(Pt_1–xCu_x)₃Cu₂B and Pt₉Cu₃B₅, the first examples of copper platinum borides. Observation of superconductivity in a novel boron filled β-Mn-type compound

New ternary copper platinum borides have been synthesized by arc melting of pure elements followed by annealing at 600 °C. The structures have been studied by X-ray single crystal and powder diffraction. (Pt_1−xCu_x)₃Cu₂B (x=0.33) forms a B-filled β-Mn-type structure (space group P4₁32; a=0.6671(1) nm). Cu atoms are distributed preferentially on the 8c atom sites, whereas the 12d site is randomly occupied by Pt and Cu atoms (0.670(4) Pt±0.330(4) Cu). Boron is located in octahedral voids of the parent β-Mn-type structure. Pt₉Cu₃B₅ (space group P-62m; a=0.9048(3) nm, c=0.2908(1) nm) adopts the Pt₉Zn₃B_5–δ-type structure. It has a columnar architecture along the short translation vector exhibiting three kinds of [Pt₆] trigonal prism columns (boron filled, boron semi-filled and empty) and Pt channels with a pentagonal cross section filled with Cu atoms. The striking structural feature is a [Pt₆] cluster in form of an empty trigonal prism at the origin of the unit cell, which is surrounded by coupled [BPt₆] and [Pt₆] trigonal prisms, rotated perpendicularly to the central one. There is no B–B contact as well as Cu–B contact in the structure. The relationships of Pt₉Cu₃B₅ structure with the structure of Ti_1+xOs_2−xRuB₂ as well as with the structure families of metal sulfides and aluminides have been elucidated. (Pt_1–xCu_x)₃Cu₂B (x=0.3) (B-filled β-Mn-type structure) is a bulk superconductor with a transition temperature of about 2.06 K and an upper critical field μ₀H_C2(0)^WHH of 1.2 T, whereas no superconducting transition has been observed up to 0.3 K in Pt₉Cu₃B₅ (Pt₉Zn₃B_5–δ-type structure) from electrical resistivity measurements. - Highlights: • First two copper platinum borides, (Pt_0.67Cu_0.33)₃Cu₂B and Pt₉Cu₃B₅ were obtained. • (Pt_0.67Cu_0.33)₃Cu₂B forms a B-filled β-Mn-type structure. • Pt₉Cu₃B₅ adopts a Pt₉Zn₃B_5–δ-type structure. • Boron atoms exhibit octahedral and trigonal prismatic coordination. • (Pt_1–xCu_x)3Cu₂B (x=0.3) is a bulk superconductor with T_c 2.06 K