北京高压科学研究中心刘浩哲团队报道了高压和低温条件下La₃Ni₂O₇超导态的结构。相关研究成果于2024年3月8日发表于国际顶尖学术期刊《美国化学会杂志》。

最近，在约15–40 GPa的条件下，在镍酸盐（La₃Ni₂O₇）中报道了一种Tc=80K的新超导体（Nature，6214932023），这是除铜酸盐之外的第二种非常规超导体，Tc高于液氮温度。

然而，该报告中绘制的相图主要基于低温和高压条件下的扩散测试，并且假设相应的X射线衍射（XRD）结果是在室温下进行的。这鼓励人们进行原位高压和低温同步加速器XRD实验，以确定哪个相是高Tc态的原因。除了从正交Amam结构到正交Fmmm结构的相变之外，当样品在40K下压缩到19GPa左右时，发现了具有I4/mmm空间群的四方相，其中超导性发生在La₃Ni₂O₇中。

基于这种四方结构的计算表明，接近费米能量的电子态主要由Ni原子的eg轨道（3d_z2和3d_x2–y2）主导，这些轨道位于氧八面体晶场中。分析了Tc与这种结构演化之间的相关性，特别是Ni–O八面体规则性和面内Ni–O–Ni键角。

该项工作为确定双层镍酸盐中最有可能产生超导电性的相提供了新的线索。

附：英文原文

Title: Structure Responsible for the Superconducting State in La3Ni2O7 at High-Pressure and Low-Temperature Conditions

Author: Luhong Wang, Yan Li, Sheng-Yi Xie, Fuyang Liu, Hualei Sun, Chaoxin Huang, Yang Gao, Takeshi Nakagawa, Boyang Fu, Bo Dong, Zhenhui Cao, Runze Yu, Saori I. Kawaguchi, Hirokazu Kadobayashi, Meng Wang, Changqing Jin, Ho-kwang Mao, Haozhe Liu

Issue&Volume: March 8, 2024

Abstract: Very recently, a new superconductor with Tc = 80 K has been reported in nickelate (La3Ni2O7) at around 15–40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I4/mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2–y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni–O octahedra regularity and the in-plane Ni–O–Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.

DOI: 10.1021/jacs.3c13094

Source: https://pubs.acs.org/doi/abs/10.1021/jacs.3c13094