Structure and composition of the superconducting phase in alkali iron selenide K$_y$Fe$_{1.6+x}$Se$_2$

We use neutron diffraction to study the temperature evolution of the average structure and local lattice distortions in insulating and superconducting potassium iron selenide K $_{y}$ Fe $_{1.6 + x}$ Se $_{2}$ . In the high temperature paramagnetic state, both materials have a single phase with crystal structure similar to that of the BaFe $_{2}$ As $_{2}$ family of iron pnictides. While the insulating K $_{y}$ Fe $_{1.6 + x}$ Se $_{2}$ forms a $\sqrt{5} \times \sqrt{5}$ iron vacancy ordered block antiferromagnetic (AF) structure at low-temperature, the superconducting compounds spontaneously phase separate into an insulating part with $\sqrt{5} \times \sqrt{5}$ iron vacancy order and a superconducting phase with chemical composition of K $_{z}$ Fe $_{2}$ Se $_{2}$ and BaFe $_{2}$ As $_{2}$ structure. Therefore, superconductivity in alkaline iron selenides arises from alkali deficient K $_{z}$ Fe $_{2}$ Se $_{2}$ in the matrix of the insulating block AF phase.

Structure and composition of the superconducting phase in alkali iron selenide K $_{y}$ Fe $_{1.6 + x}$ Se $_{2}$

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Structure and composition of the superconducting phase in alkali iron selenide K $_{y}$ Fe $_{1.6 + x}$ Se $_{2}$

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Structure and composition of the superconducting phase in alkali iron selenide Kyy​Fe1.6+x1.6+x​Se22​

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Structure and composition of the superconducting phase in alkali iron selenide Kyy​Fe1.6+x1.6+x​Se22​

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Structure and composition of the superconducting phase in alkali iron selenide K $_{y}$ Fe $_{1.6 + x}$ Se $_{2}$

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Structure and composition of the superconducting phase in alkali iron selenide K $_{y}$ Fe $_{1.6 + x}$ Se $_{2}$

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