Long-Range Structural Order in a Hidden Phase of Ruddlesden-Popper Bilayer Nickelate La 3 Ni 2 O 7 .

The recent discovery of superconductivity in the Ruddlesden-Popper bilayer nickelate, specifically La3 Ni2 O7 , 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 La3 Ni2 O7 (referred to as 2222-phase) and a previously uncharacterized phase, La3 Ni2 O7 (1313-phase). The 2222-phase is characterized by a pseudo F -centered orthorhombic lattice, featuring bilayer perovskite [LaNiO3 ] 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 [LaNiO3 ] 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.