Iron In Octahedral Coordination Research Articles

Electrochemical Energy Storage in Cs2Zn3[Fe(CN)6] and Their Use As Cathode at Na-Ion Batteries in Aqueous Media

The use of renewable energy resources represented by solar and wind must be the most ideal solution to reduce the serious energy and environmental problems. However, the available amounts of these resources are dependent of weather conditions, thus requiring energy storage systems (ESSs) for persistent power supply. While hydropower and pumped air systems are currently used as grid-scale ESSs, rechargeable batteries are expected to replace the existing technology utilizing their advantages related with higher efficiencies and simple installation. Among a variety of batteries, lithium-ion batteries (LIBs) are receiving the most significant attention due to their superior performance in various electrochemical aspects. In this context, batteries based on alternative carrier ion such as sodium (Na+) might be more suitable for grid-scale ESSs, linked with critical aspects such as abundance and cost of the raw materials. Indeed, raw materials for sodium-ion batteries (SIBs) are estimated to be about 30 times cheaper than those of LIBs. On the other hand, the use of radio-cesium (137Cs) in nuclear-power plants and for military activities have caused a critical environmental hazard as main fission product in radioactive wastes.From past few decades, transition metal hexacyanoferrates known as Prussian Blue Analogues (PBAs), have been widely used for Cs removal from waste waters. This open framework materials with the Prussian Blue crystal structure offer the high power capability and low cost synthesis. Herein, we report the synthesis of zinc hexacyanoferrate (ZHCF), which is endowed with enlarged ionic channels compared with typical PBAs. ZHCF was synthesized by precipitation method from a mixture of Na4Fe(CN)6 and Zn(NO3)2 solutions at 60°C. The solid formed was centrifuged and dried to room temperature. The solid material was mixed with a solution of CsNO3 during 24 hr. Then centrifuged again and dried. The obtained material was labeled as Cs-ZHCF. Using XRD, the crystal structure suggests that materials crystallize with a rhombohedral unit cell (R-3c space group). Their porous framework can be represented as the assembling of FeC6 octahedral and ZnN4 tetrahedral shapes. Therefore, the synthesis process promotes the assembling of octahedral-anionic blocks ([Fe(CN)6]4-) through Zn2+ cations, which links neighboring blocks at the N ends. In the structure of these materials the N–Zn–N angle is near to 108°. The formation of elliptical-like windows and ellipsoidal cavities is related to such bonding angle. The calculated cavity dimension was ca. 15.5 x 11.1 x 7.9 Å. Six elliptical windows of ca. 6.8 x 8 Å communicate neighbor cavities. Within a given cavity two exchangeable Cs-ions are found, located close to the major ellipsoid axis in order to minimize the repulsive electrostatic interactions between them.For the electrochemical characterization using cyclic voltammetry in NaNO3 solutions, the Cs-ZHCF was mixed with amorphous carbon, polyvinylidenedifluoride, and graphite in a ratio of 80:9:9:2 were prepared in THF. The obtained suspension was deposited on a glassy carbon electrode, and dried in Argon atmosphere. It was found that the i-E profiles show a reversible capacity of 56.4 mAhg-1. For the evaluation of the observed electrochemical data, it is important to note that in Cs-ZHCF, Zn is electrochemically inactive but rather only contributes to maintaining the framework structure, whereas, the iron in octahedral coordination can be oxidized and reduced during charge-discharge. These results indicate that only one Cs-ion was removed from the cavity of the material-structure, improving the diffusion of Na-ions.

