Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer

Abstract

The tetragonal phase of Fe 1+ Se (0.01 ≤  ≤ 0.04) with the anti-PbO-type crystal structure displays superconductivity (T c ~ 8.5 K) when  = 0.01 and the superconductivity is destroyed by additional interstitial Fe. 9 The compound bears close structural resemblance to LiFeAs 10 (both compounds contain FeQ 4 (Q = Se, As) tetrahedra that are highly compressed in the basal plane compared with other iron-based superconductors) and both compounds differ from the canonical iron-based superconducting system in that they superconduct when as close to stoichiometric as possible (i. The lithium/ammonia solutions were rapidly decolourised by Fe 1+ Se at -78 °C. This is consistent either with the classic method for decomposing the metastable solution of solvated electrons using a "rusty nail" as a catalyst for the formation of lithium amide and hydrogen, or it indicates. Donation of the solvated electrons from the alkali metal ammonia solution to empty bands in the solid with Li ions co-inserted to balance the charge in a reductive intercalation reaction. The product was a black powder with a much finer grain size than the parent Fe 1+ Se material. The products were extremely air sensitive. X-ray powder diffraction (XRPD) showed no evidence for the starting material or other products above the 5% level and the diffraction peaks were indexed on a body centred tetragonal unit cell with lattice parameters a = 3.8249(2) Å and c = 16.5266(9) Å at room temperature. The basal lattice parameter, a (= √2 × Fe-Fe distance) is 1.4% larger than the value of 3.7734(1) Å reported for Fe 1.01 Se. 9 The c lattice parameter of 16.5266 (9) Neutron powder diffraction (NPD) patterns were collected from samples synthesised using 0.5 moles of Li per mole of FeSe with either NH 3 or ND 3 as solvent. The XRPD patterns of these products were similar, but their NPD patterns were dramatically different (the intensities varied greatly between the two compounds because of the greatly different neutron scattering lengths for H (-3.74 fm) and D (+6.67 fm), and the H-containing material produced a characteristic incoherent background) proving that the samples contain H(D). A structural model was obtained from the deuterated sample by starting from the model suggested by the X-ray refinements with N in the site 8-coordinate by Se and computing Fourier difference maps to reveal the remaining nuclear scattering density. Refinements against data from the GEM diffractometer at room temperature and the HRPD diffractometer at 8 K on the same sample of deuterated material produced similar structural models at the two temperatures. The initial assumption of a formula (LiND 2 )Fe 2 Se 2 resulted in an apparently satisfactory fit to the low temperature HRPD data (which emphasises the short d-spacing data) using a model in which the D atoms were located on crystallographic positions (16m site: (x, x, z)) which refined freely to be about 1 Å from the N atom (2a site: (0, 0, 0)) and with the N-D bonds directed towards the selenide anions. In this initial model the...

New Superconducting Phase of Lix(C6H16N2)yFe2−zSe2 with Tc = 41 K Obtained through the Post-Annealing

“…Through the intercalation of potassium 7) and sodium 8) into FeSe, in fact, the interlayer spacing between neighboring Fe layers, d, has increased from 5.52 Å to 7.02 Å and 6.83 Å, so that T c has increased up to ~ 31 K and ~ 37 K, respectively, while T c of lithium-intercalated FeSe without increase in d has remained 8 K. 9) Further increase in d has been attained through the co-intercalation of alkali or alkaline-earth metals and ammonia, so that d and T c have increased to 7 -11 Å and 31 -46 K in A x (NH 3 ) y Fe 2-z Se 2 (A: alkali or alkaline-earth metals), respectively. 8,[10][11][12][13][14] Following ammonia, several kinds of organic molecules have also been reported to be intercalated into FeSe together with alkali or alkaline-earth metals. In pyridine-intercalated Li 16,17) Collecting these results of the FeSe-based intercalation superconductors, it has been found that T c tends to increase with increasing d and to be saturated at ~ 45 K for d ≥ 9 Å and that the carrier doping of intercalants into the conducting FeSe layer seems not to affect T c so much.…”

Correlation of superconductivity with crystal structure in (NH3)yCsxFeSe

“…Metal-doped FeSe materials can be synthesized using the high-temperature-annealing method [1,[5][6][7][8] and liquidammonia technique [2,3,[9][10][11][12][13][14], and its T c can be increased by increasing the FeSe plane spacing [2,4]. On the other hand, it was found that pressure application resulted in a high-T c phase with the T c reaching a value greater than 45 K [1,2].…”

“…Our previous study on (NH 3 ) y Na x FeSe exhibiting different T c 's, 32 and 46 K [3,12,17], suggested that a difference in the location of metal atoms in each phase resulted in a different FeSe plane spacing, leading to a different T c . This was suggested from the analogy of the FeSe plane spacing and T c of the high-T c (T c = 46 K [3,14]) phase in (NH 3 ) y Na x FeSe with those of (NH 3 ) y Li x FeSe (T c = 44 K [3,10]), while the FeSe plane spacing and T c of the low-T c phase (T c = 32K [12,17]) with those of K x FeSe (T c = 31 K [5]). The Rietveld refinement of the neutron diffraction of (NH 3 ) y Li x FeSe clarified that the Li occupied the 4c and 2b sites and the N of NH 3 occupied the 2a site [10].…”

Unified picture of the doping dependence of superconducting transition temperatures in alkali metal/ammonia intercalated FeSe

“…In an effort to investigate this latter effect, organic molecular complexes including alkalis were recently intercalated between the FeSe layers [12][13][14][15][16][17][18][19] 15 . The crystal structure of a stoichiometric version of these materials is shown in Fig.…”