Simon Cassidy

Abstract:

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li_1–xFe_x(OH)Fe_1–ySe (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–xFe_x(OH) reservoir layer to fill vacancies in the selenide layer.

Abstract:

The effect of the isovalent substitution of Sr2+ by Ca 2+ on the structure and superconducting properties of Sr 2-xCaxVO3FeAs is described in the compositional range 0 ≤ x ≤ 0.5. SQUID magnetometry measurements reveal that after an initial increase in Tc, which is maximised at 29.5 K in Sr 1.95Ca0.05VO3FeAs, a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr 2-xCaxVO3FeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in Tc with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with Tc maximised in samples with regular FeAs4-tetrahedra. © 2014 Elsevier Inc.