Cation Occupancy Research Articles

Computational studies of gramicidin permeation: An entryway sulfonate enhances cation occupancy at entry sites

The impact on the cation-transport free-energy profile of replacing the C-terminal ethanolamine in the gramicidin A channel with a taurine residue is studied using molecular dynamics simulations of gramicidin A (1JNO) embedded in a lipid bilayer (DMPC) with 1 mol/kg NaCl saline solution. The potential of mean force for ion transport is obtained by umbrella sampling. The presence of a negatively charged sulfonate group at the entrance of the gramicidin channel affects the depth and the location of the binding sites, producing a strong attraction for the cations in the bulk. The potential of mean force by the sulfonate acting directly through electrostatics and van der Waals interactions on the test ion is highly modulated by indirect effects (i.e., sulfonate effects on other components of the system that, in turn, affect the ion free-energy profile in the channel). Because the “entry” sites are located symmetrically at both entry and exit of the channel, the deeper free-energy wells should inhibit exit. Given that the channel has increased conductance experimentally, the simulation results suggest that the channel conductance is normally entry limited.

Structural features in Tutton's salts K2[M2+(H2O)6](SO4)2, with M2+ = Mg, Fe, Co, Ni, Cu, and Zn

The crystal chemistry of six crystals of general formula K 2 [M 2+ (H 2 O) 6 ](SO 4 ) 2 , with M 2+ = Mg, Fe, Co, Ni, Cu, and Zn, was investigated by single-crystal structure analysis to determine the effects of the chemical variation of M 2+ on the structural environment surrounding K, M, and S sites. Results indicate that the distortion in the SO 4 tetrahedron and the MO 6 octahedron is very small, except for CuO 6 where it is pronounced because of the Jahn-Teller effect. The KO 8 -octacoordinate polyhedron has the highest degree of distortion, and its idealized shape may be referred to as a bicapped trigonal prism. The SO 4 size is not affected by changes in cation occupancies at the adjacent M site. In contrast, changes in the KO 8 size, which are accompanied by changes in the bond valence sum at K, depend on interaction with the first and second coordination sphere of M. This interaction results by changes in M-O individual lengths, by expansion of the second coordination sphere of M, and by changes in the distribution of the bond strengths over the O atoms coordinated to K. The MO 6 size follows the expected trend from the increased ionic radius at the M site. The latter is also correlated with the unit-cell volume except for the Cu- and Mg-phase, which show a larger cell volume with respect to that expected. Although the relevant octahedral distortion around the Cu 2+ cation explains the volume excess in the Cu-phase, an expansion of the second coordination sphere of Mg 2+ , compared to those of cations of larger ionic radius (such as Zn and Co), explains the excess of the unit-cell volume in the Mg-phase. As the CuO 6 distortion can be caused by the Jahn-Teller effect, the higher ionicity of the Mg atom could be the cause for its anomalous behavior observed in Tutton’s salts. This stereochemical behavior of the Mg atom seems to be consistent with the weakening of the hydrogen bonds in the structure connected to differences in the bonding character of Mg and transition metals when coordinated by water molecules.

The magnetic and dielectric properties of multiferroic Sr-substituted Zn2-Y hexagonal ferrites

The magnetic and dielectric properties of Sr-substituted Zn2-Y hexagonal ferrites (Ba2-xSrxZn2Fe12O22, 1.0 ≤ x ≤ 1.5) are studied in this paper. Sr substitution will lead to the variation of cation occupation, which influences both the magnetic and electric properties. As Sr content x rises from 1.0 to 1.5, magnetic hysteresis loop gets wider gradually and the permeability drops rapidly due to the transformation from ferrimagnetic to antiferromagnetic phase. Moreover, permittivity rises with increasing Sr content. Under a certain external magnetic field, the phase transition of helical spin structure of Ba0.5Sr1.5Zn2Fe12O22 at about 295 K seems to open a possibility for the room-temperature ferroelectricity induced by magnetic field. But its low resistivity prevents the observation of ferroelectric and magnetoelectric properties at room-temperature.

