Sergii I. Shylin

Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system

A superconductor is a material that can conduct electricity without resistance below a superconducting transition temperature, Tc. The highest Tc that has been achieved to date is in the copper oxide system: 133 kelvin at ambient pressure and 164 kelvin at high pressures. As the nature of superconductivity in these materials is still not fully understood (they are not conventional superconductors), the prospects for achieving still higher transition temperatures by this route are not clear. In contrast, the Bardeen–Cooper–Schrieffer theory of conventional superconductivity gives a guide for achieving high Tc with no theoretical upper bound—all that is needed is a favourable combination of high-frequency phonons, strong electron–phonon coupling, and a high density of states. These conditions can in principle be fulfilled for metallic hydrogen and covalent compounds dominated by hydrogen, as hydrogen atoms provide the necessary high-frequency phonon modes as well as the strong electron–phonon coupling. Numerous calculations support this idea and have predicted transition temperatures in the range 50–235 kelvin for many hydrides, but only a moderate Tc of 17 kelvin has been observed experimentally. Here we investigate sulfur hydride, where a Tc of 80 kelvin has been predicted. We find that this system transforms to a metal at a pressure of approximately 90 gigapascals. On cooling, we see signatures of superconductivity: a sharp drop of the resistivity to zero and a decrease of the transition temperature with magnetic field, with magnetic susceptibility measurements confirming a Tc of 203 kelvin. Moreover, a pronounced isotope shift of Tc in sulfur deuteride is suggestive of an electron–phonon mechanism of superconductivity that is consistent with the Bardeen–Cooper–Schrieffer scenario. We argue that the phase responsible for high-Tc superconductivity in this system is likely to be H3S, formed from H2S by decomposition under pressure. These findings raise hope for the prospects for achieving room-temperature superconductivity in other hydrogen-based materials.The highest critical temperature of superconductivity Tc has been achieved in cuprates: 133 K at ambient pressure and 164 K at high pressures. As the nature of superconductivity in these materials is still not disclosed, the prospects for a higher Tc are not clear. In contrast the Bardeen-Cooper-Schrieffer (BCS) theory gives a clear guide for achieving high Tc: it should be a favorable combination of high frequency phonons, strong coupling between electrons and phonons, and high density of states. These conditions can be fulfilled for metallic hydrogen and covalent hydrogen dominant compounds. Numerous followed calculations supported this idea and predicted Tc=100-235 K for many hydrides but only moderate Tc~17 K has been observed experimentally. Here we found that sulfur hydride transforms at P~90 GPa to metal and superconductor with Tc increasing with pressure to 150 K at ~200 GPa. This is in general agreement with recent calculations of Tc~80 K for H2S. Moreover we found superconductivity with Tc~190 K in a H2S sample pressurized to P>150 GPa at T>220 K. This superconductivity likely associates with the dissociation of H2S, and formation of SHn (n>2) hydrides. We proved occurrence of superconductivity by the drop of the resistivity at least 50 times lower than the copper resistivity, the decrease of Tc with magnetic field, and the strong isotope shift of Tc in D2S which evidences a major role of phonons in the superconductivity. H2S is a substance with a moderate content of hydrogen therefore high Tc can be expected in a wide range of hydrogen-contain materials. Hydrogen atoms seem to be essential to provide the high frequency modes in the phonon spectrum and the strong electron-phonon coupling.

Chiral spin crossover nanoparticles and gels with switchable circular dichroism

Spin crossover complexes represent spectacular examples of molecular switchable materials. We describe a new approach towards homochiral coordination nanoparticles of [Fe(NH2trz)3](L-CSA)2 (NH2trz = 4-amino-1,2,4-triazole, L-CSA = L-camphorsulfonate) that display an abrupt switch of chiral properties associated with a cooperative spin transition. This is an original method that generates stable and additive-free colloidal solutions of nanoparticles with a spin transition around room temperature. The introduction of a chiral anion to the coordination framework makes these nanoparticles display specific chiro-optical (circular dichroism) properties that are different in high and low spin states. We also illustrate here an effect of the “chiral memory” which is caused by the hysteretic spin transition. In addition, a bistable chiral composite supramolecular system – a [Fe(NH2trz)3](L-CSA)2–CHCl3 gel – is described here.

