Temperature-dependent Structural Evolution of Ruddlesden-Popper Bilayer Nickelate La$_3$Ni$_2$O$_7$

Abstract

A recent $J. Am. Chem. Soc.$ Article (DOI: 10.1021/jacs.3c13094) details a pressure-temperature ($P$-$T$) phase diagram for the Ruddlesden-Popper bilayer nickelate La$_3$Ni$_2$O$_7$ (LNO-2222) using synchrotron X-ray diffraction. This study identifies a phase transition from $Amam$ (#63) to $Fmmm$ (#69) within the temperature range of 104 K to 120 K under initial pressure and attributes the $I\rm{4/}$$mmm$ (#139) space group to the structure responsible for the superconductivity of LNO-2222. Herein, we examine the temperature-dependent structural evolution of LNO-2222 single crystals at ambient pressure. Contrary to symmetry increase and the established $Amam$-$Fmmm$ phase boundary, we observe an enhancement in the $Cmcm$ reflections as temperature decreases. This work not only establishes a benchmark method for single crystal structure studies of LNO-2222 using laboratory X-rays, but also enhances the understanding of the complex crystallographic behavior of this system, contributing insights to further experimental and theoretical explorations.