7 research outputs found
Chlorine Insertion Promoting Iron Reduction in BaāFe Hexagonal Perovskites: Effect on the Structural and Magnetic Properties
BaFeCl<sub>0.13(2)</sub>O<sub>2.48(2)</sub> has been synthesized and studied. A proper tuning of the synthetic
route has been designed to stabilize this compound as a single phase.
The thermal stability and evolution, along with the magnetic and structural
properties are reported here. The crystal structure has been refined
from neutron powder diffraction data, and it is of the type (hhchc)<sub>2</sub>-10H. It is stable up to a temperature of 900 Ā°C, where
the composition reads BaFeCl<sub>0.13(2)</sub>O<sub>2.34(2)</sub>.
The study by electron microscopy shows that the crystal structure
suffers no changes in the whole BaFeCl<sub>0.13(1)</sub>O<sub>3ā<i>y</i></sub> (2.34 ā¤ 3 ā <i>y</i> ā¤
2.48) compositional range. Refinement of the magnetic structure shows
that the Fe is antiferromagneticaly ordered, with the magnetic moment
parallel to the <i>ab</i> plane of the hexagonal structure.
At higher temperature, a nonreversible phase transition into a (hchc)-4H
structure type takes place with overall composition BaFeCl<sub>0.13(1)</sub>O<sub>2.26(1)</sub>. Microstructural characterization shows that,
in some crystals, this phase intergrows with a seemingly cubic related
phase. Differences between these two crystalline phases reside in
the chlorine content, which keeps constant through the phase transition
for the former and disappears for the latter
Direct Atomic Observation in Powdered 4H-Ba<sub>0.8</sub>Sr<sub>0.2</sub>Mn<sub>0.4</sub>Fe<sub>0.6</sub>O<sub>2.7</sub>
A new hexagonal polytype in the BaMn<sub>1ā<i>x</i></sub>Fe<sub><i>x</i></sub>O<sub>3āĪ“</sub> system has been stabilized. Powdered Ba<sub>0.8</sub>Sr<sub>0.2</sub>Mn<sup>IV</sup><sub>0.4</sub>Fe<sup>III</sup><sub>0.6</sub>O<sub>2.70</sub> crystallizes in the 4H hexagonal polytype (space group <i>P</i>6<sub>3</sub>/<i>mmc</i>) according to X-ray
diffraction. HAADF images and chemical maps with atomic resolution
have been obtained by combining Cs-corrected electron microscopy and
EELS spectroscopy. The structure is formed by dimers of face-sharing
octahedra linked by corners. EELS data show a random distribution
of the transition metals ions identified by Fe and Mn-L2,3 chemical
maps. A systematic difference in contrast observed in the OāK
signal mapping suggests that anion deficiency is randomly located
along the hexagonal layers in agreement with ND data. The magnetic
structure consists of ferromagnetic sheets with the magnetic moments
aligned along the <i>x</i>-axis and coupled antiferromagnetically
along the <i>c</i>-axis
Nickel-Doped Sodium Cobaltite 2D Nanomaterials: Synthesis and Electrocatalytic Properties
In
this work we report a synthetic pathway to two-dimensional nanostructures
of high oxidation state lamellar cobalt oxides with thicknesses of
only few atom layers, through the combined use of precipitation in
basic water at room temperature and gentle solid state topotactic
transformation at 120 Ā°C. The 2D nanomaterials are characterized
by X-ray diffraction, nitrogen porosimetry, scanning electron microscopy,
transmission electron microscopy and especially scanning transmission
electron microscopy coupled to energy dispersive X-ray analysis and
electron energy loss spectroscopy to assess the composition of the
nanosheets and the oxidation state of the transition metal species.
We show that the nanosheets preserve high oxidation states Co<sup>3+</sup> and Co<sup>4+</sup> of high interest for electrocatalysis
of the oxygen evolution reaction (OER). By combining high Co oxidation
state, surface-to-volume ratio and optimized nickel substitution,
the 2D nanomaterials produced in a simple way exhibit high OER electrocatalytic
activity and stability in alkaline aqueous electrolyte comparable
to standard materials obtained in harsh thermal conditions
Synthesis of 4H-SrMnO<sub>3.0</sub> Nanoparticles from a Molecular Precursor and Their Topotactic Reduction Pathway Identified at Atomic Scale
Stoichiometric
4H-SrMnO<sub>3.0</sub> nanoparticles have been synthesized
from thermal decomposition of a new molecular heterometallic precursor
[SrMnĀ(edta)Ā(H<sub>2</sub>O)<sub>5</sub>]Ā·<sup>3</sup>/<sub>2</sub>H<sub>2</sub>O whose crystal structure has been solved by single
crystal X-ray diffraction. From this precursor, highly homogeneous
4H-SrMnO<sub>3.0</sub> nanoparticles, with average particle size of
70 nm, are obtained. The agglomeration of these nanoparticles maintains
the sheet-assembling morphology of the metalāorganic compound.