Crystal structures of the new isotypic compounds Rb4(M2+)(Fe3+)8[SeO3]14[SeO2(OH)]2 · 2 H2O (M = Mg, Cu) with a short review on iron(III) selenites

Single crystals of the new compounds Rb4(M2+)(Fe3+)8[SeO3]14[SeO2(OH)]2 · 2 H2O (M = Mg, Cu) have been synthesized in Teflon-lined steel vessels at 495 K. Their structures were determined from X-ray single crystal diffraction data and found to be isotypic: space group P-1, Z = 1, a = 7.716(2)/7.726(2), b = 10.325(3)/10.327(3), c = 15.764(4)/15.724(4) Å, α = 77.57(1)/77.87(1), β = 89.23(1)/89.47(1), γ = 86.09(1)/86.57(1)°, V = 1223.6(6)/1224.3(6) Å3, R1 = 0.028/0.037 for the Mg/Cu compound, respectively. The structure type is characterized by a complex polyhedral framework: the [(Fe3+)O6] octahedra are linked via all corners with typical pyramidal [SeO3]2– groups — with exception of a H2O molecule which is a ligand of one of the Fe atoms. The [(M2+)O6] polyhedron shares two corners and two edges with selenite pyramids. Further sharing corners with [FeO6] octahedra, centrosymmetric units of the type [MFe4Se4O28] may be emphasized. Voids within the framework are occupied by Rb+ cations and by the lone-pair electrons of the Se4+ cations. Three different hydrogen bonds with O···O distances in the range 2.69–2.85 Å occur. At present 28 selenites of ferric iron are known, most of them synthesized at low-hydrothermal conditions. Commonly, framework structures are formed with ferric iron in octahedral coordination. Further building units are trigonal pyramidal [SeO3]2– and [SeO2(OH)]– groups or [Se2O5]2– dimer anions, featuring a large variety of structure arrangements. Iron(III) selenites are classified based on the presence of extra cations, on the respective anionic groups and by the type of interpolyhedral linkage. In the literature, few isotypic selenites (with Fe3+ substituted by another trivalent cation) have been reported yet. Among 51 [FeOn] polyhedra studied all but one show ferric iron (Fe3+) in sixfold coordination with mean Fe—O bond lengths for 41 pure [FeO6] octahedra ranging from 1.989 to 2.044 Å with an overall mean value of 2.013(11) Å. Remarkably, only 14% of all [FeO6] poly hedra are centrosymmetric. In the oxoanions investigated (61 [SeO3]2–, 9 [SeO2(OH)]– and 6 [Se2O5]2– groups) tetravalent selenium typically forms a flattened trigonal pyramid with O—Se—O bond angles of about 100° and Se—O bond lengths of 1.70 Å, completed by the lone-pair electrons E at the selenium cation to a formal [ESeO3]2– ψ 1 tetrahedron. In hydrogenselenites and diselenites elongated Se—O distances occur for the protonated and the bridging oxygen atoms, respectively. With some rare exceptions all selenium atoms have the site symmetry 1.

Coordination structures of vanadium and iron in MCM-41 and the catalytic properties in partial oxidation of methane

The coordination structures of vanadium and iron introduced into MCM-41, a typical mesoporous silica, by both direct hydrothermal synthesis (DHT) and template–ion exchange (TIE) methods have been studied by X-ray absorption spectroscopy (XANES and EXAFS). Vanadium is tetrahedrally coordinated with oxygen in V-MCM-41 prepared by both methods but the location of vanadium is probably different; vanadium is mainly incorporated inside the framework of MCM-41 by the DHT method as the content is lower than approximately 1 wt%, while the vanadyl tetrahedra are probably dispersed on the wall surface of MCM-41 over the samples by the TIE method. In the case of Fe-MCM-41, the DHT method results in iron atoms mainly in tetrahedral coordination and isolated from each other in the framework of MCM-41. However, aggregated iron oxides with iron in octahedral coordination are mainly observed in the TIE samples. The V-MCM-41 by the TIE method shows better catalytic performances than that by the DHT method in the partial oxidation of methane to formaldehyde with oxygen. However, the Fe-MCM-41 by the DHT method exhibits remarkably higher methane conversion and formaldehyde selectivity than that by the TIE method.

Mössbauer spectroscopic investigation of some iron-containing sodium phosphate glasses

57Fe Mössbauer spectroscopy has been used to investigate the effect of an addition of an increasing amount of Fe2O3 on the iron environment in a number of sodium phosphate glasses of composition (1 - x)NaPO3 · xFe2O3 (x = 0.05, 0.1, 0.15), prepared in air under the same conditions. High-spin ferric and ferrous iron in octahedral coordination is present in all the samples. Two sites, with different disorder and octahedral distortion, are occupied by ferric ions. An increasing iron content results in an increase in the distortion and disorder of the ferric sites and, in addition, in a redistribution of the iron atoms between the two ferric sites.

