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Like many complex intermetallic phases, the crystal structures of REZn_5+xcompounds (RE = lanthanide or Group 3 element) based on the EuMg₅type have gradually unfolded. The original reports described a complex hexagonal structure with an unusual combination of tetrahedrally close-packed regions and open spaces, as well as observations of superstructure reflections. More recently, we reinvestigated the structure of YZn₅, reclassifying it as the EuMg_5+x-type compound YZn_5+x(x ≃ 0.2), in which disordered channels run alongcthrough the spaces formerly considered open. In addition, DFT-chemical pressure (DFT-CP) analysis of ordered models of YZn_5+xhighlighted paths for communication between neighboring channels setting the stage for superstructure formation. Herein, the experimental elucidation of this effect is presented with the synthesis and structure determination of a modulated form of YZn_5+x. By slow-cooling samples of YZn_5+xfrom the annealing temperature, crystals were obtained that exhibit satellite reflections with the modulation wavevectorq= {1\over 3}a*+ {1\over 3}b*+ 0.3041c*. Structure solution and refinement using a (3+1)D model in superspace groupP31c({1\over 3}\,\!{1\over 3}σ₃)00sreveals incommensurate order in the structure's channels. Here, two Zn sites associated with the channels are present, each with discontinuous atomic domains that are slanted in thex₃x₄plane. Their slanting corresponds to adjustments along thecaxis for the presence or absence of close neighbors along that axis, while the occupation patterns of neighboring channels are shifted by {1\over 3} of the modulation period. These features follow earlier predictions from CP analysis, highlighting how this approach can be used predictively in search of new phenomena. 

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 CaCu₅type. 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, YZn₅exhibits tightened coordination environments around the cations. However, it also contains an unusually inhomogeneous atomic packing, particularly in channels running between the Y atoms alongc. Our structural reinvestigation of YZn₅reveals a disordered occupation pattern of Zn atoms within these channels, consistent with the EuMg_5+xtype, a disordered variant of the EuMg₅type. DFT‐CP analysis indicates that the transition from the CaCu₅type to the YZn_5+xstructure 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 ErZn₅corresponding to a √3×√3×3 supercell.