[论文解读] Design and synthesis of three-dimensional hybrid Ruddlesden-Popper nickelate single crystals

Feiyu Li, Ning Guo+9 more|arXiv (Cornell University)|Dec 13, 2023

Iron-based superconductors research参考文献 41被引用 4

一句话总结

本研究首次报道了具有交替La₂NiO₄（n=1）和La₃Ni₂O₇（n=2）层的三维杂化Ruddlesden-Popper镍酸盐单晶的合成，实现了新型A_{n+1}B_nX_{3n+1}A'_{m+1}B'_mX'_{3m+1}结构。XRD和STEM证实其为正交相Immm（编号71）结构，具有交替的单层和双层NiO₆堆积；电阻率测量显示在约140 K附近出现独特的绝缘体-金属转变，DFT计算进一步证实该转变源于电荷转移诱导的金属性行为。

ABSTRACT

Advancement of technologies relies on discovery of new materials with emerging physical properties that are determined by their crystal structures. Ruddlesden-Popper (R-P) phases with formula of $A_{n+1}$$B_n$$X_{3n+1}$ (n=1,2,3...) are among one of the most widely studied class of materials due to their electrical, optical, magnetic, thermal properties and their combined multifunctional properties(Ref.1-6). In R-P phases, intergrowth is well-known in the short range(Ref.7-9); however, no existing compounds have been reported to have different n mixed in bulk single crystals. Here we design a hybrid R-P nickelate $La_2NiO_4$$La_3Ni_2O_7$ by alternatively stacking bilayers, which is the active structural motif in the newly discovery high-$T_c$ superconductor $La_3Ni_2O_7$ and single layers of the antiferromagnetic insulator $La_2NiO_4$. We report the successful synthesis of $La_2NiO_4$$La_3Ni_2O_7$ single crystals, and X-ray diffraction and real-space imaging vis STEM show that the crystal structure consists of single layers and bilayers of $NiO_6$ octahedral stacking alternatively perpendicular to the ab plane, characterized by the orthorhombic Immm (No.71) space group. Resistivity measurements indicate a peculiar insulator-to-metal transition around 140 K on cooling. Correlated density functional theory calculations corroborate this finding, and reveal that the single layer becomes paramagnetic metallic due to charge transfer via LaO layers. The discovery of $La_2NiO_4$$La_3Ni_2O_7$ opens a door to access a completely new family of 3D hybrid R-P phases with the formula of $A_{n+1}$$B_n$$X_{3n+1}$$A'_{m+1}$$B'_m$$X'_{3m+1}$ which potentially host a plethora of emerging physical properties for various applications.

研究动机与目标

• 设计并合成一种具有单晶中交替n=1和n=2相的三维杂化Ruddlesden-Popper镍酸盐。
• 通过在单晶中实现不同n值的可控堆叠，克服块体R-P相中长期存在的互渗混合难题。
• 探索由杂化结构引发的新兴电子性质，特别是单层与双层NiO₆单元之间的相互作用。
• 建立一类新型三维杂化R-P相，具有多功能应用潜力。

提出的方法

• 通过交替La₂NiO₄（n=1）和La₃Ni₂O₇（n=2）层设计杂化R-P相，利用高温超导相和反铁磁绝缘相的结构特征。
• 在受控氧压下通过固相反应法合成，以稳定单相杂化结构。
• 采用实空间X射线衍射（XRD）和扫描透射电子显微镜（STEM）确认晶体结构和层堆叠。
• 通过温度依赖的电阻率测量探测电子相变。
• 采用密度泛函理论（DFT）计算分析电子结构和电荷转移机制。

实验结果

研究问题

• RQ1 能否以单晶形式合成具有交替n=1和n=2层的三维杂化Ruddlesden-Popper镍酸盐？
• RQ2 该杂化R-P相的晶体结构和对称性是什么？层堆叠如何有序排列？
• RQ3 该杂化结构是否表现出新颖的电子相变？若是，其起源为何？
• RQ4 层间电荷转移如何影响单层组分的电子态？

主要发现

• 成功合成具有交替单层和双层NiO₆单元的杂化R-P镍酸盐La₂NiO₄La₃Ni₂O₇单晶。
• XRD和STEM分析证实该晶体结构属于正交相Immm（编号71）空间群，且垂直于ab面的层堆叠具有高度有序性。
• 电阻率测量显示在冷却过程中约140 K附近出现明显的绝缘体-金属转变，表明存在关联电子相变。
• DFT计算表明，单层组分因通过中间LaO层的电荷转移而变为顺磁性并呈现金属性。
• 本工作确立了一类新型三维杂化R-P相，通式为A_{n+1}B_nX_{3n+1}A'_{m+1}B'_mX'_{3m+1}，为探索新颖量子现象开辟了新途径。