Crystal structure of La-438

The quasi-2D nickelate La4Ni3O8 (La-438), consisting of trilayer networks of square planar Ni ions, is a member of the so-called T′ family, which is derived from the Ruddlesden–Popper (R-P) parent compound La4Ni3O10−x by removing two oxygen atoms and rearranging the rock salt layers to fluorite-type layers. Although previous studies on polycrystalline samples have identified a 105-K phase transition with a pronounced electronic and magnetic response but weak lattice character, no consensus on the origin of this transition has been reached. Here, we show using synchrotron X-ray diffraction on high-pO2 floating zone-grown single crystals that this transition is associated with a real space ordering of charge into a quasi-2D charge stripe ground state. The charge stripe superlattice propagation vector, q = (2/3, 0, 1), corresponds with that found in the related 1/3-hole doped single-layer R-P nickelate, La5/3Sr1/3NiO4 (LSNO-1/3; Ni2.33+), with orientation at 45° to the Ni-O bonds. The charge stripes in La-438 are weakly correlated along c to form a staggered ABAB stacking that reduces the Coulomb repulsion among the stripes. Surprisingly, however, we find that the charge stripes within each trilayer of La-438 are stacked in phase from one layer to the next, at odds with any simple Coulomb repulsion argument.

Junjie Zhang, Yu-Sheng Chen, D. Phelan, Hong Zheng, M. R. Norman, and J. F. Mitchell
Stacked charge stripes in the quasi-2D trilayer nickelate La4Ni3O8
Materials Science Division, Argonne National Laboratory, Argonne, IL 60439
ChemMatCARS, The University of Chicago, Argonne, IL 60439

PNAS, 2016, 113 (32), pp 8945-8950