7 的化合物在极端条件下可以采用无序的等轴萤石型结构（A，B）4O7。尽管无序过程非常重要，但对于局部有序对能量景观的作用，人们的理解仍然有限。我们使用中子总散射来表明，无序萤石（由组成/化学计量比内在诱导或由高能辐射产生的远离平衡条件引起）由重复的局部正交结构单元组成，具有准平移对称性，因此正交和等轴阵列以不同的长度尺度共存。我们还表明，等轴尖晶石中的反演也通过类似的过程发生。这一见解为理解对于 Pu 固定、快速离子传导和热障涂层等应用很重要的有序到无序转变提供了新的基础。
