Chemical Torque in Y₁₄Ag_39.3Zn_12.1: Unwinding the Disordered Triangles of the Gd₁₄Ag₅₁ Type

Elemental substitution is commonly used in materials chemistry to tune the Fermi energy of a compound or impart Chemical Pressure (CP) to atomic environments. In this Article, we illustrate how such substitution can also allow one to tune the relative orientations of geometrical units within a crystal structure, using the Gd14Ag51 structure type as a demonstration. A key feature of this structure type is the presence of hexagonal columns based on the CaPd5+x structure type, containing triangles of atoms disordered over two orientations. Synthesis and structure solution of a Zn-substituted variant of this structure, Y14Ag39.3Zn12.1, reveal that the incorporation of Zn atoms into these triangles leads to their rotation relative to their surroundings. Distinct triangle orientations are found depending on whether they are Ag3 or Zn3 units. A DFT-CP analysis of unsubstituted parent compound Y14Ag51 elucidates these observations. The Ag3 triangles lie within a large hexagon of Y atoms, with each triangle corner being able to gain close contacts with up to two of these Y atoms depending on the orientation. Obtaining an optimal alignment of the Ag3 triangles with respect to these interactions, however, is prevented by repulsion from other Ag atoms in the columns derived from the CaPd5+x type. Instead, the triangles are twisted toward the Y neighbors with larger negative CP features. The replacement of Ag atoms with smaller Zn atoms provides the opportunity to relieve these packing tensions, allowing the triangles to turn to a position that better optimizes their interactions with the surrounding Y atoms. These results point to simple guidelines for identifying, with CP analysis, rigid units within structures that may be manipulated through elemental substitution.