Chimie Métallurgique des Terres Rares, Institut de Chimie et des Matériaux Paris-Est, CNRS, Université Paris-Est Créteil, 2-8 rue Henri Dunant, 94320 Thiais Cedex, France.
Inorg Chem. 2012 Mar 5;51(5):3071-8. doi: 10.1021/ic202479y. Epub 2012 Feb 22.
The site occupancies of the Mo-Ni-Re σ phase have been studied as a function of the composition in the ternary homogeneity domain by both experimental measurements and calculations. Because of the possible simultaneous occupancy of three elements on the five sites of the crystal structure, the experimental determination of the site occupancies was achieved by using combined Rietveld refinement of X-ray and neutron diffraction data, whereas calculation of the site occupancies was carried out by using the density functional theory results of every ordered (i.e., 3(5) = 243) configuration appearing in the ternary system. A comparison of the experimental and calculation results showed good agreement, which suggests that the topologically close-packed phases, such as the σ phase, could be described by the Bragg-Williams approximation (i.e., ignoring the short-range-order contributions). On the other hand, the atomic distribution on different crystallographic sites of the Mo-Ni-Re σ phase was found to be governed by the atomic sizes. Ni, having the smallest atomic size, showed a preference for low-coordination-number (CN) sites, whereas Mo, being the largest in atomic size, preferred occupying high-CN sites. However, the preference of Re, having intermediate atomic size, varied depending on the composition, and a clear reversal in the preference of Re as a function of the composition was evidenced in both the calculated and experimental site-occupancy results.
已经通过实验测量和计算研究了 Mo-Ni-Re σ 相在三元同成分域中随组成变化的占位情况。由于晶体结构的五个位置上可能同时占据三个元素，因此通过结合 X 射线和中子衍射数据的 Rietveld 精修实验确定了占位情况，而通过使用出现在三元体系中的每个有序（即 3(5) = 243）构型的密度泛函理论结果计算了占位情况。实验结果和计算结果的比较表明，具有拓扑密堆积相的 σ 相可以用布拉格-威廉姆斯近似（即忽略短程有序贡献）来描述。另一方面，发现 Mo-Ni-Re σ 相不同晶格格位上的原子分布受原子尺寸控制。Ni 的原子尺寸最小，优先占据低配位数（CN）位，而 Mo 的原子尺寸最大，优先占据高配位数位。然而，具有中等原子尺寸的 Re 的偏好取决于组成，在计算和实验占位结果中都证明了 Re 偏好随组成的明显反转。
