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

    A bimetallic hydroxychalcogenide, BaZn2Se2(OH)2, was synthesized through hydrothermal pouch methods. The single crystal X‐ray diffraction and electron diffraction indicates that the phase crystallizes in the orthorhombic space groupPnmaand is composed of anionic layers [ZnSe3/3(OH)1/1]that are separated and charged balanced by Ba2+cations. The [ZnSe3/3(OH)1/1]layer comprises two unique Zn sites, which form interpenetrating zigzag chains with an in‐plane dipole moment and adopts a brownmillerite‐type structural motif. The adjacent layers contain tetrahedrally coordinated Zn chains of opposite handedness related by an inversion center, which cancel the microscopic dipoles to minimize the macroscopic electric polarization. The adoption of a brownmillerite structural motif in BaZn2Se2(OH)2can be rationalized by the distinct charge difference between Se2−and OHanions, which creates a sufficient dipole moment in the ZnSe3(OH) tetrahedra to allow the occurrence of twisted chains. FTIR spectroscopy confirms the existence of OHanions and DFT calculations indicate that BaZn2Se2(OH)2is a semiconductor with a direct band gap. This work expands the chemistry of the brownmillerite family from traditional homoanionic oxides to multianion hydroxychalcogenides, offering a new opportunity to explore tunable structural complexity for better design of functional materials.

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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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  4. Abstract

    Intermetallic phases have been known to exhibit a wide diversity since Pauling's seminal investigations into NaCd2in the 1920s that, along with Cd3Cu4and Mg2Al3, was shown by Samson to crystallize with a giant cubic cell containing >1000 atoms. The concept of structural plasticity – the notion that complex structures emerge from the release of internal stresses that would arise in simpler structures – has recently been used to account for one family of intermetallics, tracing the structures of Ca2Ag7, Ca14Cd51, CaPd5, and CaCd6to chemical pressure (CP) issues in the CaCu5type. Here, we extend the ideal of structural plasticity closer to the giant cells elucidated by Pauling and Samson through its application to a series of Mo−Fe−Cr Frank‐Kasper phases. We begin with a DFT‐CP analysis of the MgZn2‐type phase MoFe2, which serves as a parent structure toμ‐Mo6Fe7andχ‐Mo5Cr6Fe18. The analysis reveals negative CPs around the Mo atoms arising from collisions between the Fe atoms. Tighter Mo coordination is provided in theμ‐ orχ‐phases by substituting some of the Friauf polyhedra of MoFe2with eitherμ‐ andχ‐phase units, resulting in layers or blocks of Laves‐like connectivity. Sites preferences in theμ‐phase and the role of Cr substitution in theχ‐phase are explained through the dual lenses of CP and electronegativity. Parallels to the features of NaCd2hint that such giant‐unit‐celled intermetallics can represent striking manifestations of structural plasticity.

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