Soft Chemical Control of Superconductivity in Lithium Iron Selenide Hydroxides Li_1–xFe_x(OH)Fe_1–ySe

Abstract

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se (x ∼ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li(1-x)Fe(x)(OH) reservoir layer to fill vacancies in the selenide layer.

Y89 NMR observation of ferromagnetic and antiferromagnetic spin fluctuations in the collapsed tetragonal phase of YFe2 (Ge,Si) Srpcic1, Jeglič2, Felner3 et al. 2017Phys. Rev. B6351View full textAdd to dashboardCite

“…4a). To explain the presence of localized paramagnetic impurities we refer here to a common feature frequently encountered also in iron-pnictide and iron-chalcogenide samples [27,47,48] Curie-like dependence is anticipated on the nearest neighboring 89 Y sites in agreement with the experiment. We stress that the presence of such localized moments can provide a very efficient channel for magnetic impurity pair-breaking effects thus explaining the large variation of T c 's in different YFe 2 Ge 2 samples [9,[12][13][14].…”

Magnetic interactions in iron superconductors: A review

“…Ferromagnetic order is much less usual than antiferromagnetism in Fe superconductors but it has been observed in a few cases [15-19, 23, 24, 269, 270]. [(Li 1−x Fe x )OH]FeSe based [15][16][17][18][19] [23,24]. This variety of magnetic patterns indicates the magnetic order in these systems is soft, namely, it can be affected by small changes in parameters.…”

Nontrivial Role of Interlayer Cation States in Iron-Based Superconductors

“…Although these models usually reproduce the Fermi surface very well, they do not reflect the physical instabilities of the actual materials because they neglect the interaction with the spacer between the FeX layers. Even though bulk FeSe does not contain spacer layers, our arguments may be relevant for intercalates [31][32][33], alkali-dosed thick films [34] and FeSe monolayers on SrTiO 3 [3] Materials and Methods. Fig.…”