Investigations of the Mn–Ni–Sm ternary system by means of anomalous X-ray diffraction, X-ray absorption and DFT calculations

Various applications benefit from the use of compounds combining lanthanides and 3d metals because of their fundamental physicochemical properties. To tune compositions for dedicated applications and to overcome the criticality of raw materials or geopolitical tension, a better understanding of the binary, ternary and even quaternary systems is needed. The present work focuses on the ternary Mn-Ni-Sm system, which is particularly interesting for energy storage applications as it forms hydrogen absorbing intermetallic compounds. The fruitful combination of standard X-ray diffraction, anomalous X-ray diffraction close to the Mn 3d edge, X-ray absorption near edge structure, electron probe micro-analysis and first-principles calculations provides a better understanding of the physicochemical properties and phase equilibrium of this ternary system. In particular, the atomic substitution of Mn atoms in the Ni sites has been investigated for the stacking AB2/AB5 Sm2MnyNi7-y system. The Mn atoms substitute preferentially in the AB5 sub-units, whereas Sm vacancies are observed in the AB2 sub-units inducing an antagonist variation in cell volume units. In the SmMnyNi12-y system, the preferential localization of Mn atoms has been highlighted in site 8i for the SmMn4Ni8 compound.