Mössbauer spectroscopic investigation of iron in sodium phosphate glasses

We studied three sodium phosphate glasses (1 − x) NaPO 3 + Fe 2 O 3 ( x = 0.05, 0.1, 0.15) prepared in air in the same conditions using 57Fe Mössbauer spectroscopy in order to investigate the effect of a growing Fe 2O 3 addition on the features of iron sites. High spin ferric and ferrous iron in octahedral coordination is present in all the samples. We found that the distortion, compared to a regular octahedron, of the ferric and ferrous iron sites increases from the low to the high iron content samples.

X-ray powder diffraction and57Fe Mössbauer spectroscopy study of the thermal breakdown of vivianite, Fe3(PO4)2.8H2O

Ferric phosphates resulting from the thermal breakdown of natural Fe3(PO4)2.8H2O are studied by X-ray powder diffraction and Mossbauer spectroscopy. Total iron oxidation and water release give rise to an amorphous material. During heating, a transitional cristobalite-type ferric orthophosphate recrystallizes prior to the stable berlinite-type. The main additional final product is a poorly characterized “Fe3PO7” phase with iron in octahedral coordination.

SCF–Xα scattered wave MO studies of the electronic structure of ferrous iron in octahedral coordination with sulfur

SCF–Xα scattered wave calculations are presented for the FeS10−6 cluster at Fe–S distances of 2.50, 2.38, and 2.26 Å for quintet and singlet spin states and the transition state intermediate between them. These results, together with those of recent calculations on S−12, are used to interpret the x-ray and optical spectra of FeS2 (pyrite) and FeS (troilite). A crossover of the internal energies of the quintet and singlet states of FeS10−6 is predicted at R (Fe–S) =2.28 Å and the ease of spin pairing in FeS is related to its high compressibility and its large volume change upon spin pairing.

Bonding in SiO2 and Fe2O3 by Oxygen Kα X-ray Emission Spectra

SILICON in tetrahedral and iron in octahedral coordination are fundamental units in a wide range of silicate minerals. Thus investigations of the nature of the Si–O and Fe–O bond in simple oxides such as SiO2 and Fe2O3 may provide some basis for the better understanding of these bonds in more complex silicate structures. Recently, attempts have been made to interpret certain aspects of the LII,III X-ray emission spectra of some transition metal compounds1–3 and the Kβ spectra of third row metal compounds3–5, using molecular orbital (MO) theory. Initial results have been most encouraging and MO theory seems to have great potential in the interpretation of spectra arising from transitions from valence orbitals. We present here the initial results of investigations of oxygen Kα emission spectra from SiO2 (quartz) and Fe2O3 (haematite), which are readily interpretable within the framework of simple MO theory.

Analysis Of Iron In Layer Silicates By Mossbauer Spectroscopy

AbstractAnalysis of Mossbauer effect in layer silicates provides a spectroscopic method for determining valences and coordination of iron. In this study Mossbauer spectra were obtained for amesite, cronstedtite, nontronite, two glauconites, biotite, lepidomelane, chlorite, minnesotaite, vermiculite. stilpnomelane, and chloritoid.Trivalent iron was detected in tetrahedral coordination. Abundant trivalent iron in octahedral coordination apparently causes quadrupole splitting values of divalent iron in the same mineral to decrease. This phenomenon was noted in cronstedtite and glauconite. In cases where divalent iron predominates in the mineral, the quadrupole splitting is larger. It is generally accepted that ferrous iron is largely in octahedral coordination. This suggests that the octahedral sites may be more distorted when ferric iron is present in the octahedral sheet. In biotite, quadrupole splitting of divalent iron is decreased when trivalent iron is present in tetrahedral sheets. This suggests that there is also more distortion in the octahedral sheet because of iron in tetrahedral positions.