NiFe2O4Nanoparticles Dispersed in an Aerogel Silica Matrix: An X-ray Absorption Study

The formation of NiFe2O4 nanoparticles dispersed in an aerogel silica matrix was investigated as a function of calcination temperature by X-ray absorption fine structure and X-ray absorption near edge structure at both the Fe and Ni K-edges. In particular, nanocomposite aerogels containing a relative NiFe2O4 amount of 10 wt % and calcined at 450, 750 (1 h and 20 h), and 900 °C were studied. A quantitative determination of the relative occupancy of iron and nickel cations in the octahedral and tetrahedral sites of the spinel structure was obtained. It has been found that nickel ferrite prepared by sol−gel has the classical inverted spinel structure found in bulk materials with nickel(II) cations fully occupying the octahedra sites and iron(III) equally distributed between octahedra and tetrahedra sites.

Investigation on the hydrated and dehydrated forms of the ion-exchanged microporous stannosilicate EMS-2

The ion-exchange properties and characterization of EMS-2, a new stannosilicate isostructural with the natural zirconosilicate lemoynite, are reported. EMS-2 samples were characterized by high resolution synchrotron X-ray diffraction, elemental, thermogravimetric and differential scanning calorimetry analyses. The crystal structure of Mg, Ca, Sr and Ba exchanged samples was refined revealing the positions and occupancies of the extraframework cations and water molecules. After dehydration, the crystal structure of ion-exchanged EMS-2 was investigated by in-situ high temperature X-ray powder diffraction. The experiments indicated that the materials are stable at least up to 400 °C. Mg- and Ca-EMS-2 possess a very rigid structure and only slight variations of the unit cell parameters were noted after dehydration. At a difference, at high temperature Sr- and Ba-exchanged samples undergo a phase transition. The new structures were determined and the phase transition was rationalized as an extensive reorganization of cations after loosing the coordinated water molecules. This process is possibly driven by the need to attain a coordination environment as favourable as possible. This produces a strong framework distortion, associated to a structure contraction in the b direction, which drives to doubling the c cell parameter.

SU-57 – An aluminosilicogermanate with a DFT topology and variable composition

An aluminosilicogermanate with a DFT zeotype framework (denoted as SU-57) was synthesized for the first time by ethanol-assisted hydrothermal synthesis at 160 and 170 °C. The compound was characterized by single crystal and powder X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy (EDS), thermogravimetry (TG) and solid state nuclear magnetic resonance (NMR) spectroscopy. SU-57 crystallizes in space group P4 2/ n with estimated a ≈ 10.38–10.46 Å, c ≈ 8.88–8.91 Å, V ≈ 957–975 Å 3. It has variable Al–Si–Ge composition with an approximate formula |C 2H 10N 2| [Al(Si x Ge 1− x )O 4] 2 ( x ≈ 0.3–0.9), which results in a super-structure originated from different cation occupancies of the two unique tetrahedral (T) sites. Single crystal X-ray structure refinements, together with results from X-ray powder diffraction (XRPD) and EDS analysis, showed that (i) the AlO 4 and (Si, Ge)O 4 tetrahedra are only partially ordered over the DFT framework and do not follow a strict alternating manner. (ii) Al resides predominantly on one T site and Si and Ge predominantly on the other. (iii) The Al cation concentration (Al/(Al + Si + Ge)) is nearly constant and slightly less than 50 at%, while the Si and Ge cation concentrations vary over a large range. (iv) Al and Ge occupy both T sites. The cation disorder was confirmed by 27Al and 29Si solid state NMR. TG analysis and in situ XRPD showed that SU-57 was stable up to 375 °C in N 2 atmosphere.

Monodisperse Zn ferrite nanospheres and its Ba substitution nanosheets: Synthesis and magnetic properties

Monodisperse Zn ferrite nanospheres and its Ba substitution nanosheets were successfully prepared by a facile solvothermal method. The size, morphology, and structure of the as-prepared products were investigated in detail by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Magnetic studies revealed that Zn ferrite nanospheres are ferrimagnetic with a saturation magnetization ( M s ) of 49.6 emu /g and M s increased from 49.6 emu /g to 84.2 emu /g when Ba 2+ was substituted into Zn ferrite to form BaZn ferrite nanosheets. To explain the variations of morphology and magnetic properties, the strong preferential occupancy of cations in chemically equivalent A and B sites and the metastable cation distribution in nanoparticles are invoked.

