A wide variety of compositions adopt the isometric spinel structure (AB₂O₄), in which the atomic-scale ordering is conventionally described according to only three structural degrees of freedom. One, the inversion parameter, is traditionally defined as the degree of cation exchange between the A- and B-sites. This exchange, a measure of intrinsic disorder, is fundamental to understanding the variation in the physical properties of different spinel compositions. Based on neutron total scattering experiments, we have determined that the local structure of Mg_1–xNi_xAl₂O₄ spinel cannot be understood as simply being due to cation disorder. Rather, cation inversion creates a local tetragonal symmetry that extends over sub-nanometer domains. Consequently, the simple spinel structure is more complicated than previously thought, as more than three parameters are needed to fully describe the structure. This new insight provides a framework by which the behavior of spinel can be more accurately modeled under the extreme environments important for many geophysics and energy-related applications, including prediction of deep seismic activity and immobilization of nuclear waste in oxides.

中文翻译：

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Mg _{1– x} Ni _x Al ₂ O ₄尖晶石的反转：局部结构的新见解

各种各样的成分采用等轴尖晶石结构（AB ₂ O ₄），其中通常仅根据三个结构自由度描述原子级有序。一个参数，即反演参数，传统上定义为A位和B位之间的阳离子交换程度。这种交换是内在无序性的一种度量，对于理解不同尖晶石成分的物理特性的变化至关重要。根据中子全散射实验，我们确定了Mg _{1– x} Ni _x Al ₂ O ₄的局部结构尖晶石不能被理解为仅仅是由于阳离子紊乱引起的。相反，阳离子反演会在亚纳米域上产生局部四边形对称性。因此，简单的尖晶石结构比以前认为的要复杂，因为要完整描述该结构需要三个以上的参数。这一新的见解提供了一个框架，通过该框架可以在对许多地球物理学和能源相关应用重要的极端环境下，更准确地对尖晶石的行为进行建模，包括预测深部地震活动和将核废料固定在氧化物中。