Synthesis and crystal growth of Cs_0.8(FeSe_0.98)₂: a new iron-based superconductor with T_c = 27 K

We report on the synthesis of large single crystals of a new FeSe layer superconductor Cs_0.8(FeSe_0.98)₂. X-ray powder diffraction, neutron powder diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs_0.8(FeSe_0.98)₂ with those of the recently discovered potassium intercalated system K_xFe₂Se₂. The new compound, Cs_0.8(FeSe_0.98)₂, shows a slightly lower superconducting transition temperature (T_c = 27.4 K) in comparison to 29.5 in (K_0.8(FeSe_0.98)₂). The volume of the crystal unit cell increases by replacing K by Cs—the c parameter grows from 14.1353(13) to 15.2846(11) Å. For the alkali metal intercalated layered compounds known so far, (K_0.8Fe₂Se₂ and Cs_0.8(FeSe_0.98)₂), the T_c dependence on the anion height (distance between Fe layers and Se layers) was found to be analogous to those reported for As-containing Fe superconductors and Fe(Se_{1 − x}Ch_x), where Ch = Te, S.