La₂₆Ge₁₉M₅O₅ (M = Ag, Cu): Rare-Earth Metal Suboxide Superconductors with [La₁₈O₅] Cluster Units

Herein we report two reduced rare-earth metal-based superconductors, La₂₆Ge₁₉M₅O₅ (M = Ag, Cu), that feature an unprecedented [La₁₈O₅] cluster composed of five oxygen-centered [La₆O] octahedra condensed through shared faces and capped with [Ge₄] butterfly rings. The structure, determined by single-crystal X-ray diffraction, crystallizes in a tetragonal space group (P4/nmm), with a = 15.508(2) Å and c = 11.238(2) Å. Resistivity and magnetic susceptibility measurements show onsets of superconductivity at T_c = 5.4 and 6.4 K for the Ag and Cu compounds, respectively. Applying high pressures, up to 1.3 GPa, results in increased superconducting transition temperatures (T_c = 6.8 K for Ag and 7.2 K for Cu compounds), with no sign of saturation.