Distinct microbial populations are tightly linked to the profile of dissolved iron in the methanic sediments of the Helgoland mud area, North Sea

Iron reduction in subseafloor sulfate-depleted and methane-rich marine sediments is currently a subject of interest in subsurface geomicrobiology. While iron reduction and microorganisms involved have been well studied in marine surface sediments, little is known about microorganisms responsible for iron reduction in deep methanic sediments. Here, we used quantitative PCR-based 16S rRNA gene copy numbers and pyrosequencing-based relative abundances of bacteria and archaea to investigate covariance between distinct microbial populations and specific geochemical profiles in the top 5 m of sediment cores from the Helgoland mud area, North Sea. We found that gene copy numbers of bacteria and archaea were specifically higher around the peak of dissolved iron in the methanic zone (250–350 cm). The higher copy numbers at these depths were also reflected by the relative sequence abundances of members of the candidate division JS1, methanogenic and Methanohalobium/ANME-3 related archaea. The distribution of these populations was strongly correlated to the profile of pore-water Fe2+ while that of Desulfobacteraceae corresponded to the pore-water sulfate profile. Furthermore, specific JS1 populations also strongly co-varied with the distribution of Methanosaetaceae in the methanic zone. Our data suggest that the interplay among JS1 bacteria, methanogenic archaea and Methanohalobium/ANME-3-related archaea may be important for iron reduction and methane cycling in deep methanic sediments of the Helgoland mud area and perhaps in other methane-rich depositional environments.

Enantioselective Guest Effect on the Spin State of a Chiral Coordination Framework.

The diversity of spin crossover (SCO) complexes that, on the one hand, display variable temperature, abruptness and hysteresis of the spin transition, and on the other hand, are spin-sensitive to the various guest molecules, makes these materials unique for the detection of different organic and inorganic compounds. We have developed a homochiral SCO coordination polymer with a spin transition sensitive to the inclusion of the guest 2-butanol, and these solvates with (R)- and (S)-alcohols demonstrate different SCO behaviours depending on the chirality of the organic analyte. A stereoselective response to the guest inclusion is detected as a shift in the temperature of the transition both from dia- to para- and from para- to diamagnetic states in heating and cooling modes respectively. Furthermore, the Mössbauer spectroscopy directly visualizes how the metallic centres in a chiral coordination framework differently sense the interaction with guests of different chiralities.

Synthesis of Nanocrystals and Particle Size Effects Studies on the Thermally Induced Spin Transition of the Model Spin Crossover Compound [Fe(phen)2(NCS)2].

Surfactant-free nanocrystals of the model spin-crossover compound [Fe(phen)2(NCS)2] (phen: 1,10-phenanthroline) have been synthesized applying the reverse micelle technique. The morphology of the nanocrystals, characterized by scanning electronic microscopy, corresponds to rhombohedric platelets with dimensions ranging from 203 × 203 × 106 nm to 142 × 142 × 74 nm. Variation of the concentration of the Fe(BF4)2·6H2O salt in the synthesis has been found to have little influence on the crystallite size. In contrast, the solvent-surfactant ratio (ω) is critical for a good particle growth. The spin transition of the nanocrystals has been characterized by magnetic susceptibility measurements and Mössbauer spectroscopy. The nanocrystals undergo an abrupt and more cooperative spin transition in comparison with the bulk compound. The spin transition is centered in the interval of temperature of 175-185 K and is accompanied by 8 K of thermal hysteresis width. The crystallite quality more than the crystallite size is responsible for the higher cooperativity. The magnetic properties of the nanocrystals embedded in organic polymers such as polyethylene glycol, nujol, glycerol, and triton have been studied as well. The spin transition in the nanocrystals is affected by the polymer coating. The abrupt and first-order spin transition transforms into a more continuous spin transition as a result of the chemical pressure asserted by the organic polymers on the Fe(II) centers.

