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

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(1-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), (57)Fe Mössbauer spectroscopy, magnetization measurements, and transmission electron microscopy. Above 300 °C, ms-FeSe decomposes irreversibly to β-FeSe(1-x) and Fe(7)Se(8). 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-FeSe(1-x). The Mössbauer spectrum rules out interstitial iron atoms or additional phases. Magnetization data suggest canted antiferromagnetism below T(N) = 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.

Doping effects of Se vacancies in monolayer FeSe

“…While some studies [21] conclude that anion vacancies suppress the superconductivity, others [18,19,22] conclude that they are an essential ingredient. To resolve this debate the microscopic effect of the anion vacancies on the Fe-d carriers must be known.…”

In‐situ Electrochemical X‐ray Diffraction: A Rigorous Method to Navigate within Phase Diagrams Reveals β‐Fe1+xSe as Superconductor for All x

“…[17] But there are several reports which find that the b-Fe 1+x Se structure shows no superconductivity down to 2K. [12,15,16] Therefore,w e considered initial hints of another phase transition which might be responsible for b-Fe 1+x Se becoming non-superconducting. [12] Heating experiments:T oi nvestigate this further, presynthesised samples of b-Fe 1+x Se were prepared in 0.3 mm silica capillaries of which one was sealed under argon, one was sealed after exposing it to air and therefore containing afixed amount of oxygen, and one was left open.…”

“…Most prominently,t he existence of the tetragonal-to-orthorhombic transition at around 80 Kwas found to be critical for b-Fe 1+x Se to become superconducting at temperatures around 8K. [13,14] If this transition is not observed, one finds atetragonal-to-triclinic transition at 60 Ka nd no superconducting state down to 2.0 K. [15,16] Theonly difference that leads to this changed behaviour is that the latter samples were grown hydrothermally,w hile the superconducting samples resulted Figure 1. (a) Unit cell of the b-Fe 1+x Se structure (anti-PbO type) (b) Fe-Se phase diagraminthe vicinity of the 1:1composition.…”

Elektrochemische In‐situ‐Röntgenbeugung: Eine präzise Methode zur Navigation in Phasendiagrammen enthüllt β‐Fe1+xSe als Supraleiter für alle x

“…Dies stimmt mit ersten Hinweisen aus EELS‐Messungen überein . Im Allgemeinen können diese Ergebnisse nicht nur die “Sauerstoffvergiftung” des β‐Fe 1+ x Se erklären, sondern auch warum Proben aus Hydrothermalsynthesen keine Supraleitung zeigen . In Bezug auf Letztere spekulieren wir, dass der Sauerstoff, so nicht molekular im Wasser gelöst, durch eine Redoxreaktion oberhalb von 100 °C von β‐Fe 1+ x Se mit H 2 O unter Bildung von winzigen Mengen H 2 bereitgestellt wird [Gl.…”

“…Dies führt dazu, dass der supraleitende Übergang wieder beobachtet wird. Unsere Ergebnisse geben Hinweise zu der lange Zeit nicht erklärbare Tatsache, dass hydrothermal oder anderweitig wässrig synthetisiertes β‐Fe 1+ x Se keinen supraleitenden Übergang zeigt …”