EPR STUDY OF NANOSTRUCTURED ZINC FERRITE

Bulk zinc ferrite has been studied in the past. In the bulk form zinc ferrite crystallizes in the normal spinel structure with all the zinc ions occupying the tetrahedral and all the iron occupying the octahedral sites, respectively. But, nanosize zinc ferrite is believed to crystallize in a different way. It is now well established that in a nanosize zinc ferrite there is partial inversion in the cation occupancy, leading to some kind of magnetic ordering in this system much above its Neel temperature of 10 K. Hence, there is ample scope of maneuvering the electrical and magnetic properties of this ferrite. In the present study nanosize zinc ferrite was prepared by a chemical reaction involving the nitrates of Zn and Fe and using the citric acid as the host. The sintering of the precursor was done for 1 h at various temperatures ranging from 300°C to 1000°C. These samples were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), and electron paramagnetic resonance (EPR). The average particle sizes in these sintered samples measured by TEM are found to vary from ≈ 10 to 62 nm. The XRD measurements show the formation of single-phase spinel structure in all the samples. The X-band EPR spectrum of the precursor specimen at room temperature consists of two EPR signals: an intense signal at g eff = 2.02 having peak-to-peak line width △H PP = 425 Gauss and other a weak signal at g eff = 4.51 having an asymmetric line shape. As the samples are heat-treated at different temperatures, the EPR signal corresponding to the intense peak gets stabilized at g eff ≈ 2.03, whereas the weak signal gets suppressed progressively. The spin–spin relaxation time in these samples have values ≈ 3 × 10-10 s . The effect of sintering temperature on the properties of the nanostructured specimen of zinc ferrite and the underlying mechanism would be discussed.

Cation mass-valence sum (CM-VS) approach to assigning OH-bending bands in dioctahedral smectites

AbstractThe assignments of OH-bending bands in the infrared (IR) spectra of dioctahedral smectites (montmorillonites, ferruginous smectites, and nontronites) have been revisited using a cation mass-valence sum (CM-VS) approach to quantify octahedral cation occupancy. The CM-VS approach enabled prediction of OH-bending band positions (in wavenumbers) related to OH-sharing octahedral cation pairs that had valence sums of 4 and 5, and cation masses associated with Fe(II) and Mg. Application of rules for the relationship enabled determination of the location of OH-sharing octahedral cation pairs containing Mg and Fe(II) for which previous assignments have been considered controversial, e.g. Fe(III)Mg-OH and MgMg-OH, or for which assignments have been missing due to lack of spectroscopic evidence, e.g. AlFe(II)-OH, Fe(III)Fe(II)-OH, Fe(II)Mg-OH, and Fe(II)Fe(II)-OH. Examples of these bands from several natural ferruginous smectites and nontronites are discussed. Quantification of IR spectra was used to develop a better understanding of the octahedral cation occupancy of this important class of Fe(III)-enriched smectites. While Fe(II) contents may be somewhat overestimated by the IR technique, those for four of the six ferruginous smectites studied here agree well with data from Mössbauer spectroscopy.

Cation site occupancy of biogenic magnetite compared to polygenic ferrite spinels determined by X-ray magnetic circular dichroism

Ferrite spinels, especially magnetite (Fe3O4), can be formed either by geological, biological or chemical processes leading to chemically similar phases that show different physical characteristics. We compare, for the first time, magnetite produced by these three different methods using X-ray magnetic circular dichroism (XMCD), a synchrotron radiation based technique able to determine the site occupancy of Fe cations in the ferrite spinels. Extracellular nanoscale magnetite produced by different Fe(Ill)reducing bacteria was shown to have different degrees of stoichiometry depending on the bacteria and the method of formation, but all were oxygen deficient due to formation under anoxic conditions. Intracellular nano-magnetite synthesized in the magnetosomes of magnetotactic bacteria was found to have a Fe cation site occupancy ratio most similar to stoichiometric magnetite, possibly due to the tight physiological controls exerted by the magnetosome membrane. Chemically-synthesised nano-magnetite and bulk magnetite produced as a result of geological processes were both found to be cation deficient with a composition between magnetite and maghemite (oxidised magnetite).

