Soft chemical control of superconductivity in lithium iron selenide hydroxides Li1-xFex(OH)Fe1-ySe

TL;DR

This study demonstrates that controlling iron vacancy concentration and oxidation state in lithium iron selenide hydroxides via hydrothermal synthesis enables tuning of superconductivity up to 40 K.

Contribution

It introduces a method to control superconductivity in lithium iron selenide hydroxides through chemical manipulation of iron vacancies and oxidation states.

Findings

Superconductivity occurs at up to 40 K in samples with low iron vacancy concentration.

Reducing iron oxidation state below +2 promotes superconductivity.

Post-synthetic lithiation enhances superconducting temperature above 40 K.

Abstract

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li1-xFex(OH)Fe1-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 synthesised 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 emphasised by the demonstration that reductive post-synthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li1-xFex(OH) reservoir layer to fill vacancies in the selenide layer