Synthesis and topotactic transformation of nickelate single-crystals

In contrast to the superconducting ground state found in thin films of Sr- or Ca-doped infinite-layer nickelates RNiO₂ (R = Pr, Nd, La), polycrystalline powder samples show insulating behavior. Whether epitaxy to a substrate is required for superconductivity in infinite-layer nickelates remains an open question. So far, high-quality bulk single-crystalline samples were not available, although they might host properties distinct from powders. Recently, we therefore synthesized perovskite La_1-xCa_xNiO₃ single-crystals using flux growth under high-pressure (HP) via a Walker-type multi-anvil apparatus. The perovskite crystals were transformed to the infinite-layer phase by topotactic reduction and showed metallic properties, reminiscent of weakly doped thin films. Furthermore, we synthesized perovskite single-crystals with R = Pr and Nd using the HP method as well as optical floating zone growth, which were also reduced towards the infinite-layer phase. We will present details on the synthesis and reduction, together with scanning transmission electron microscopy, x-ray diffraction, magnetic susceptibility, electronic transport, and Raman spectroscopy measurements.