Temperature-Dependent Structural Transition in Cu-Intercalated Trigonal CuYbSe ₂

Rare-earth delafossites, ARCh₂; A = alkali metal, R = rare-earth, Ch = chalcogen which consist of intercalated rare-earth metal dichalcogenides, host frustrated triangular lattices that are fertile ground for exotic phenomena. In most cases, the triangular rare-earth sublattice arises from <i>R</i>-3<i>m</i> (No. 166) structures with three layers of rare-earth metal dichalcogenide octahedra or <i>P</i>6₃/<i>mmc</i> (No. 194) structures with two such layers, analogous to those found in transition metal dichalcogenides. Substituting the alkali metal with Cu⁺ yields a distinct trigonal crystal symmetry<i>P</i>-3<i>m</i>1 (No. 164)in these structures. This symmetry change alters the coordination environment from ASe₆ octahedra in <i>R</i>-3<i>m</i> AYbSe₂ to CuSe₄ tetrahedra in CuYbSe₂, resulting in shortened rare-earth to rare-earth separations and significantly reduced interlayer distances. Using X-ray single-crystal diffraction, powder neutron diffraction, resistance, and specific heat measurements, a structural transition slightly below room temperature (258 K) is observed. The low-temperature structure is a lower-symmetry <i>I</i>2/<i>m</i> structure, accompanied by partial Cu-site vacancy ordering. The combination of Cu disorder and the triangular lattice geometry in CuYbSe₂ provides a promising platform for investigating frustrated magnetism and unconventional transport phenomena.