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.
@article{arxiv.2502.01501,
title = {Damage of bilayer structure in La3Ni2O7-d induced by high pO2 annealing},
author = {Yulin Zhang and Cuiying Pei and Ning Guo and Longlong Fan and Mingxin Zhang and Lingzhen Wang and Gongting Zhang and Feiyu Li and Yunong Wang and Chao Ma and Wenyong Cheng and Shanpeng Wang and Qiang Zheng and Yanpeng Qi and Junjie Zhang},
journal= {arXiv preprint arXiv:2502.01501},
year = {2026}
}