Phase co-existence and structured diffuse scattering seen by X-ray 3D mapping of reciprocal space for Cs0.8Fe1.6Se2

AxFe2-ySe2 (A=K, Rb, Cs) superconductors may show a complex mixture of structural phases even in a single crystal form. A full sphere of the diffraction intensity has been collected, with the help of synchrotron radiation, for Cs0.8Fe1.6Se2 . In addition to the expected pattern for the tetragonal phase with ordered Fe vacancies, a diffuse scattering from Cs occupational disorder has been observed, together with an extra Bragg contribution from a minor phase. The minor phase, in agreement with previous findings, is compressed in the tetragonal a-b plane and expanded along the c-direction; a set of modulated Bragg rods evidences a planar disorder. Fourfold splitting of the rods as well as the main Bragg peaks for L\neq0 imply that symmetry of the minor phase is not higher than monoclinic. The monoclinic distortion was estimated to be 90.7 degrees. Structured diffuse scattering, observed on top of the Bragg component, relates to the major phase and is attributed to a correlated distribution of Cs ions. Elastic nature of the observed scattering was confirmed by additional inelastic X-ray scattering experiment. Diffuse scattering forms 3D objects that have topological similarity with what one would expect, under assumptions discussed in the text, for a Fermi surface nesting.Comment: 17 pages, 6 figure

Temperature and pressure evolution of the crystal structure of Ax(Fe1-ySe)2 (A = Cs, Rb, K) studied by synchrotron powder diffraction

Temperature-dependent synchrotron powder diffraction on Cs0.83(Fe0.86Se)2 revealed first order I4/m to I4/mmm structural transformation around 216{\deg}C associated with the disorder of the Fe vacancies. Irreversibility observed during the transition is likely associated with a mobility of intercalated Alkali atoms. Pressure-dependent synchrotron powder diffraction on Cs0.83(Fe1-ySe)2, Rb0.85(Fe1-ySe)2 and K0.8(Fe1-ySe)2 (y ~ 0.14) indicated that the I4/m superstructure reflections are present up to pressures of 120 kbar. This may indicate that the ordering of the Fe vacancies is present in both superconducting and non-superconductive states.Comment: 11 pages, 5 figures, 1 tabl

Conformity of spin fluctuations in alkali-metal iron selenide superconductors inferred from the observation of a magnetic resonant mode in K(x)Fe(2-y)Se(2)

Spin excitations stemming from the metallic phase of the ferrochalcogenide superconductor K(0.77)Fe(1.85)Se(2) (T_c=32 K) were mapped out in the ab plane by means of the time-of-flight neutron spectroscopy. We observed a magnetic resonant mode at Q_res=(1/2 1/4), whose energy and in-plane shape are almost identical to those in the related compound Rb(0.8)Fe(1.6)Se(2). This lets us infer that there is a unique underlying electronic structure of the bulk superconducting phase K(x)Fe(2)Se(2), which is universal for all alkali-metal iron selenide superconductors and stands in contrast to the doping-tunable phase diagrams of the related iron pnictides. Furthermore, the spectral weight of the resonance on the absolute scale, normalized to the volume fraction of the superconducting phase, is several times larger than in optimally doped BaFe(2-x)Co(x)As(2). We also found no evidence for any additional low-energy branches of spin excitations away from Q_res. Our results provide new input for theoretical models of the spin dynamics in iron based superconductors

Synthesis, crystal and magnetic structure of iron selenide BaFe2Se3 with possible superconductivity at Tc=11K

We report on synthesis of single crystals of BaFe2Se3 and study of 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 FeSe4 edge connected tetrahedra intercalated by barium. Below 240 K long range block-spin checkerboard antiferromagnetic (AFM) order is developed. The magnetic structure is similar to one observed in A0.8Fe1.6Se2 (A=K, Rb or Cs) superconductors. The crystals exhibit a transition to the diamagnetic state with an onset transition temperature of Tc ~11 K. Though we observe FeSe as an impurity phase (<0.8% mass fraction) the diamagnetism unlikely can be attributed to the FeSe-superconductor which has Tc\approx8.5K.Comment: 12 pages, 6 figures, added erratum (page 12) for Figure 4b showing tau2 structur