We have examined structural ordering of a Cu3Au-type compound ScRh3Bx (x = 0-1) by electron diffraction and high-resolution transmission electron microscopy (HRTEM). The electron diffraction patterns reveal 1/2 1/2 1/2-type superlattice reflections in a B content range of 0.5 ≤ x ≤ 0.75, where an anomalous drop of hardness was observed. This behavior is quite similar in a CeRh3Bx phase. Conclusively, ordering of B atoms in Rh-octahedra followed by the appearance of the long-ranged modulated structure could lead to the anomalous behavior of hardness in ScRh3Bx. The observed HRTEM image of ScRh3B0.5 and ScRh3B0.75 taken with a diffraction pattern showing the superlattice reflections along the [0 1 1] direction shows a domain structure. The superlattice is due to vacancy ordering.
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