Local Stability to Periodicity in the EuMg_5+x Type: Chemical Pressure, Disordered Channels, and Predicted Superstructure in YZn_5.225

Abstract

A central theme in the structural chemistry of intermetallic phases is that complex structures can be derived from variations on simpler ones. This is vividly demonstrated by the variety of structure types that can be connected to chemical pressure (CP)‐driven transformations of the simple CaCu5 type. In this Article, we investigate an intriguing addition to this family: the EuMg5‐type intermetallics, as exemplified by YZn5. As expected from the large negative CPs around the cations in CaCu5‐type structures, YZn5 exhibits tightened coordination environments around the cations. However, it also contains an unusually inhomogeneous atomic packing, particularly in channels running between the Y atoms along c. Our structural reinvestigation of YZn5 reveals a disordered occupation pattern of Zn atoms within these channels, consistent with the EuMg5+x type, a disordered variant of the EuMg5 type. DFT‐CP analysis indicates that the transition from the CaCu5 type to the YZn5+x structure indeed creates more compact Y environments, but strong tensions remain within the Zn sublattice. These include CP features on the channel walls that provide a mechanism for the communication of structural information between the channels and favorable cooperation in their occupation patterns. Based on these results, a structural model is proposed that explains an earlier observation of superstructure reflections in the diffraction patterns of ErZn5 corresponding to a √3×√3×3 supercell.

Self-Consistent Chemical Pressure Analysis: Resolving Atomic Packing Effects through the Iterative Partitioning of Space and Energy

“…This map is then interpreted and visualized in terms of interatomic pressures that arise from atomic packing frustrations. With this approach, a range of phenomena have been elucidated or predicted, including superstructures, , intergrowth structures, − incommensurate modulations, − elemental site preferences, ,− high pressure phases, , and chemical bonding interactions and bond formation. − …”

Interface Nuclei in the Y–Ag–Zn System: Three Chemical Pressure-Templated Phases with Lamellar Mg2Zn11- and CaPd5+x-Type Domains

“…The complex structures reported for the Zn- and Ag-rich sides of the Y–Ag and Y–Zn binary systems − inspired us to explore the potential for new structural chemistry in the Y–Ag–Zn ternary system. In our syntheses, we prepared pressed pellets of the elements under Ar, sealed them in evacuated fused silica tubes, subjected them to a preannealing step (24 h at 400 °C), and then annealed them at 700 °C for 1 week.…”

“…The architecture of these channels is not new. Analogous channels based on the same arrays of tricapped trigonal prisms and flattened octahedra are prominent in the EuMg 5 structure type, as well as its disordered (EuMg 5+ x -type) ,, and modulated (YZn 5+ x -type) variants. In the Y­(Ag/Zn) 5+ x compounds, we have an opportunity to see how elemental substitution on the channel walls can influence the occupation pattern.…”

“…This suppression of the triangular face occupation by high Ag content is understandable from our earlier CP analysis of the analogous channels in YZn 5+x . 31 Zn channel atoms placed near the triangular faces are subject to positive CPs from the triangle vertices, indicating that the atoms at these positions experience a tight fit. As Ag atoms are larger than Zn ones, this tension is expected to be higher in the Ag-rich regions of the channel walls.…”

“…During our synthetic investigations of the Y–Ag–Zn system, we encountered a series of new compounds relevant to this strategy. Our synthetic exploration here was originally inspired by the structural intricacy of transition-metal-rich Y–Ag and Y–Zn phases, such as the incommensurately modulated and disordered forms of YZn 5+ x − and the Gd 14 Ag 51 -type phase Y 14 Ag 51 . − The region of the ternary composition space near the Ag–Zn edge then seemed rich in new possibilities. As we will describe below, our solid-state reactions yielded new phases with compositions near Y­(Ag/Zn) 5+ x with complex orthorhombic crystal structures, featuring superstructuring or disorder.…”