Chemical Torque in Y14Ag39.3Zn12.1: Unwinding the Disordered Triangles of the Gd14Ag51 Type

Abstract

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 CaPd_5+x structure type, containing triangles of atoms disordered over two orientations. Synthesis and structure solution of a Zn-substituted variant of this structure, Y₁₄Ag_39.3Zn_12.1, 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 CaPd_5+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.