We report on the synthesis of large single crystals of a new FeSe-layer
superconductor Cs0.8(FeSe0.98)2. X-ray powder diffraction, neutron
powder-diffraction and magnetization measurements have been used to compare the
crystal structure and the magnetic properties of Cs0.8(FeSe0.98)2 with those of
the recently discovered potassium intercalated system KxFe2Se2. The new
compound Cs0.8(FeSe0.98)2 shows a slightly lower superconducting transition
temperature (Tc=27.4 K) in comparison to 29.5 in K0.8(FeSe0.98)2). The volume
of the crystal unit cell increases by replacing K by Cs - the c-parameter grows
from 14.1353(13) {\AA} to 15.2846(11) {\AA}. For the so far known alkali metal
intercalated layered compounds (K0.8Fe2Se2 and Cs0.8(FeSe0.98)2) the Tc
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(Se1-xChx), where Ch=Te, S.Comment: 8 pages, 4 figure
