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.Comment: 6 pages, 5 figures, 1 tabl

Pressure-induced magnetic collapse and metallization of TlFe1.6Se2\mathrm{TlF}{\mathrm{e}}_{1.6}\mathrm{S}{\mathrm{e}}_{2}

The crystal structure, magnetic ordering, and electrical resistivity of TlFe1.6Se2 were studied at high pressures. Below ~7 GPa, TlFe1.6Se2 is an antiferromagnetically ordered semiconductor with a ThCr2Si2-type structure. The insulator-to-metal transformation observed at a pressure of ~ 7 GPa is accompanied by a loss of magnetic ordering and an isostructural phase transition. In the pressure range ~ 7.5 - 11 GPa a remarkable downturn in resistivity, which resembles a superconducting transition, is observed below 15 K. We discuss this feature as the possible onset of superconductivity originating from a phase separation in a small fraction of the sample in the vicinity of the magnetic transition.Comment: 12 pages, 5 figure

Pyridinium bis(pyridine-κN)tetrakis(thiocyanato-κN)ferrate(III) -pyrazine-2-carbonitrile-pyridine (1/4/1)

In the title compound, (C5H6N)[Fe(NCS)4(C5H5N)2]- 4C5H3N3C5H5N, the FeIII ion is located on an inversion centre and is six-coordinated by four N atoms of the thiocyanate ligands and two pyridine N atoms in a trans arrangement, forming a slightly distorted octahedral geometry. A half-occupied H atom attached to a pyridinium cation forms an N—HN hydrogen bond with a centrosymmetrically-related pyridine unit. Four pyrazine-2-carbonitrile molecules crystallize per complex anion. In the crystal, – stacking interactions are present [centroid–centroid distances = 3.6220 (9), 3.6930 (9), 3.5532 (9), 3.5803 (9) and 3.5458 (8) A˚ ].peerReviewe

Photoinduced hole transfer from tris(bipyridine)ruthenium dye to a high-valent iron-based water oxidation catalyst

An efficient water oxidation system is a prerequisite for developing solar energy conversion devices. Using advanced time-resolved spectroscopy, we study the initial catalytic relevant electron transfer events in the light-driven water oxidation system utilizing [Ru(bpy)(3)](2+) (bpy = 2,2 '-bipyridine) as a light harvester, persulfate as a sacrificial electron acceptor, and a high-valent iron clathrochelate complex as a catalyst. Upon irradiation by visible light, the excited state of the ruthenium dye is quenched by persulfate to afford a [Ru(bpy)(3)](3+)/SO4- pair, showing a cage escape yield up to 75%. This is followed by the subsequent fast hole transfer from [Ru(bpy)(3)](3+) to the Fe-IV catalyst to give the long-lived Fe-V intermediate in aqueous solution. In the presence of excess photosensitizer, this process exhibits pseudo-first order kinetics with respect to the catalyst with a rate constant of 3.2(1) x 10(10) s(-1). Consequently, efficient hole scavenging activity of the high-valent iron complex is proposed to explain its high catalytic performance for water oxidation

Solvent-dependent SCO Behavior of Dinuclear Iron(II) Complexes with a 1,3,4-Thiadiazole Bridging Ligand

Two dinuclear iron­(II) complexes [Fe₂(μ-L)₂]­X₄*4DMF (X = BF₄^– (<b>1·4DMF</b>) and ClO₄^– (<b>2·4DMF</b>)) with a 1,3,4-thiadiazole bridging ligand have been synthesized and show a very distinct solvent-depending SCO behavior. The complexes represent new solvatomorphs of the first dinuclear iron­(II) complexes with the ligand L (2,5-bis­[(2-pyridylmethyl)­amino]­methyl-1,3,4-thiadiazole). The incorporated lattice DMF molecules directly affect the spin state of these complexes. This behavior reveals a structural insight into the role of the solvent molecules on the spin transition

Hofmann-Like Frameworks Fe(2-methylpyrazine)n[M(CN)2]2 (M = Au, Ag) : Spin-Crossover Defined by the Precious Metal