Local structural information, provided by atomically resolved microscopy
techniques, shows that 4H-SrMnO<sub>3.0</sub> nanoparticles exhibit
the same general structural features as the bulk material, although
structural disorder, due to edge dislocations, is observed. The nanometric
particle size enables a topotactic reduction process at low temperature
stabilizing a metastable 4H-SrMnO<sub>2.82</sub> phase. The oxygen
deficiency is accommodated through extra cubic layers breaking the
...chch... 4H-sequence. These defect areas are Mn<sup>3+</sup> rich,
as evidenced by high energy resolution EELS data. Magnetic characterization
of nano-SrMnO<sub>3.0</sub> shows significant variations with respect
to the bulk material. Besides the dominant antiferromagnetic interactions,
a weak ferromagnetic contribution as well as exchange bias and a glassy-like
component are present. After the reduction process, the stabilization
of Mn<sup>3+</sup> in the 4H-structure gives rise to magnetic anomalies
in the 40ā60 K temperature range. The origin of such magnetic
features is discussed
Unknown Aspects of Self-Assembly of PbS Microscale Superstructures
A lot of interesting and sophisticated examples of nanoparticle (NP) self-assembly (SA) are known. From both fundamental and technological standpoints, this field requires advancements in three principle directions: (a) understanding the mechanism and driving forces of three-dimensional (3D) SA with both nano- and microlevels of organization; (b) understanding disassembly/deconstruction processes; and (c) finding synthetic methods of assembly into continuous superstructures without insulating barriers. From this perspective, we investigated the formation of well-known star-like PbS superstructures and found a number of previously unknown or overlooked aspects that can advance the knowledge of NP self-assembly in these three directions. The primary one is that the formation of large seemingly monocrystalline PbS superstructures with multiple levels of octahedral symmetry can be explained only by SA of small octahedral NPs. We found five distinct periods in the formation PbS hyperbranched stars: (1) nucleation of early PbS NPs with an average diameter of 31 nm; (2) assembly into 100ā500 nm octahedral mesocrystals; (3) assembly into 1000ā2500 nm hyperbranched stars; (4) assembly and ionic recrystallization into six-arm rods accompanied by disappearance of fine nanoscale structure; (5) deconstruction into rods and cuboctahedral NPs. The switches in assembly patterns between the periods occur due to variable dominance of pattern-determining forces that include van der Waals and electrostatic (chargeācharge, dipoleādipole, and polarization) interactions. The superstructure deconstruction is triggered by chemical changes in the deep eutectic solvent (DES) used as the media. PbS superstructures can be excellent models for fundamental studies of nanoscale organization and SA manufacturing of (opto)electronics and energy-harvesting devices which require organization of PbS components at multiple scales
SrMnO<sub>3</sub> Thermochromic Behavior Governed by Size-Dependent Structural Distortions
The influence of particle size in
both the structure and thermochromic
behavior of 4H-SrMnO<sub>3</sub> related perovskite is described.
Microsized SrMnO<sub>3</sub> suffers a structural transition from
hexagonal (<i>P</i>6<sub>3</sub>/<i>mmc</i>) to
orthorhombic (<i>C</i>222<sub>1</sub>) symmetry at temperature
close to 340 K. The orthorhombic distortion is due to the tilting
of the corner-sharing Mn<sub>2</sub>O<sub>9</sub> units building the
4H structural type. When temperature decreases, the distortion becomes
sharper reaching its maximal degree at ā¼125 K. These structural
changes promote the modification of the electronic structure of orthorhombic
SrMnO<sub>3</sub> phase originating the observed color change. nano-SrMnO<sub>3</sub> adopts the ideal 4H hexagonal structure at room temperature,
the orthorhombic distortion being only detected at temperature below
170 K. A decrease in the orthorhombic distortion degree, compared
to that observed in the microsample, may be the reason why a color
change is not observed at low temperature (77 K)
Critical Influence of Redox Pretreatments on the CO Oxidation Activity of BaFeO<sub>3āĪ“</sub> Perovskites: An in-Depth Atomic-Scale Analysis by Aberration-Corrected and in Situ Diffraction Techniques
A BaFeO<sub>3āĪ“</sub> (Ī“ ā 0.22) perovskite
was prepared by a solāgel method and essayed as a catalyst
in the CO oxidation reaction. BaFeO<sub>3āĪ“</sub> (0.22
ā¤ Ī“ ā¤ 0.42) depicts a 6H perovskite hexagonal
structural type with Fe in both III and IV oxidation states and oxygen
stoichiometry accommodated by a random distribution of anionic vacancies.
The perovskite with the highest oxygen content, BaFeO<sub>2.78</sub>, proved to be more active than its lanthanide-based counterparts,
LnFeO<sub>3</sub> (Ln = La, Sm, Nd). Removal of the lattice oxygen
detected in both temperature-programmed oxidation (TPO) and reduction
(TPR) experiments at around 500 K and which leads to the complete
reduction of Fe<sup>4+</sup> to Fe<sup>3+</sup>, i.e. to BeFeO<sub>2.5</sub>, significantly decreases the catalytic activity, especially
in the low-temperature range. The analysis of thermogravimetric experiments
performed under oxygen and of TPR studies run under CO clearly support
the involvement of lattice oxygen in the CO oxidation on these Ba-Fe
perovskites, even at the lowest temperatures. Atomically resolved
images and chemical maps obtained using different aberration-corrected
scanning transmission electron microscopy techniques, as well as some
in situ type experiments, have provided a clear picture of the accommodation
of oxygen nonstoichiometry in these materials. This atomic-scale view
has revealed details of both the cation and anion sublattices of the
different perovskites that have allowed us to identify the structural
origin of the oxygen species most likely responsible for the low-temperature
CO oxidation activity