A. Krzton-Maziopa

Synthesis of a new alkali metal–organic solvent intercalated iron selenide superconductor with Tc ≈ 45 K

We report on a new iron selenide superconductor with a T(c) onset of 45 K and the nominal composition Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2), synthesized via intercalation of dissolved alkaline metal in anhydrous pyridine at room temperature. This superconductor exhibits a broad transition, reaching zero resistance at 10 K. Magnetization measurements reveal a superconducting shielding fraction of approximately 30%. Analogous phases intercalated with Na, K and Rb were also synthesized and characterized. The superconducting transition temperature of Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2) is clearly enhanced in comparison to those of the known superconductors FeSe(0.98) (T(c) ~ 8 K) and A(x)Fe(2-y)Se(2) (T(c) ~ 27-32 K) and is in close agreement with critical temperatures recently reported for Li(x)(NH(3))(y)Fe(2-z)Se(2). Post-annealing of intercalated material (Li(x)(C(5)H(5)N)(y)Fe(2-z)Se(2)) at elevated temperatures drastically enlarges the c-parameter of the unit cell (~44%) and increases the superconducting shielding fraction to nearly 100%. Our findings indicate a new synthesis route leading to possibly even higher critical temperatures for materials in this class: by intercalation of organic compounds between Fe-Se layers.

Superconductivity in a new layered bismuth oxyselenide: LaO0.5F0.5BiSe2

We report superconductivity at T(c) ≈ 2.6 K in a new layered bismuth oxyselenide LaO(0.5)F(0.5)BiSe2 with the ZrCuSiAs-type structure composed of alternating superconducting BiSe2 and blocking LaO layers. The superconducting properties of LaO(0.5)F(0.5)BiSe2 were investigated by means of dc magnetization, resistivity and muon-spin rotation experiments, revealing the appearance of bulk superconductivity with a rather large superconducting volume fraction of ≈ 70% at 1.8 K.

Iron-vacancy superstructure and possible room-temperature antiferromagnetic order in superconducting CsyFe2! xSe2

A). 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 group 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 all possible symmetry adapted magnetic configurations and found that the presence of antiferromagnetic ordering with the ordered magnetic moment of Fe with # 2µB does not contradict 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 )p lane or alongc axis. The superstructure is destroyed above Ts # 500 K by a first-order-like transition.

Study of electrorheological properties of poly(p-phenylene) dispersions

Samples of powdered poly(p-phenylene) lightly doped with ferric chloride were dispersed in silicone oil and the observed electrorheological (ER) effect was studied. The conjugated polymers were obtained in three different synthetic procedures resulting in materials of different crystallinity, which was then additionally modified by annealing in vacuum. The polymer samples were carefully characterized and their electric conductivity and permittivity, doping level, x-ray diffractograms, Fourier-transform infrared spectra, and grain size distribution were determined. The influence of these properties on the magnitude of the ER phenomenon was examined. It was found that the ER activity of the suspensions depended strongly on the crystallinity of a dispersed polymer. This observation was correlated with the ionic conductivity of the material leading to the conclusion that the ER effect in suspensions of FeCl3 doped polyphenylene resulted from bulk polarization processes relying on movement of ions within the p...

Intrinsic crystal phase separation in the antiferromagnetic superconductor RbyFe2−xSe2: a diffraction study

The crystal and magnetic structures of the superconducting iron-based chalcogenides Rb(y)Fe(2-x)Se(2) have been studied by means of single-crystal synchrotron x-ray and high-resolution neutron powder diffraction in the temperature range 2-570 K. The ground state of the crystal is an intrinsically phase-separated state with two distinct-by-symmetry phases. The main phase has the iron vacancy ordered √5 × √5 superstructure (I4/m space group) with AFM ordered Fe spins. The minority phase does not have √5 × √5-type of ordering and has a smaller in-plane lattice constant a and larger tetragonal c-axis and can be well described by assuming the parent average vacancy disordered structure (I4/mmm space group) with the refined stoichiometry Rb(0.60(5))(Fe(1.10(5))Se)(2). The minority phase amounts to 8-10% mass fraction. The unit cell volume of the minority phase is 3.2% smaller than the one of the main phase at T = 2 K and has quite different temperature dependence. The minority phase merges with the main vacancy ordered phase on heating above the phase separation temperature T(P) = 475 K. The spatial dimensions of the phase domains strongly increase above T(P) from 1000 to >2500 Å due to the integration of the regions of the main phase that were separated by the second phase at low temperatures. Additional annealing of the crystals at a temperature T = 488 K, close to T(P), for a long time drastically reduces the amount of the minority phase.

Superconductivity in alkali metal intercalated iron selenides.

Alkali metal intercalated iron selenide superconductors A x Fe2-y Se2 (where A  =  K, Rb, Cs, Tl/K, and Tl/Rb) are characterized by several unique properties, which were not revealed in other superconducting materials. The compounds crystallize in overall simple layered structure with FeSe layers intercalated with alkali metal. The structure turned out to be pretty complex as the existing Fe-vacancies order below ~550 K, which further leads to an antiferromagnetic ordering with Néel temperature fairly above room temperature. At even lower temperatures a phase separation is observed. While one of these phases stays magnetic down to the lowest temperatures the second is becoming superconducting below ~30 K. All these effects give rise to complex relationships between the structure, magnetism and superconductivity. In particular the iron vacancy ordering, linked with a long-range magnetic order and a mesoscopic phase separation, is assumed to be an intrinsic property of the system. Since the discovery of superconductivity in those compounds in 2010 they were investigated very extensively. Results of the studies conducted using a variety of experimental techniques and performed during the last five years were published in hundreds of reports. The present paper reviews scientific work concerning methods of synthesis and crystal growth, structural and superconducting properties as well as pressure investigations.

Superconductivity and magnetism in RbxFe2−ySe2: Impact of thermal treatment on mesoscopic phase separation

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed Rb

Crystal structure of BaFe2Se3 as a function of temperature and pressure: phase transition phenomena and high-order expansion of Landau potential

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Compressibility and pressure-induced disorder in superconducting phase-separatedCs0.72Fe1.57Se2

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Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2.

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