Damage of bilayer structure in La3Ni2O7-d induced by high pO2 annealing

TL;DR

This study investigates how high oxygen pressure annealing damages the bilayer structure in La3Ni2O7-d, affecting its superconductivity, and suggests optimal annealing conditions to preserve the structure and enhance oxygen content.

Contribution

It provides a systematic analysis of the structural damage caused by high pO2 annealing in La3Ni2O7-d and links this damage to changes in superconducting properties.

Findings

High pO2 annealing causes damage to the bilayer structure.

Damage correlates with weakened superconductivity.

Near atmospheric oxygen pressure preserves the bilayer structure.

Abstract

The discovery of superconductivity with onset temperature of ~80 K in pressurized bilayer Ruddlesden-Popper La3Ni2O7-d has attracted much attention. Despite intense research, determination of the exact oxygen content and understanding of the relationship between superconductivity and oxygen content remain a big challenge. Here, we report a systematical study on the structure and physical properties of La3Ni2O7-d polycrystalline powders which were prepared using sol-gel method at ambient pressure and then annealed under various oxygen pressure. We found that high pO2 annealing with slow cooling results in a new phase, which can be modeled using the hybrid single-layer-trilayer La3Ni2O7 or the tetragonal bilayer La3Ni2O7. Scanning transmission electron microscopy (STEM) measurements revealed significant single layers and trilayers after high oxygen pressure annealing, evidencing damage of the bilayer structure. The superconducting transition under high pressure became weak for high pO2 annealed samples, which is consistent with the damage of the bilayer structure. Our results reveal that the bilayer structure is fragile and post-annealing under near atmosphere pressure of oxygen is suitable to maintain bilayer structure and increase oxygen content at the same time.