Structure puzzle revealed for the high-pressure superconducting in La4Ni3O10−δ - Dr. Wenge Yang

New study from a team of scientists led by Dr. Wenge Yang from the Center for High Pressure Science and Technology Advance Research investigated the evolution of the crystal and electronic structures of the trilayer nickelate La₄Ni₃O_10-d under high-pressure and low-temperature conditions, using an in-situ high-pressure/low-temperature X-ray diffraction and X-ray absorption spectroscopy system. This study established, for the first time, the high-pressure–low-temperature phase diagram of La₄Ni₃O_10-d and revealed that pressure-induced crystal-structure distortion and electronic-structure reconstruction are key factors driving the superconducting transition in La₄Ni₃O_10-d . The results were published in JACS  under the title “Crystal and Electronic Structure Studies of La₄Ni₃O_10-d under High Pressure and Low-Temperature Conditions.”

In recent years, the discovery of high-temperature superconductivity in bilayer and trilayer nickelates with Ruddlesden–Popper structures has attracted tremendous global interest in the condensed-matter physics community, sparking a new wave of research on nickel-based superconductors. Considered the “sibling system” of cuprate superconductors, nickelates possess unique electronic configurations and layered crystal frameworks that may hold the key to understanding unconventional superconductivity. Therefore, precisely determining how their crystal structures and electronic states evolve under varying pressures and temperatures is essential for uncovering the intrinsic nature of superconductivity in nickelates.

By combining in-situ high-pressure/low-temperature X-ray diffraction with X-ray absorption spectroscopy, Yang’s team systematically investigated the crystal and electronic structure evolution of the trilayer nickelate La₄Ni₃O_10-d  under these conditions. Their approach enabled simultaneous, in-situ monitoring of both the crystal structure and electronic states within the superconducting-critical regime, thereby revealing the structural mechanism underlying the superconducting transition in trilayer La₄Ni₃O_10-d.

At ambient pressure, La₄Ni₃O_10-d adopts a monoclinic phase. As the pressure increases, it gradually transitions into a tetragonal phase; when the pressure exceeds approximately 48 GPa and the temperature is further lowered, La₄Ni₃O_10-d ultimately enters an orthorhombic phase. This result deviates significantly from the widely held expectation that a tetragonal structure corresponds to the superconducting state, indicating that the structural evolution of this system under high pressure is more complex than previously recognized and that its superconducting mechanism may involve structural degrees of freedom not identified before. Further analysis shows that below 48 GPa at low temperatures, the NiO₆ octahedra undergo pronounced distortion, leading to substantially enhanced hybridization between Ni 3d and O 2p orbitals. Such electronic-structure reconstruction is likely crucial to the superconducting transition in La₄Ni₃O_10-d. In addition, high pressure raises the overall Ni valence state, which remains stable at low temperatures, providing an electronic environment necessary for superconductivity.

Caption: The phase diagram of  La₄Ni₃O_10-d at high pressure and low temperature based on the in situ XRD results. The region below the purple dashed line represents the superconducting state.

This study provides the first systematic construction of the high-pressure–low-temperature phase diagram of La₄Ni₃O_10-d  from both crystal-structure and electronic-structure perspectives, and analyzes the structural mechanism behind its superconducting transition. The work offers valuable data that may aid in uncovering the underlying mechanism of high-temperature superconductivity in nickelates.

近年来，双层与三层Ruddlesden–Popper结构的镍酸盐高温超导的发现，引发了全球凝聚态物理领域的强烈关注，并开启了镍基超导研究的新热潮。镍酸盐被视为“铜酸盐超导体的兄弟体系”，其独特的电子结构和层状晶体框架，被认为可能是揭示非常规超导的核心机制。因此，要真正理解镍酸盐的超导本质，获得材料在不同压力与温度下精确的晶体结构和电子态演变是关键所在。杨文革团队结合原位高压-低温 X 射线衍射技术和X 射线吸收谱技术，系统研究了三层镍酸盐La₄Ni₃O_10−δ在高压-低温下的晶体及电子结构演变，实现了对其在超导临界区域内晶体结构和电子态的原位同步监测，从而揭示了三层La₄Ni₃O_10−δ超导转变的结构机制。