Comprehensive Study of Oxygen Storage in YbFe2O4+x (x ≤ 0.5): Unprecedented Coexistence of FeOn Polyhedra in One Single Phase

Rational design, crystal structure, and frustrated magnetism of the Ge-containing YbFe2O4-type layered oxides In2Zn3-xCoxGeO8 (0 ≤ x ≤ 3)

YbFe₂O₄-type layered oxides have attracted tremendous interest because the unique crystal comprises two distinct geometrically frustrated triangular cation-sublattices. Herein, a series of YbFe₂O₄-type materials In₂Zn_3-xCo_xGeO₈ (0 ≤ x ≤ 3) were rationally designed and experimentally synthesized for the first time. The crystal structures of In₂Zn_3-xCo_xGeO₈ were investigated comprehensively by Rietveld refinements against high-resolution monochromatic Cu K_α1 XRD data. Zn²⁺, Co²⁺, and Ge⁴⁺ cations are distributed randomly on the [MO]₂ bilayer and possess a trigonal bipyramid (TBP) coordination geometry. Because Co²⁺ has an unpaired electron in the d_z2 orbital and a larger electronegativity than Zn²⁺, Co²⁺-to-Zn²⁺ equivalent substitution in In₂Zn_3-xCo_xGeO₈ results in more compact MO₅-TBPs, which is the origin of anisotropic lattice expansion and contraction along the a and c axes, respectively. The Co²⁺ moments in the [MO]₂ bilayer are strongly AFM coupled and geometrically frustrated, therefore resulting in a spin-glass magnetic transition at around T_g = 20 K for In₂ZnCo₂GeO₈, while a long-range AFM ordering is established for In₂Co₃GeO₈ with a Néel temperature of 53 K, attributed to the significantly enhanced AFM interactions and increased In³⁺/Co²⁺ anti-site disordering, as compared to those in In₂ZnCo₂GeO₈.

Modulated structure determination and ion transport mechanism of oxide-ion conductor CeNbO4+δ

CeNbO_4+δ, a family of oxygen hyperstoichiometry materials with varying oxygen content (CeNbO₄, CeNbO_4.08, CeNbO_4.25, CeNbO_4.33) that shows mixed electronic and oxide ionic conduction, has been known for four decades. However, the oxide ionic transport mechanism has remained unclear due to the unknown atomic structures of CeNbO_4.08 and CeNbO_4.33. Here, we report the complex (3 + 1)D incommensurately modulated structure of CeNbO_4.08, and the supercell structure of CeNbO_4.33 from single nanocrystals by using a three-dimensional electron diffraction technique. Two oxide ion migration events are identified in CeNbO_4.08 and CeNbO_4.25 by molecular dynamics simulations, which was a synergic-cooperation knock-on mechanism involving continuous breaking and reformation of Nb₂O₉ units. However, the excess oxygen in CeNbO_4.33 hardly migrates because of the high concentration and the ordered distribution of the excess oxide ions. The relationship between the structure and oxide ion migration for the whole series of CeNbO_4+δ compounds elucidated here provides a direction for the performance optimization of these compounds.