Hofmann-like cyanometalates constitute a large class of spin-crossover iron(II) complexes with variable switching properties. However, it is not yet clearly understood how the temperature and cooperativity of a spin transition are influenced by their structure. In this paper, we report the synthesis and crystal structures of the metal–organic coordination polymers {FeII(Mepz)[AuI(CN)2]2} ([Au]) and {FeII(Mepz)2[AgI(CN)2]2} ([Ag]), where Mepz = 2-methylpyrazine, along with characterization of their spin-state behavior by variable-temperature SQUID magnetometry and Mössbauer spectroscopy. The compounds are built of cyanoheterometallic layers, which are pillared by the bridging Mepz ligands in [Au] but separated in [Ag]. The complex [Au] exhibits an incomplete stepped spin transition as a function of the temperature with TSCO1 = 170 K and TSCO2 = 308 K for the two subsequent steps. In contrast, the complex [Ag] attains the high-spin state over the whole temperature range. In the crystal structure of [Ag], weak interlayer contacts (Ag−π, Me−π, and Ag–N) are found that may be responsible for an unusual axial elongation of the FeN6 polyhedra. We propose that this structural distortion contributes to the trapping of iron in its high-spin state

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

1D iron(ii)-1,2,4-triazolic chains with spin crossover assembled from discrete trinuclear complexes

Temperature-induced spin crossover has been found in a molecular ferrous complex of 4-amino-1,2,4-triazole for the FeN6 centres, while the FeN3O3 centres are always HS

Crystal structure of a water oxidation catalyst solvate with composition (NH4)2[FeIV(L-6H)]·3CH3COOH (L = clathrochelate ligand)

The synthetic availability of molecular water oxidation catalysts containing high-valent ions of 3d metals in the active site is a prerequisite to enabling photo- and electrochemical water splitting on a large scale. Herein, the synthesis and crystal structure of diammonium {μ-1,3,4,7,8,10,12,13,16,17,19,22-dodecaazatetracyclo[8.8.4.13,17.18,12]tetracosane-5,6,14,15,20,21-hexaonato}ferrate(IV) acetic acid trisolvate, (NH4)2[FeIV(C12H12N12O6)]·3CH3COOH or (NH4)2[FeIV(L–6H)]·3CH3COOH is reported. The FeIV ion is encapsulated by the macropolycyclic ligand, which can be described as a dodeca-aza-quadricyclic cage with two capping triazacyclohexane fragments making three five- and six six-membered alternating chelate rings with the central FeIV ion. The local coordination environment of FeIV is formed by six deprotonated hydrazide nitrogen atoms, which stabilize the unusual oxidation state. The FeIV ion lies on a twofold rotation axis (multiplicity 4, Wyckoff letter e) of the space group C2/c. Its coordination geometry is intermediate between a trigonal prism (distortion angle φ = 0°) and an antiprism (φ = 60°) with φ = 31.1°. The Fe—N bond lengths lie in the range 1.9376 (13)–1.9617 (13) Å, as expected for tetravalent iron. Structure analysis revealed that three acetic acid molecules additionally co-crystallize per one iron(IV) complex, and one of them is positionally disordered over four positions. In the crystal structure, the ammonium cations, complex dianions and acetic acid molecules are interconnected by an intricate system of hydrogen bonds, mainly via the oxamide oxygen atoms acting as acceptors

Pyridinium bis(pyridine-&#954;N)tetrakis(thiocyanato-&#954;N)ferrate(III)

In the title compound, (C5H6N)[Fe(NCS)4(C5H5N)2], the FeIII ion is coordinated by four thiocyanate N atoms and two pyridine N atoms in a trans arrangement, forming an FeN6 polyhedron with a slightly distorted octahedral geometry. Charge balance is achieved by one pyridinium cation bound to the complex anion via N&#8212;H...S hydrogen bonding. The asymmetric unit consists of one FeIII cation, four thiocyanate anions, two coordinated pyridine molecules and one pyridinium cation. The structure exhibits &#960;&#8211;&#960; interactions between pyridine rings [centroid&#8211;centroid distances = 3.7267&#8197;(2), 3.7811&#8197;(2) and 3.8924&#8197;(2)&#8197;&#197;]. The N atom and a neighboring C atom of the pyridinium cation are statistically disordered with an occupancy ratio of 0.58&#8197;(2):0.42&#8197;(2)