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(x)Fe(2)Se(2). 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(0.8)Fe(2)Se(2) 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.

We report on muon-spin rotation and relaxation (µSR), electrical resistivity, magnetization and differential scanning calorimetry measurements performed on a high-quality single crystal of Cs0.8(FeSe0.98)2. Whereas our transport and magnetization data confirm the bulk character of the superconducting state below Tc = 29.6(2) K, the µSR data indicate that the system is magnetic below TN = 478.5(3) K, where a first-order transition occurs. The first-order character of the magnetic transition is confirmed by differential scanning calorimetry data. Taken all together, these data indicate in Cs0.8(FeSe0.98)2 a microscopic coexistence between the superconducting phase and a strong magnetic phase. The observed TN is the highest reported to date for a magnetic superconductor.

No abstract

Neutron and x-ray powder and single crystal synchrotron diffraction of CsyFe2−xSe2 show the presence of superstructure reflections with propagation vector k=[ 2 5 , 1 5 , 1] with respect to the average crystal structure I4/mmm (a = 4, c = 15Å). The propagation vector star corresponds to the 5 times bigger unit cell given by transformation A=2a+b, B= -a+2b, C= c. A solution for the atomic structure is found in the space groups P 42/n and I4/m with an ordered pattern of iron vacancies corresponding to the iron deficiency x = 0.29 and Cs stoichiometry y = 0.83. The superstructure satellites are more pronounced in the neutron diffraction patterns suggesting that they can have some magnetic contribution. We have sorted out possible symmetry adapted magnetic configurations and found that the presence of AFM ordering with the ordered magnetic moment of Fe with 2µB does not contradict to the experimental data. However, the solutions space is highly degenerate and we cannot choose a specific solution. Instead we propose possible magnetic configurations with the Fe magnetic moments in (ab)-plane or along c-axis. The superstructure is destroyed above Ts 500 K by a first-order-like transition.PACS numbers: 75.50. Ee, 75.25.-j, 61.05.C-, 74.90.+n arXiv:1102

We report on the synthesis of single crystals of BaFe(2)Se(3) and study their crystal and magnetic structures by means of synchrotron single-crystal x-ray and neutron powder diffraction. The crystal structure has orthorhombic symmetry and consists of double chains of FeSe(4) edge connected tetrahedra intercalated with barium. Below 240 K, long range spin-block checkerboard antiferromagnetic order is developed. The magnetic structure is similar to one observed in A(0.8)Fe(1.6)Se(2) (A = K, Rb or Cs) superconductors. The crystals exhibit a transition to the diamagnetic state with an onset transition temperature of T(c) ∼ 11 K. Though we observe FeSe as an impurity phase (<0.8% mass fraction) it is not likely that the diamagnetism is attributable to the FeSe superconductor, which has T(c) ≈ 8.5 K.