  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.
