Detailed structural analysis and the role of oxygen deficiency in the Nickelate superconductor, La3Ni2O7-δ

The recent discovery of superconductivity in Ruddlesden Popper (R-P) structured lanthanum nickelates has revitalised research into high-temperature superconductivity. In particular, La3Ni2O7 exhibits superconductivity near 80 K under uniaxial pressures of 15 to 40 GPa. Concurrent studies have revealed evidence of a charge density wave (CDW) a possible precursor to superconductivity under both ambient and pressurised conditions. However, the crystallographic and magnetic structures of La3Ni2O7 remain debated, with reports of bilayer (Cmcm) and monolayer trilayer (Fmmm) stacking polymorphs. Using high-resolution TEM, we have confirmed that our high-quality single crystals, grown by the floating zone method, adopt the monolayer trilayer structure. Preliminary neutron diffraction measurements (WISH, RB2500049) were inconclusive regarding CDW formation. We therefore propose a detailed structural investigation of the monolayer trilayer La3Ni2O7 polymorph using the SXD instrument to elucidate the role of nickel oxygen layering and oxygen defects in stabilising the CDW. Measurements will be performed across the CDW temperature range to identify possible structural anomalies. This study will provide essential insights into the structural origins of superconductivity in nickelates, utilising rare, high-quality single crystals and the advanced capabilities of SXD for precise structural determination.

鲁德斯登-波珀（Ruddlesden-Popper，R-P）结构镍酸镧中近期发现的超导性，重新激活了高温超导领域的研究热潮。其中，La₃Ni₂O₇在15至40 GPa的单轴压力下，于约80 K附近表现出超导特性。同期开展的多项研究表明，在环境及加压条件下均存在电荷密度波（Charge Density Wave，CDW）的相关证据，其或为超导性的前驱现象。然而，La₃Ni₂O₇的晶体学与磁学结构仍存在争议，已有报道分别提出了双层（Cmcm）与单层三层（Fmmm）堆叠的多型体。本研究借助高分辨透射电子显微镜（High-Resolution Transmission Electron Microscopy，TEM），确认了通过浮区法（Floating Zone Method）制备的高质量单晶体采用单层三层堆叠结构。初步中子衍射（Neutron Diffraction）实验（WISH，RB2500049）未能明确电荷密度波的形成情况。因此，我们提议利用SXD衍射仪对单层三层型La₃Ni₂O₇多型体开展详细的结构研究，以阐明镍氧层排布与氧缺陷在稳定电荷密度波过程中的作用。实验将覆盖电荷密度波的特征温度区间，以识别可能存在的结构异常。本研究将依托稀缺的高质量单晶体，结合SXD衍射仪在精准结构测定方面的先进能力，为揭示镍酸盐超导性的结构起源提供关键认知。