  The recent discovery of superconductivity in Ruddlesden-Popper bilayer
nickelate, specifically La$_3$Ni$_2$O$_7$, has generated significant interest
in the exploration of high-temperature superconductivity within this material
family. In this study, we present the crystallographic and electrical
resistivity properties of two distinct Ruddlesden-Popper nickelates: the
bilayer La$_3$Ni$_2$O$_7$ (referred to as 2222-phase) and a previously
uncharacterized phase, La$_3$Ni$_2$O$_7$ (1313-phase). The 2222-phase is
characterized by a pseudo $F$-centered orthorhombic lattice, featuring bilayer
perovskite [LaNiO$_3$] layers interspaced by rock salt [LaO] layers, forming a
repeated ...2222... sequence. Intriguingly, the 1313-phase, which displays
semiconducting properties, crystallizes in the $Cmmm$ space group and exhibits
a pronounced predilection for a $C$-centered orthorhombic lattice. Within this
structure, the perovskite [LaNiO$_3$] layers exhibit a distinctive long-range
ordered arrangement, alternating between single- and trilayer configurations,
resulting in a ...1313... sequence. This report contributes to novel insights
into the crystallography and the structure-property relationship of
Ruddlesden-Popper nickelates, paving the way for further investigations into
their unique physical properties.
