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  1. Free, publicly-accessible full text available March 20, 2024
  2. Abstract

    Enumerating the potential stacking sequences of layers is a fundamental way to account for the structure diversity of solid state compounds. In many cases, these stacking variations represent polymorphs with only small energetic differences. Here, we examine a compound for which the preferred stacking pattern instead reveals key aspects about its chemical bonding: Pd5InAs. Its structure is based on the intergrowth of slabs of the AuCu3and PtHg2(or alternatively, fluorite) structure types. Two basic stacking arrangements are available to this compound, represented by the Pd5TlAs and HoCoGa5structure types. DFT total energy calculations reveal that the former outcompetes the latter by a staggering 0.65 eV/formula unit. Through a combination of DFT‐reversed approximation Molecular Orbital (DFT‐raMO) and DFT‐Chemical Pressure (DFT‐CP) analysis we trace this preference to two factors. First, with DFT‐raMO analysis, we derive a Zintl‐like bonding scheme of Pd5InAs. This scheme, along with the inspection of selected crystal orbitals, is then connected to preferred stacking through the coordination environments of the Pd atoms at the interface between the Pd−In and Pd−As layers. In the hypothetical HoCoGa5‐type and observed Pd5InAs‐type structures, different Pd coordination environments arise at the interfaces. The hypothetical structure features square planar PdIn2As2units, in each of which the same 4d‐orbital serves in the Pd sublattice's role as both Lewis acid (for interactions with the As) and Lewis base (for interactions with the In). In the observed structure, tetrahedral PdIn2As2units occur instead, so that these contradictory roles are distributed to separate 4d‐orbitals, leading to more effective bonding. DFT‐CP analysis illustrates that this driving force for the Pd5TlAs‐type arrangement is supplemented by a favorable alignment of the packing tensions in the parent structures. Altogether, the resulting picture demonstrates how the reaction of simple intermetallic structures to form intergrowths can be guided by recognizable chemical interactions.

     
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  3. 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 EuMg5+xtype, a disordered variant of the EuMg5type. DFT‐CP analysis indicates that the transition from the CaCu5type to the YZn5+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.

     
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