Room-Temperature Synthesis, Hydrothermal Recrystallization, and Properties of Metastable Stoichiometric FeSe

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-FeSe1–x 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), 57Fe Mössbauer spectroscopy, magnetization
measurements, and transmission electron microscopy. Above 300 °C,
ms-FeSe decomposes irreversibly to β-FeSe1–x and Fe7Se8. The structural
parameters of ms-FeSe (P4/nmm, a = 377.90(1) pm, c = 551.11(3) pm, Z = 2), obtained by Rietveld refinement, differ significantly
from literature data for β-FeSe1–x. The Mössbauer spectrum rules out interstitial iron
atoms or additional phases. Magnetization data suggest canted antiferromagnetism
below TN = 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.