Polymorphism in Ruddlesden-Popper La_3Ni_2O_7: Discovery of a Hidden Phase with Distinctive Layer Stacking

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_3Ni_2O_7adopts 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_3Ni_2O_7 (space group Amam) and the trilayer RP phase, La_4Ni_3O_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_3Ni_2O_7.