Semimetallic Paramagnetic Nano-Bi₂Ir and Superconducting Ferromagnetic Nano-Bi₃Ni by Microwave-Assisted Synthesis and Room Temperature Pseudomorphosis

Uniform nanocrystals of the intermetallic compounds Bi₂Ir (diameter ≥ 50 nm) and Bi₃Ni (typical size 200x600 nm) were obtained by a microwave-assisted polyol process at 240 °C. The method was also applied to the spatially confined reaction environment in the microporous exo-template SBA-15 resulting in Bi₃Ni particles of about 6 nm. Non-crystalline bundles of parallel Bi₃Ni nanofibres that have an individual diameter of less than 1 nm were obtained by reductive pseudomorphosis of the subiodide Bi₁₂Ni₄I₃ at room temperature. Magnetic susceptibility measurements demonstrate coexistence of ferromagnetism and superconductivity in a single phase for the nanostructured Bi₃Ni materials. Curie temperature, coercive field, remnant magnetization, saturation moment, diamagnetic screening, and critical field vary with particle size. The crystal structure of Bi₂Ir was determined by Rietveld refinement of powder X-ray diffraction data. Bi₂Ir crystallizes in the monoclinic arsenopyrite type (space group P2₁/c), a superstructure of the markasite type, with a = 690.11(1), b = 678.85(1), c = 696.17(1) pm, and β = 116.454(1)°. In contrast to most of the other phases of this type, the Bi₂Ir is not a diamagnetic semiconductor but a weakly paramagnetic semimetal. Conductivity measurements down to 4 K and magnetization measurements in a field of μ₀H = 10 mT down to 1.8 K give no evidence for a transition into the superconducting state. Bonding analysis shows prevailing contribution of Bi–Bi interactions to the conduction, whereas Bi–Ir bonding is mostly covalent and localized.