167 research outputs found
Intercalated Iron Chalcogenides: Phase Separation Phenomena and Superconducting Properties
Organic molecule-intercalated layered iron-based monochalcogenides are presently the subject of intense research studies due to the linkage of their fascinating magnetic and superconducting properties to the chemical nature of guests present in the structure. Iron chalcogenides have the ability to host various organic species (i.e., solvates of alkali metals and the selected Lewis bases or long-chain alkylammonium cations) between the weakly bound inorganic layers, which opens up the possibility for fine tuning the magnetic and electrical properties of the intercalated phases by controlling both the doping level and the type/shape and orientation of the organic molecules. In recent years, significant progress has been made in the field of intercalation chemistry, expanding the gallery of intercalated superconductors with new hybrid inorganic–organic phases characterized by transition temperatures to a superconducting state as high as 46 K. A typical synthetic approach involves the low-temperature intercalation of layered precursors in the presence of liquid amines, and other methods, such as electrochemical intercalation, intercalant or ion exchange, and direct solvothermal growths from anhydrous amine-based media, are also being developed. Large organic guests, while entering a layered structure on intercalation, push off the inorganic slabs and modify the geometry of their internal building blocks (edge-sharing iron chalcogenide tetrahedrons) through chemical pressure. The chemical nature and orientation of organic molecules between the inorganic layers play an important role in structural modification and may serve as a tool for the alteration of the superconducting properties. A variety of donor species well-matched with the selected alkali metals enables the adjustment of electron doping in a host structure offering a broad range of new materials with tunable electric and magnetic properties. In this review, the main aspects of intercalation chemistry are discussed, involving the influence of the chemical and electrochemical nature of intercalating species on the crystal structure and critical issues related to the superconducting properties of the hybrid inorganic–organic phases. Mutual relations between the host and organic guests lead to a specific ordering of molecular species between the host layers, and their effect on the electronic structure of the host will be also argued. A brief description of a critical assessment of the association of the most effective chemical and electrochemical methods, which lead to the preparation of nanosized/microsized powders and single crystals of molecularly intercalated phases, with the ease of preparation of phase pure materials, crystal sizes, and the morphology of final products is given together with a discussion of the stability of the intercalated materials connected with the volatility of organic solvents and a possible degradation of host materials
Microstructural analysis of phase separation in iron chalcogenide superconductors
The interplay between superconductivity, magnetism and crystal structure in
iron-based superconductors is a topic of great interest amongst the condensed
matter physics community as it is thought to be the key to understanding the
mechanisms responsible for high temperature superconductivity. Alkali metal
doped iron chalcogenide superconductors exhibit several unique characteristics
which are not found in other iron-based superconducting materials such as
antiferromagnetic ordering at room temperature, the presence of ordered iron
vacancies and high resistivity normal state properties. Detailed
microstructural analysis is essential in order to understand the origin of
these unusual properties. Here we have used a range of complementary scanning
electron microscope based techniques, including high-resolution electron
backscatter di raction mapping, to assess local variations in composition and
lattice parameter with high precision and sub-micron spatial resolution. Phase
separation is observed in the Csx Fe2-ySe2 crystals, with the minor phase
distributed in a plate-like morphology throughout the crystal. Our results are
consistent with superconductivity occurring only in the minority phase.Comment: Accepted for publication in a special edition of Supercond. Sci.
Techno
Field-induced transition of the magnetic ground state from A-type antiferromagnetic to ferromagnetic order in CsCo2Se2
We report on the magnetic properties of CsCoSe with ThCrSi
structure, which we have characterized through a series of magnetization and
neutron diffraction measurements. We find that CsCoSe2 undergoes a
phase transition to an antiferromagnetically ordered state with a N\'eel
temperature of 66 K. The nearest neighbour interactions are
ferromagnetic as observed by the positive Curie-Weiss temperature of 51.0 K. We find that the magnetic structure of CsCoSe consists
of ferromagnetic sheets, which are stacked antiferromagnetically along the
tetragonal \textit{c}-axis, generally referred to as A-type antiferromagnetic
order. The observed magnitude of the ordered magnetic moment at = 1.5 K is
found to be only 0.20(1)/Co. Already in comparably small
magnetic fields of (5K) 0.3 T, we observe a
metamagnetic transition that can be attributed to spin-rearrangements of
CsCoSe, with the moments fully ferromagnetically saturated in a
magnetic field of (5K) 6.4 T. We discuss the entire
experimentally deduced magnetic phase diagram for CsCoSe with respect
to its unconventionally weak magnetic coupling. Our study characterizes
CsCoSe, which is chemically and electronically posed closely to the
superconductors, as a host of versatile magnetic
interactions
Superconductivity in a new layered bismuth oxyselenide: LaO0.5F0.5BiSe2
We report superconductivity in a new layered bismuth oxyselenide
LaO0.5F0.5BiSe2 with the ZrCuSiAs-type structure composed of alternating
superconducting BiSe2 and blocking LaO layers. The superconducting transition
temperature is TC = 2.6K, as revealed from DC magnetization, resistivity and
muon spin rotation (muSR) experiments. DC magnetization measurements indicate a
superconducting volume fraction of approximately 80%, which is at least twice
higher in comparison to that found in corresponding sulphide LaO0.5F0.5BiS2.
Importantly, the bulk character of superconductivity in LaO0.5F0.5BiSe2 was
confirmed by muSR
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