Design and synthesis of three-dimensional hybrid Ruddlesden-Popper nickelate single crystals

The advancement of technologies relies on the discovery of new materials with emerging physical properties that are determined by their crystal structures. Ruddlesden-Popper (R-P) phases with a formula of $A_{n+1}{B}_n{X}_{3n+1}$ ($n=1,2,3,...,\infty$) are among one of the most widely studied classes of materials due to their electrical, optical, magnetic, and thermal properties as well as their combined multifunctional properties. In R-P phases, intergrowth is well known in the short range; however, no existing compounds have been reported to have different $n$ mixed in bulk single crystals. Here we design a hybrid R-P nickelate $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ by alternatively stacking bilayers, which is the active structural motif in the $\text{high-}{T}_{\mathrm{c}}$ superconductor $\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ and single layers of the antiferromagnetic insulator $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4$. We report the successful synthesis of $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ single crystals, and x-ray diffraction and real-space imaging via scanning transmission electron microscopy (STEM) show that the crystal structure consists of single layers and bilayers of $\mathrm{Ni}{\mathrm{O}}_6$ octahedral stacking alternatively perpendicular to the $ab$ plane, characterized by the orthorhombic Immm (No. 71) space group. Resistivity measurements indicate a metallic ground state and a peculiar resistivity maximum around 140 K. Density functional theory (DFT+$U$) calculations corroborate this finding and reveal that both the bilayer and the single layer are metallic and that the single layer becomes paramagnetic metallic due to the charge transfer via LaO layers. The discovery of $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ opens a door to access a family of three-dimensional hybrid R-P phases with the formula of $A_{n+1}{B}_n{X}_{3n+1}·A_{m+1}^{\prime }{B}_m^{\prime }{X}_{3m+1}^{\prime }$ ($n\ne m$), which potentially host a plethora of emerging physical properties for various applications.

Figure 1

Materials design strategy for hybrid $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ and calculated electronic structure. (a) Illustration of our design strategy to hybridize bilayer, the structural motif that is responsible for high-temperature superconductivity in $\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$, and single layer in the three-dimensional (3D) antiferromagnetic $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4$ in the long-range order. (b) Orbital projected weights of the bilayer $\mathrm{Ni}{\mathrm{O}}_2$ in the hybrid $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ crystal given by the density functional theory. (c) Orbital projected weights of the single $\mathrm{Ni}{\mathrm{O}}_2$ in the hybrid $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ crystal given by the density functional theory. (d) Schematic of the 3D body-centered orthorhombic Brillouin zone. The red lines correspond to the paths of the electronic bands in (b) and (c). (e) Energy levels of the unit with two $\mathrm{N}{\mathrm{i}}^{2.5+} \left(3d^{7.5}\right)$ in the bilayer under the crystal field. The $d_{z2}$ orbitals of two Ni cations in the neighboring layers form the bonding and antibonding states. (f) Energy levels of the unit with $\mathrm{N}{\mathrm{i}}^{2+} \left(3d^8\right)$ in the single layer under the crystal field. The dotted circle indicates a hole generated by charge transfer from $d_{z2}$ to $d_{x2-y2}$ orbitals.

Figure 2

Crystal structure of the single-layer and bilayer hybridized nickelate $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$. (a) A SEM image of an as-grown single crystal with dimensions of $110\times 80\times 50\mu {\mathrm{m}}^3$. (b) Reconstructed ($hk0$) planes from in-house x-ray single-crystal diffraction data collected at 296(2) K. (c) Reconstructed ($h0l$) planes from in-house x-ray single-crystal diffraction data collected at 296(2) K. (d) Structural model obtained from x-ray single-crystal diffraction. (e) Ball-and-stick drawings of the $\mathrm{Ni}{\mathrm{O}}_6$ octahedrons with bond distances and bond angles in the bilayer $\mathrm{Ni}{\mathrm{O}}_2$. (f) Ball-and-stick drawing of the $\mathrm{Ni}{\mathrm{O}}_6$ octahedrons with bond distances in the single-layer $\mathrm{Ni}{\mathrm{O}}_2$. (g) Rietveld refinement on powder diffraction data collected on pulverized single crystals obtained from flux growth. Structural model in (d) was used as a starting model.

Figure 3

Real-space imaging of the structure and physical properties of $\mathrm{L}{\mathrm{a}}_2\mathrm{N}\mathrm{i}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$. (a) A typical atomic-scale HAADF-STEM image in the projection of [110] with overlaid crystal structure model; the right panel is the line intensity profile for rows of all atomic columns. (b) EDS maps for La and Ni and mixed color map of them. (c) Magnetic susceptibility of as-grown single crystals. (d) Resistivity on warming under an external magnetic field of 0 and 5 T. Note, a single crystal is too small to put four leads on; polycrystalline pellets were used and treated at ${900}^{\circ }\mathrm{C}$ in air prior to making contacts.

Figure 4

Hybrid Ruddlesden-Popper (R-P) phases as a class of quantum materials that potentially host emerging physical properties. (a) Conventional R-P nickelates with the formula of $\mathrm{L}{\mathrm{a}}_{n+1}\mathrm{N}{\mathrm{i}}_n{\mathrm{O}}_{3n+1}$ ($n=1$,2,3…). Note $n$ are integers and only one $n$ exists in a single compound. (b) Hybrid R-P nickelate family with the formula of $\mathrm{L}{\mathrm{a}}_{n+1}\mathrm{N}{\mathrm{i}}_n{\mathrm{O}}_{3n+1}·\mathrm{L}{\mathrm{a}}_{m+1}\mathrm{N}{\mathrm{i}}_m{\mathrm{O}}_{3m+1}$ ($m,n=1$,2,3… and $m\ne n$). $\mathrm{L}{\mathrm{a}}_2\mathrm{Ni}{\mathrm{O}}_4·\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ is one member of this family with $n=1$ and $m=2$. (c) General R-P phases with the formula of $A_{n+1}{B}_n{X}_{3n+1}·A_{m+1}^{\prime }{B}_m^{\prime }{X}_{3m+1}^{\prime }$ ($m,n=1$,2,3…). In these compounds, $A$ and ${\mathrm{A}}^{\prime }$ are usually alkaline-earth- or rare-earth-metal ions (e.g., Ca, Sr, Ba, La-Lu) or organic groups [e.g., $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3, \mathrm{CH}{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2$]; $B$ and ${\mathrm{B}}^{\prime }$ are transition metal ions (e.g., Ti, V, Mn, Fe, Co, Ni, Nb, Zr, Ru, Sn, Pb); $X$ and ${\mathrm{X}}^{\prime }$ are anions (e.g., O, Cl, Br, I).