Cation occupancy at the A1/A2 sites in strontium barium niobate micro-crystals grown from molten NaCl and KCl salts

The cation chemistry in two sets of strontium barium niobate (SBN) micro-crystals, synthesized at 1100 °C via molten NaCl and KCl salts was investigated using analytical electron microscopy. Na + and K + ions were found to enter the tungsten bronze (TB) structure to form the filled TB phase Sr 4− m Ba m Na 2Nb 10O 30 or the partially filled TB phase (Sr,Ba) 4+ n K 2−2 n □ n Nb 10O 30. Small Na + ions fill the A2 sites, leaving Sr 2+ and Ba 2+ ions to fill the A1 sites. The shifts in the actual compositions ( m) from the nominal ones imply there may be morphotropic phase boundaries for the given composition range. In contrast, the large K + ions partially fill two of the A1 sites, creating vacancies and results in the formation of (Sr 4− m Ba m )(Ba n K 2−2 n □ n ) Nb 10O 30. A correlation between Ba, Sr and K contents reveals that m and n have a linear relation ( m ≈ 3 n). This leads to the new formula Sr 4−3 y/4 Ba y K 2−2 y/4 □ y/4 Nb 10O 30 (0 < y < 4), which illustrates a balance in the solution energy.

Control of the cation occupancies of MnZn ferrite synthesized via reverse micelles

Variations in cation occupancy in mixed metal ferrite systems can affect their electronic and magnetic properties. It is known that different synthesis parameters can lead to various cation distributions and the ability to tune these distributions is of great interest. This study uses the extended x-ray-absorption fine structure–IR relationship to investigate the effect of various Fe2+∕Fe3+ ratios in initial synthesis conditions on cation distribution for manganese zinc ferrite (MZFO). Differences in the precipitated material before firing could lead to differences in the final material if fired under similar conditions. This work uses several different ratios of Fe3+∕Fe2+, which will affect the initial cell potential for the reaction, to synthesize nano MZFO. All samples were fired for 5h at 500°C under flowing nitrogen. Transmission electron microscopy micrographs reveal highly crystalline uniform nanoparticles of 16±2nm. The x-ray diffraction revealed single phase crystalline MZFO with an average crystallite size of around 14nm. The saturation magnetization ranged from 43to68emu∕g as measured by vibrating-sample magnetometry. The Fourier transform infrared (FTIR) analysis was used to determine the cation occupancies while changing the initial Fe3+∕Fe2+ ratios from 10∕90 to 90∕10. The FTIR spectra revealed a shift in the first absorption region in the far IR from 566.98to549.62cm−1 corresponding to the octahedral occupancies. This shift corresponds to a change in the percentage of octahedral sites occupied by manganese from roughly 25% to 12%. This change in manganese occupancy is also observed in the iron occupancies, which in turn help to explain the variation in saturation magnetization.

Mössbauer investigations of palygorskite from Xuyi, China

The original palygorskite clay mineral has been collected from Longwangshan, Xuyi, Jiangsu Province, China. XRF analysis was performed to study its chemical compositions. Thermogravimetric analysis and differential thermogravimetric analysis (TGA/DTA) were used to study the weight loss due to dehydration and dehydroxylation. Then eight samples were prepared by heating at various temperatures. X-ray diffraction (XRD) patterns and Mössbauer spectra were measured to study the crystal evolvement and the occupation of iron cations. It revealed that the iron ions occupied the M1 and M2 with the ratio closed to 3:1. Hematite has been observed when heated at 650 °C, which indicated that iron cations mainly occupy M1 sites. An unknown silicate was identified when heated at 750 °C and dominated at 850 °C.

