The iron vacancy order and the block antiferromagnetic order exist in the new iron 245 superconductors [1,2]. The appearance of the superconductivity crucially depends on the perfectness of the vacancy order [3]. The magnetic and vacancy orders in superconducting (Tl,Rb)2Fe4Se5 (245) single-crystals were investigated using high-pressure neutron diffraction technique [4]. Similar to the temperature effect, the block antiferromagnetic order gradually decreases upon increasing pressure while the Fe vacancy superstructural order remains intact before its precipitous drop at the critical pressure Pc =8.3 Gpa. Combining with previously determined Pc for superconductivity, our phase diagram under pressure reveals an intimate connection among the block antiferromagnetic order, the Fe vacancy order and superconductivity for the 245 superconductor. Similar connection between the perfectness of crystalline order and superconductivity has been previously demonstrated in our neutron scattering study on related Fe based superconductors [5,6].
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