Room-Temperature Synthesis,
Hydrothermal Recrystallization,
and Properties of Metastable Stoichiometric FeSe
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Abstract
Room-temperature precipitation from aqueous solutions
yields the
hitherto unknown metastable stoichiometric iron selenide (ms-FeSe)
with tetragonal anti-PbO type structure. Samples with improved crystallinity
are obtained by diffusion-controlled precipitation or hydrothermal
recrystallization. The relations of ms-FeSe to superconducting β-FeSe<sub>1–<i>x</i></sub> and other neighbor phases of the
iron–selenium system are established by high-temperature X-ray
diffraction, DSC/TG/MS (differential scanning calorimetry/thermogravimetry/mass
spectroscopy), <sup>57</sup>Fe Mössbauer spectroscopy, magnetization
measurements, and transmission electron microscopy. Above 300 °C,
ms-FeSe decomposes irreversibly to β-FeSe<sub>1–<i>x</i></sub> and Fe<sub>7</sub>Se<sub>8</sub>. The structural
parameters of ms-FeSe (<i>P</i>4/<i>nmm</i>, <i>a</i> = 377.90(1) pm, <i>c</i> = 551.11(3) pm, <i>Z</i> = 2), obtained by Rietveld refinement, differ significantly
from literature data for β-FeSe<sub>1–<i>x</i></sub>. The Mössbauer spectrum rules out interstitial iron
atoms or additional phases. Magnetization data suggest canted antiferromagnetism
below <i>T</i><sub>N</sub> = 50 K. Stoichiometric non-superconducting
ms-FeSe can be regarded as the true “parent” compound
for the “11” iron-chalcogenide superconductors and may
serve as starting point for new chemical modifications