Experimental results and first-principles calculation show that the stabilization of Z3-Fe(Pd,In d) 3 is induced by not a nanosize or a kinetic effect but a specific inter-element miscibility of In, which is miscible with Pd but immiscible with Fe. In this work, we create the Fe–Pd–In ternary alloy phase (11 < In/(Fe+Pd+In)< 14 at.%) with a Z3-based structure by using nanoparticulate precursor powders, because the uniformly mixing of Fe, Pd, and In at the nanoscale is crucial for the formation of Z3-type structure. This indicates the difficulty in producing other Fe–Pd–In ternary alloys with a small amount of In composition (e.g., 1.2 < In/(Fe+Pd+In) < 20 at.%).
However, according to previous reports, Fe–Pd–In ternary alloys have been rarely reported except for the In-rich ternary alloys (In/(Fe+Pd+In) >20 atomic percent (at.%)) 14, 15 and the In-poor L1 2-type FePd 3 ternary alloy phases (In/(Fe+Pd+In) <1.2 at.%) 16. Such a structural change would, in turn, alter the physical properties. Between the well-known L1 2-FePd 3 and L1 0-FePd phases in the Fe–Pd system, we chose the L1 2-FePd 3 as a target crystal phase because of its likeliness to change crystal structure from an isotropic L1 2 structure to a highly anisotropic structure when selectively locating the In atoms at specific sites. The element In was selected as a substitutional third element. To demonstrate this idea, we focused on the binary alloy of Pd and Fe, because many third elements can be miscible with Pd and immiscible with Fe in the binary phase diagram 8. This structural transformation caused by the interelement affinity of an interstitial N with Fe and Ni inspired us to analogically consider that the transformation from well-known to unexplored structures of a binary alloy could be achieved by the difference in the miscibility of a substitutional third element with the two kinds of elements in binary alloys. The Fe–Ni–N compound formed during this process possessed a crystal structure with a unit cell composed of one Fe–N layer sandwiched by two Ni layers in such a way as to reflect the better binding affinity of Fe with N, such as N 2 dissociation by Fe catalyst 13. Recently, the transformation from A1- to L1 0-FeNi structures was achieved by introducing and withdrawing interstitial N atoms 12. 1, we paid attention to the inter-element miscibility-the miscibility between two elements at an atomic level-in a binary-alloy phase diagram 8. To create an ordered alloy with such an atomic arrangement as the layered structures shown in Supplementary Fig. Therefore, a synthetic strategy of stabilizing a crystal structure is deserved.
For instance, in the case of the FePd 3 alloy system targeted in this research, only the thermodynamically stable L1 2 (Cu 3Au-type) phase has been experimentally obtained 5, 7, 8. Although this geometrical possibility means that there is still room to investigate crystal-structure control to realize a fine or drastic tuning of material properties, only thermodynamically stable phases at low and high temperatures have been commonly formed which follow the phase diagrams 8, except for the recently reported cases under extreme conditions, such as size 9, pressure 10, and lattice strain 11.
In a multicomponent alloy system, even with a specific atomic ratio, a number of geometrically available crystal structures can be devised 6, 7. The crystal structure is a crucial factor for determining the physical and chemical properties because the atomic arrangement dominates the atomic-orbital hybridization 1, 2, 3, 4, 5.
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