Temperature-Dependent Structural Transition in Cu-Intercalated Trigonal CuYbSe₂

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 R-3m (No. 166) structures with three layers of rare-earth metal dichalcogenide octahedra or P6₃/mmc (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P-3m1 (No. 164)in these structures. This symmetry change alters the coordination environment from ASe₆ octahedra in R-3m 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 I2/m 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.