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, . . . , infinity) are among one of the most widely studied classes ofmaterials 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 La2NiO4 center dot La3Ni2O7 by alternatively stacking bilayers, which is the active structural motif in the high-T-c superconductor La3Ni2O7 and single layers of the antiferromagnetic insulator La2NiO4. We report the successful synthesis of La2NiO4 center dot La3Ni2O7 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 NiO6 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 La2NiO4 center dot La3Ni2O7 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) center dot A(m+1)' B-m ' X3m+1 ' (n not equal m), which potentially host a plethora of emerging physical properties for various applications.