Structure characterization of novel alluminides in the Nd-Re-Al system by electron crystallography methods

In the Al-rich A-Re-Al alloys (where A = lanthanide or actinide), several unique structure types of ternary alluminides crystalize. In the course of the Nd-Re-Al phase diagram investigation, the new Nd₂Re₃Al₁₅ phase was revealed. Despite the prolonged heat treatment, the studied alloy did not attain equilibrium state, and contained multiple intermetallic phases, resulting in severe peak overlapping in the powder X-ray diffractogram. Thus, structure analysis was performed using combination of several electron crystallography approaches. First part of this work includes an investigation of the major phase constituting the studied alloy. Using electron diffraction tomography (EDT), atomic model of a pseudo binary (Nd,Re)Al₁₂ phase was determined and found to be in perfect agreement with the reported earlier binary structure, solved from neutron data. Following the successful structure solution of the pseudo-binary phase, the new Nd₂Re₃Al₁₅ structure was analyzed using the same methodology. The unit cell geometry was determined as orthorhombic with a = 12.801 Å, b = 22.545 Å and c = 9.103 Å. The space group was evaluated as Cc2m and partial model was obtained. It was revealed that the new Nd₂Re₃Al₁₅ structure is related to A_7+xRe₁₂Al_61+y. Typical topological features, observed in many Al-rich intermetallic phases, were also revealed in the atomic model of the new phase. Moreover, these features were clearly observed on the high resolution transmission electron micrographs, providing indirect confirmation of the proposed partial model.