Interface Nuclei in the Y-Ag-Zn System: Three Chemical Pressure-Templated Phases with Lamellar Mg₂Zn₁₁- and...

The interface nucleus approach was recently presented as a framework for understanding and predicting the emergence of modular intermetallic phases, i.e., complex structures derived from the assembly of units from simpler parent structures. Here, we present the synthesis and crystal structures of three new modular intermetallics in the Y-Ag-Zn system that support this strategy: YAg_2.79Zn_2.80 (I), YAg_2.44Zn_3.17 (II), and YAg_2.71Zn_2.71 (III). Each of these structures is derived from an intergrowth of slabs of the Mg₂Zn₁₁ and CaPd₅₊ types, with the chief differences being in the thickness and degree of disorder within the CaPd₅₊-type domains. The merging of the parent structure domains is facilitated by their sharing a common geometrical unit, a double hexagonal antiprism. The use of this motif as an interface nucleus mirrors its role in another family of structures: an intergrowth series combining the CaCu₅ and Laves phase structure types, as in the PuNi₃-type phase YNi₃. However, there is a key difference between the two series. While in the CaCu₅/Laves intergrowths, the interface between the parent structures arises perpendicular to the interface nucleus's unique (hexagonal) axis, in the Mg₂Zn₁₁/CaPd₅₊-type intergrowths revealed here, the interfaces run parallel to this axis. Using CP analysis of the Mg₂Zn₁₁/CaPd₅-type parent structures, we trace this behavior to the different directions of high-CP compatibility of the interface nuclei in the Mg₂Zn₁₁/CaPd₅₊ and CaCu₅/Laves structure type pairs. In this way, the Y(Ag/Zn)₅₊ phases highlight the role that interface nuclei play in directing the domain morphologies of modular intermetallic phases.