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.

TRANSFORMATION OF THE NATURE OF THE CONDUCTIVITY OF HIGHLY CONDUCTIVE CHALCOGENIDE MELTS TO TOWARDS THEIR ELECTROLYTIC CAPACITY

Ibe aim of this work is to study the nature of the conductivity of melts of iron chalcogenides (Fe-Se, Fe-Te, FeS-Na2S) and the conditions of its possible transformation in order to bring the high-conductivity melt to a state suitable for electrolytic decomposition into metal and chalcogen. To solve this problem, a set of methods was used - electrical conductivity (ж), thermo-EMF (a), polarization characteristics (І-V) and electrolysis (n). ж of high-conductivity melts was investigated by the four-probe method on a direct current in a quartz U-shaped cell of capillary type with graphite electrodes and molybdenum current leads in the direct current mode. The systematic error due to the accuracy class of the devices is 1%; random - does not exceed 1%. Low ж melts were studied by the two-probe AC method. Due to the high chemical aggressiveness of FeS-Na2S melts, a measuring cell with an alundum capillary was developed. Random error when measuring ж two-probe method on alternating current is 3%, systematic - is within 2%. Thermo-EMF was measured by the differential method with respect to tungsten probes. The temperature difference between the probes was recorded by a differential thermocouple. Systematic error in measuring thermo-EMF was 2%, random - 1.5%. Polarization characteristics were removed by the method of direct current in a simple cell without separation of electrode spaces. In cells of this type in an argon medium over the melt and experiments on electrolysis were performed. It is established that all samples are characterized by significant values of electrical conductivity (of the order of thousands of Sm/cm) and its negative temperature coefficients; the absolute thermo-EMF of all samples is units of цУ/deg. Melts are qualified as electron-ion (polyfunctional) conductors with a predominant metallic contribution to the conductivity. The introduction of Na2S into the FeS melt leads to a decrease in electrical conductivity and the transformation of its temperature dependence to the inherent ionic compounds. The melt electrolysis of the FeS-Na2S system with the production of iron was carried out. Thus the possibility of transformation of the nature of conductivity of highly conductive chalcogenide melts of iron towards their electrolytic ability is proved. The obtained results must be taken into account when using electrochemical technology of processing polymetallic sulfide ores, which include metal chalcogenides of iron family. KEYWORDS: MELTS, IRON CHALCOGENIDES, NATURE OF CONDUCTIVITY, ELECTRIC CONDUCTIVITY, TRANSFORMATION, ELECTROLYTIC CAPACITY.

The dependence of the critical temperature on pressure

The aim of this research is to study the critical temperature depending on the pressure of one-band superconductor. We derive the exact equation of the critical temperature [Formula: see text] by using the BCS-like model. The effect of pressure and pseudogap on critical temperature has been investigated. The analytic form and the approximation of the critical temperature are shown. First, we consider the effect of pressure on the critical temperature and find that the critical temperature increased as pressure is increased which fits well with the experimental data of Tl-based and Bi-based superconductors. Second, the effect of the pseudogap on the critical temperature is considered. We found that the critical temperature is decreased as pressure increased which agrees to the data of [Formula: see text], the iron chalcogenides [Formula: see text], intermetallic compounds [Formula: see text] and [Formula: see text] superconductors.

Proton and ammonia intercalation into layered iron chalcogenides

Hydrothermal synthesis of ammonia-intercalated iron chalcogenides using an alkali-free route.