ABSTRACT
The Ni1+/Ni2+ states of nickelates have the identical (3d(9)/3d(8)) electronic configuration as Cu2+/Cu3+ in the high temperature superconducting cuprates, and are expected to show interesting properties. An intriguing question is whether mimicking the electronic and structural features of cuprates would also result in superconductivity in nickelates. Here we report experimental evidence for a bulklike magnetic transition in La4Ni3O8 at 105 K. Density functional theory calculations relate the transition to a spin density wave nesting instability of the Fermi surface.
ABSTRACT
Single-phase Ln4Ni3O8 (Ln = La, Nd) nickelates were synthesized and their crystal structures were determined by Rietveld refinement of powder neutron diffraction data. The crystal structures of these mixed-valent Ni1+/Ni2+ phases belong to the T'-type and are built by intergrowth of LnO2 fluorite layers with triple NiO2 infinite-layer structural blocks. The major driving force of transformation of the LnO rock-salt block of the parent Ln4Ni3O10-delta Ruddlesden-Popper phases to the fluorite arrangement in the reduced Ln4Ni3O8 phases is attributed to internal structural stress. This transformation allows longer Ni-O bonds in Ln4Ni3O8 without overstretching of the Ln-O bonds, especially in the equatorial plane. The observed displacement of Ni atoms from the outer NiO2 planes toward the Ni atom of the central NiO2 plane in Ln4Ni3O8 is ascribed to large electrostatic repulsion from the fluorite part of the structure. X-ray absorption spectra near the K-edge of Ni suggest that the charge density on the nickel ion is similar for all members of the T'-type Lnn+1NinO2n+2 homologous series, which correlates with nearly constant Ni-O bond lengths observed in all the reduced nickelates. This suggests that the formal changes in the valence state of Ni affect the covalency of the Ni-O bond.
ABSTRACT
New mixed-valent, Ni1+/Ni2+, metastable nickelate, La3Ni2O6, was synthesized by low-temperature reduction of La3Ni2O7 with CaH2. The crystal structure of La3Ni2O6 (space group: I4/mmm, a = 3.9686(1) A and c = 19.3154(6) A) was determined from powder neutron diffraction data by Rietveld analysis. The structure can be described as an intergrowth of LaO2 fluorite and double infinite layer (LaNiO2)2 blocks and represents the n = 2 homologue of the T'-type series Lan+1NinO2n+2. Such double T'-type structural arrangement has never been observed before. The 3d9/3d8 electronic configuration of Ni1+/Ni2+ and the presence of NiO2 infinite layers resemble electronic and structural features of the superconducting cuprates. X-ray absorption spectroscopy supports the 1+/2+ oxidation state and planar coordination of Ni in agreement with the structure determination.
ABSTRACT
The D2 clathrate hydrate crystal structure was determined as a function of temperature and pressure by neutron diffraction for the first time. The hydrogen occupancy in the (32+X)H2.136H(2)O, x=0-16 clathrate can be reversibly varied by changing the large (hexakaidecahedral) cage occupancy between two and four molecules, while remaining single occupancy of the small (dodecahedral) cage. Above 130-160 K, the guest D2 molecules were found in the delocalized state, rotating around the centers of the cages. Decrease of temperature results in rotation freezing followed by a complete localization below 50 K.
