Long-Range Structural Order in a Hidden Phase of Ruddlesden-Popper Bilayer Nickelate La$_3$Ni$_2$O$_7$

TL;DR

This study uncovers a long-range ordered hidden phase in La$_3$Ni$_2$O$_7$, revealing new structural details and contrasting electrical properties between two Ruddlesden-Popper nickelate phases, advancing understanding of their structure-property relationships.

Contribution

It identifies and characterizes a previously unreported 1313-phase with long-range order and distinct structure, expanding knowledge of Ruddlesden-Popper nickelate phases.

Findings

The 2222-phase has a pseudo orthorhombic lattice with bilayer perovskite layers.

The 1313-phase exhibits semiconducting behavior and a C-centered orthorhombic structure.

The 1313-phase shows a long-range ordered arrangement of perovskite layers.

Abstract

The recent discovery of superconductivity in Ruddlesden-Popper bilayer nickelate, specifically La$_3$Ni$_2$O$_7$, has generated significant interest in the exploration of high-temperature superconductivity within this material family. In this study, we present the crystallographic and electrical resistivity properties of two distinct Ruddlesden-Popper nickelates: the bilayer La$_3$Ni$_2$O$_7$ (referred to as 2222-phase) and a previously uncharacterized phase, La$_3$Ni$_2$O$_7$ (1313-phase). The 2222-phase is characterized by a pseudo $F$-centered orthorhombic lattice, featuring bilayer perovskite [LaNiO$_3$] layers interspaced by rock salt [LaO] layers, forming a repeated ...2222... sequence. Intriguingly, the 1313-phase, which displays semiconducting properties, crystallizes in the $Cmmm$ space group and exhibits a pronounced predilection for a $C$-centered orthorhombic lattice. Within this structure, the perovskite [LaNiO$_3$] layers exhibit a distinctive long-range ordered arrangement, alternating between single- and trilayer configurations, resulting in a ...1313... sequence. This report contributes to novel insights into the crystallography and the structure-property relationship of Ruddlesden-Popper nickelates, paving the way for further investigations into their unique physical properties.