Synthesis and crystal growth of Cs sub(0.8)(FeSe sub(0.98)) sub(2): a new iron-based superconductor with T sub(c) = 27 K

We report on the synthesis of large single crystals of a new FeSe layer superconductor Cs sub(0.8)(FeSe sub(0.98)) sub(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 sub(0.8)(FeSe sub(0.98)) sub(2) with those of the recently discovered potassium intercalated system K sub(x)Fe sub(2)Se sub(2). The new compound, Cs sub(0.8)(FeSe sub(0.98)) sub(2), shows a slightly lower superconducting transition temperature (T sub(c) = 27.4 K) in comparison to 29.5 in (K sub(0.8)(FeSe sub(0.98)) sub(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) [Angstrom]. For the alkali metal intercalated layered compounds known so far, (K sub(0.8)Fe sub(2)Se sub( 2) and Cs sub(0.8)(FeSe sub(0.98)) sub(2)), the T sub(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 sub(1-x)Ch sub(x)), where Ch = Te, S.