Carr, Scott V.Louca, DespinaSiewenie, JoanHuang, Q.Wang, AifengChen, XianhuiDai, Pengcheng2017-06-052017-06-052014Carr, Scott V., Louca, Despina, Siewenie, Joan, et al.. "Structure and composition of the superconducting phase in alkali iron selenide KyFe1.6+xSe2." <i>Physical Review B,</i> 89, no. 13 (2014) American Physical Society: https://doi.org/10.1103/PhysRevB.89.134509.https://hdl.handle.net/1911/94794We use neutron diffraction to study the temperature evolution of the average structure and local lattice distortions in insulating and superconducting potassium iron selenide KyFe1.6+xSe2. In the high temperature paramagnetic state, both materials have a single phase with a crystal structure similar to that of the BaFe2As2 family of iron pnictides. While the insulating KyFe1.6+xSe2 forms a √5×√5 iron vacancy ordered block antiferromagnetic (AF) structure at low temperature, the superconducting compounds spontaneously phase separate into an insulating part with √5×√5iron vacancy order and a superconducting phase with chemical composition of KzFe2Se2 and BaFe2As2 structure. Therefore, superconductivity in alkaline iron selenides arises from alkali deficient KzFe2Se2 in the matrix of the insulating block AF phase.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlehttps://doi.org/10.1103/PhysRevB.89.134509