Intercalation effect on hyperfine parameters of Fe in FeSe superconductor with Tc = 42 K

57Fe-Mossbauer spectra of superconducting beta-FeSe, the Li/NH3 intercalate product and a subsequent sample of this intercalate treated with moist He gas have been measured in temperature range 4.7 - 290 K. A correlation is established between hyperfine parameters and critical temperature Tc in these phases. A strong increase of isomer shift upon intercalation is explained by a charge transfer from the Li/NH3 intercalate to the FeSe layers resulting in an increase of Tc up to 42 K. A significant decrease of the quadrupole splitting above 240 K has been attributed to diffusive motion of Li+ ions within the interlamellar space.

High temperature spin crossover in [Fe(pyrazine){Ag(CN)2}2] and its solvate

A high temperature spin crossover (Tup = 367 K) was detected in a metal–organic framework [Fe(pz){Ag(CN)2}2]·MeCN (pz = pyrazine). Upon heating, this solvate released acetonitrile guest molecules, which slightly shifted the transition temperature of the complex (Tup = 370 K and Tdown = 356 K).

Indefinitely stable iron(IV) cage complexes formed in water by air oxidation

In nature, iron, the fourth most abundant element of the Earths crust, occurs in its stable forms either as the native metal or in its compounds in the +2 or +3 (low-valent) oxidation states. High-valent iron (+4, +5, +6) compounds are not formed spontaneously at ambient conditions, and the ones obtained synthetically appear to be unstable in polar organic solvents, especially aqueous solutions, and this is what limits their studies and use. Here we describe unprecedented iron(IV) hexahydrazide clathrochelate complexes that are assembled in alkaline aqueous media from iron(III) salts, oxalodihydrazide and formaldehyde in the course of a metal-templated reaction accompanied by air oxidation. The complexes can exist indefinitely at ambient conditions without any sign of decomposition in water, nonaqueous solutions and in the solid state. We anticipate that our findings may open a way to aqueous solution and polynuclear high-valent iron chemistry that remains underexplored and presents an important challenge.

Pressure effect on superconductivity in FeSe0.5Te0.5

Due to the simple layered structure, isostructural FeSe and FeSe0.5Te0.5 are clue compounds for understanding the principal mechanisms of superconductivity in the family of Fe-based superconductors. High-pressure magnetic, structural and Mossbauer studies have been performed on single-crystalline samples of superconducting FeSe0.5Te0.5 with Tc = 13.5 K. Susceptibility data have revealed a strong increase of Tc up to 19.5 K for pressures up to 1.3 GPa, followed by a plateau in the Tc(p) dependence up to 5.0 GPa. Further pressure increase leads to a disappearance of the superconducting state around 7.0 GPa. X-ray diffraction and Mossbauer studies explain this fact by a tetragonal-to-hexagonal structural phase transition. Mossbauer parameters of the non-superconducting high-pressure phase indicate less covalency of FeSe bonds. Based on structural and susceptibility data, we conclude about a common character of Tc(p) diagrams for both FeSe and FeSe0.5Te0.5 superconductors.

The surface chemistry of iron oxide nanocrystals: surface reduction of γ-Fe2O3 to Fe3O4 by redox-active catechol surface ligands

The effect of surface functionalization on the structural and magnetic properties of catechol-functionalized iron oxide magnetic (γ-Fe2O3) nanocrystals was investigated. γ-Fe2O3 nanocrystals (NCs) were synthesized from iron acetyl acetonate in phenyl ether with 1,2-tetradecanediol, oleic acid, and oleylamine. X-ray powder diffraction in combination with Mossbauer spectroscopy revealed the presence of γ-Fe2O3 (maghemite) particles only. Replacement of oleic acid (OA) with catechol-type 3,4-dihydroxyhydrocinnamic acid (DHCA) or polydentate polydopamine acrylate (PDAm) surface ligands leads to a pronounced change of the magnetic behavior of the γ-Fe2O3 nanocrystals and separated them into two distinctive magnetic entities. XPS and Mossbauer spectroscopy revealed the shell to be reduced with a magnetite (Fe3O4) contribution of up to 33% of the total mass while the core remained maghemite (γ-Fe2O3). The magnetic interaction between the maghemite core and the magnetite shell strongly reduced the anisotropy constant of the nanocrystals and the effective magnetization. Our experiments show that the surface chemistry strongly affects the phase distribution and the macroscopic magnetic properties of iron oxide nanopowders.