Topochemical Reduction of YMnO₃ into a Composite Structure

Abstract

Topochemical modification methods for solids have shown great potential in generating metastable structures inaccessible through classical synthetic routes. Here, we present the enhanced topotactic reduction of the multiferroic compound YMnO. At moderate temperature in ammonia flow, the most reduced YMnO (δ = 0.5) phase could be stabilized. XRD, PND, and HREM results show that phase separation occurs into two intimately intergrown layered sublattices with nominal compositions [YMnO] and [YMnO] containing versatile Mn coordinations. The former sublattice shows original AA stacking between Mn layers, while AB stacking in the latter results from oxygen removal from the parent YMnO crystal structure.

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

“… the topochemical oxidation by exsolution of Fe 2 O 3 starting from Fe 2+ -based oxo-anion compounds, e.g., Fe 3 (PO 4 ) 2 →Fe 2 (PO 4 ) 2 , LiFe­(PO 4 ) → LiFe 1‑ x (PO 4 ), BaFe 2 (PO 4 ) 2 → BaFe 1.33 (PO 4 ) 2 . , It involves long Fe diffusion paths generally activated around ∼300 °C in the case of iron phosphates. the restacking of the cation-based layers concerned in this work. This process is rare but was recently reported during the reversible reduction/re-oxidation of the layered YMnO 2.5 into YMnO 3 , see Figure a,b. In this case, the change of coordination from tetrahedral Mn 2+ to TBP Mn 3+ is accompanied by a [BAB] to [CAB] restacking very similarly to the YbFe 2 O 4 oxidation, see Figure c,d.…”

“…See details about RMS and MEM conclusions in the Supporting Information, S4. Similar arguments have to be kept in mind, dealing with Yb off-centering in YbFe 2 O 4.5 . After incorporation of 0.5 oxygen, the Yb site conserves its flattened octahedral coordination (6 × 2.24 Å) PND while O int stands at its vertices (d­(Yb–O int ) = 2.89 Å) PND similarly to the 6 + 1 coordination of yttrium in YMnO 3 (apical Y–O distance of <2.33–2.43 Å>). , However, the FeO 5 parent TBP is strongly distorted due to the vicinity of O int. . One can derive from the MEM maps a general trend: for all atoms a huge increase of the spread of the MEM nuclear densities (drawn at the same spatial scale and same isosurface level on Figure ) is observed with regard to YbFe 2 O 4.0 .…”

“…After incorporation of 0.5 oxygen, the Yb site conserves its flattened octahedral coordination (6 × 2.24 Å) PND while O int stands at its vertices (d­(Yb–O int ) = 2.89 Å) PND similarly to the 6 + 1 coordination of yttrium in YMnO 3 (apical Y–O distance of <2.33–2.43 Å>). , However, the FeO 5 parent TBP is strongly distorted due to the vicinity of O int. .…”

“…For instance, besides the reduction of YMn 3+ O 3 into YMn 2/3+ O 3‑ x intermediates using hydride reactants, we have recently reported a reconstructive and reversible reduction into YMn 2+ O 2 , after ammonolysis treatment. The structural rearrangement is drastic, leading to an original disordered composite structure between ∞ [YMn 2+ O 2+ x ] (1–2 x )+ and ∞ [YMn 2+ O 3‑ x ] (1–2 x )– blocks with a mixture of various Mn 2+ O n coordinations.…”

Pure and Zr-doped YMnO3+δas a YSZ-compatible SOFC cathode: a combined computational and experimental approach

“…Transmission Electron Microscopy, in diffraction mode, was also used to characterize those samples, since this technique is well known to be very sensitive to oxygen stoichiometry deviations through the appearance or disappearance of superstructure reflections (see for instance 56 It is worth noting that additional oxygen in the P63/mmc SG was clearly evidenced with the sample prepared by Pechini technique, and not by solid state synthesis, probably evidencing the importance of the powder microstructure (grain size in particular) on the kinetics of oxygen absorption, making it difficult to observe in our first experiments 35 . Cooling rate and, in this case, characteristic diffusion length often determines at what temperature equilibration with the atmosphere is no longer retained and the oxygen content effectively 'freezes in'.…”