Superconducting Lanthanum Nickel Oxides with Bilayered and Trilayered Crystal Structures

Abstract

In 2023, superconductivity in La$_3$Ni$_2$O$_7$ was discovered under high pressures above approximately 14 GPa. In addition to its high transition temperature ($T_{\mathrm{c}} \simeq 80$ K), the structural resemblance to high-$T_{\mathrm{c}}$ cuprates has strongly stimulated research, soon followed by the discovery of superconductivity in La$_4$Ni$_3$O$_{10}$. These compounds belong to the Ruddlesden--Popper phases, comprising double- and triple-layered NiO$_2$ square lattices separated by LaO rock-salt slabs. Research on these systems has rapidly developed along three major directions, as in other prominent families of superconductors such as the cuprates and iron arsenides: expanding the chemical variety of compounds, enhancing $T_{\mathrm{c}}$ through elemental substitution, and elucidating the superconducting mechanism. These challenges, being closely interconnected, continue to drive the field. The clarification of the pairing mechanism encounters a particular difficulty, since the key experiments must be performed under high pressures. This situation highlights the significance of developing nickel oxides that exhibit superconductivity at much lower pressures, ideally at ambient pressure, which would in turn broaden the scope of chemical tuning and detailed physical characterization. In this context, it is timely and meaningful to summarize the present state of knowledge. Here, we emphasize sample synthesis and characterization, which are already well established and often decisive for progress in unconventional superconductors, while providing a brief overview of the currently available electronic properties.

Figures (9)

• Figure 1: Crystal structures of La$_3$Ni$_2$O$_7$ (a) and La$_4$Ni$_3$O$_{10}$ (b), with orthorhombic $Amam$ and monoclinic $P2_1/a$ symmetries, respectively. Panel (c) shows a top view of the perovskite and rock-salt slabs. Green, red, and blue circles represent La, Ni, and O atoms, respectively. Solid and dashed black lines in panel $c$ indicate the actual unit cell of La$_3$Ni$_2$O$_7$ and a virtual unit cell assuming tetragonal $I4/mmm$ symmetry, respectively.
• Figure 2: (a) Average Ni--O--Ni bond angle in perovskite nickel oxides, and (b) average Ni--Ni distance together with $\sqrt{2}(r_R+r_O)$, plotted as functions of the ionic radius of the rare-earth ion for ninefold coordination estimated by Shannon ShannonActaCrystA1976.
• Figure 3: Local structure of La$_2$NiO$_{4.25}$DemourguesJSSC1993. Numbered circles represent La atoms at their respective sites, while blue and pink atoms denote excess and original oxygen atoms, respectively.
• Figure 4: Crystal structure of the newly discovered 1313 phase of lanthanum nickelate.
• Figure 5: XRD pattern ($a$), SEM (scanning electron microscopy) image ($b$), and EDX (energy-dispersive X-ray spectroscopy) mapping of La ($c$) and Ni ($d$) in the sample reduced by hydrogen gas after preliminary synthesis.