  We report the discovery of a novel form of Ruddlesden-Popper (RP) oxide,
which stands as the first example of long-range, coherent polymorphism in this
class of inorganic solids. Rather than the well-known, uniform stacking of
perovskite blocks ubiquitously found in RP phases, this newly discovered
polymorph of the bilayer RP phase $La_{3}Ni_{2}O_{7}$adopts a novel stacking
sequence in which single and trilayer blocks of $NiO_{6}$ octahedra alternate
in a 1313 sequence. Crystals of this new polymorph are described in space group
Cmmm, although we note evidence for a competing Imcm variant. Transport
measurements at ambient pressure reveal metallic character with evidence of a
charge density wave transition with onset at T = 134 K, which lies intermediate
between that of the standard 2222 polymorph of $La_{3}Ni_{2}O_{7}$ (space group
Amam) and the trilayer RP phase, $La_{4}Ni_{3}O_{10}$. The discovery of such
polymorphism could reverberate to the expansive range of science and
applications that rely on RP materials, particularly the recently reported
signatures of superconductivity with $T_{c}$ as high as 80 K above 14 GPa in
bilayer $La_{3}Ni_{2}O_{7}$.
