Synthesis of 4H-SrMnO<sub>3.0</sub> Nanoparticles
from a Molecular Precursor and Their Topotactic Reduction Pathway
Identified at Atomic Scale
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Abstract
Stoichiometric
4H-SrMnO<sub>3.0</sub> nanoparticles have been synthesized
from thermal decomposition of a new molecular heterometallic precursor
[SrMn(edta)(H<sub>2</sub>O)<sub>5</sub>]·<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 metal–organic 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 40–60 K temperature range. The origin of such magnetic
features is discussed
