Synthesis and physical properties of LaNiO$_2$ crystals

TL;DR

This study reports the successful synthesis of large LaNiO$_2$ crystals via topotactic reduction, and thoroughly characterizes their structural, magnetic, and electronic properties, comparing them with powders and thin films.

Contribution

It demonstrates a new method for producing large LaNiO$_2$ single crystals and provides comprehensive physical property analysis, advancing understanding of nickelate superconductors.

Findings

Large LaNiO$_2$ crystals were synthesized using direct contact reduction.

The physical properties of crystals were characterized and compared to powders and films.

Lattice parameters evolve under high pressure, revealing structural insights.

Abstract

Infinite-layer (IL) nickelates are an emerging family of superconductors whose similarities and differences to cuprate superconductors are under intense debate. To date, the IL phase of nickelates can only be reached via topotactic oxygen reduction of the perovskite phase, using H$_2$ gas or reducing agents such as CaH$_2$. While the topotactic reduction method has been widely employed on thin film and polycrystalline powder samples, the reduction of La$_{1-x}$Ca$_x$NiO$_3$ single-crystals with lateral dimensions up to 150 $\mu$m was achieved only recently, using an indirect contact method with CaH$_2$. Here we report the topotactic transformation of much larger LaNiO$_3$ crystals with lateral dimensions of more than one millimeter, via direct contact with CaH$_2$. We characterize the crystalline, magnetic, and electronic properties of the obtained IL LaNiO$_{2}$ crystals by powder and single-crystal x-ray diffraction (XRD), magnetometry, electrical transport, and x-ray photoelectron spectroscopy (XPS) measurements. The amount of incorporated topotactic hydrogen due to the reduction process is determined by a gas extraction method. In addition, we investigate the evolution of the lattice parameters under hydrostatic pressure up to 12 GPa, using high-resolution synchrotron XRD. Furthermore, we provide a direct comparison of several physical properties of the LaNiO$_{2}$ crystals to their powder and thin film counterparts.