STRUCTURAL AND CHEMICAL CONTRASTS BETWEEN PRISMATIC AND FIBROUS FLUORO-EDENITE FROM BIANCAVILLA, SICILY, ITALY

Fluoro-edenite [NaCa2Mg5(Si7Al)O22F2], a new end-member of the calcic amphibole group, is present with both prismatic and fi brous morphologies in the altered volcanic products of the Monte Calvario locality, Biancavilla, on the fl anks of Mount Etna, Sicily, Italy. Whereas the crystal chemistry of the prismatic variety was described recently, the fi brous variety has not been well characterized. We report original crystal-chemical and structural data on this fi brous amphibole. Because of the diffi culty in analyzing this micrometric material, specifi c procedures of sample preparation were developed. SEM–EDS microanalyses and Mossbauer spectroscopy indicate a variable composition of the fi bers, ranging mostly from fl uoro-edenite to, subordinately, winchite, tremolite, and richterite compositions. Given the impossibility of using a single-crystal X-ray method, the powderdiffraction data were evaluated by the Rietveld method. By combining the microchemical data, Mossbauer Fe 3+ /Fetot values, and with a refi nement of the structure, the following chemical formula was obtained from an averaged composition of all the fi brous amphibole analyzed: (Na0.307K0.157)� 0.464 (Ca1.505Na0.495)� 2.000 ( VI Al0.104Fe 3+ 0.333Fe 2+ 0.162Mg4.255Ti0.062Mn0.063)� 4.980 (Si7.520 IV Al0.480)� 8.000 O22 (F1.970Cl0.020)� 1.990. The cell parameters, positional parameters for all the atoms, and site scattering for O3, M1, M2, M3, M4, A, and A(m) were refi ned. In agreement with the site occupancies of cations based on both XRPD and Mossbauer data, Fe 2+ was assigned to the M2 and M3 sites, and all Fe 3+ was assigned to the M2 site. In particular, Fe 2+ content of each M2 site is far greater (0.125 apfu) than that of M3 (0.037 apfu). The analytical procedures utilized here can be adopted for all types of mineral fi bers, whether they belong to the asbestiform group of amphiboles or not. In addition to their mineralogical interest, these results are also relevant for environmental and health aspects. The interactions between mineral fi ber and the biological environment are strongly related to the crystal chemistry of the fi brous materials.

Nonstoichiometric complex of gramicidin D with KI at 0.80 Å resolution

The crystal structure of a nonstoichiometric complex of gramicidin D (gD) with KI has been determined at 100 K using synchrotron radiation. The final R factor was 0.106 for 83 988 observed reflections (Friedel pairs were not merged) collected to 0.80 A. The structure consists of four independent pentadecapeptides and numerous solvent molecules and salt ions. The general architecture of the antiparallel double-stranded gramicidin dimers in the crystal (a right-handed antiparallel DSbetaH(R) form) closely resembles that of previously published cation complexes of gD. However, a significantly different mixture of gramicidin isomers is found in the crystal of the KI complex, including partial occupancy of phenylalanine at position 11. Only three sites in each of the two crystallographically independent channels are partially occupied by potassium cations instead of the commonly observed seven sites. The sum of the partial occupancies of K(+) (1.10 per two dimers) is consistent with the sum of the iodide occupancies (1.095 over eight sites), which is also confirmed by the anomalous signal of the iodide. There was a significant asymmetry of the distribution and occupancies of cations in the crystallographically independent gramicidin channels, in contrast to the distribution found in the rubidium chloride complex with gD.

Cation occupancies in Mg, Co, Ni, Zn, Al ferrite spinels: a multi-element EXAFSstudy

The distribution of cations between tetrahedral (A) sites and octahedral(B) sites in ferrite spinels has been studied using K-edge x-ray absorptionspectroscopy. The samples include natural and synthetic end-member magnetites(Fe3O4), a naturalMn- and Zn-rich magnetite (franklinite) and synthetic binary, ternary and quaternary ferrites of stoichiometryM2+M23+O4, whereM2+ = Mg, Co, Ni,Zn and M3+ = Fe, Al. XAS data were obtained for all metals. Complete, unfiltered, EXAFS spectra were refinedto determine the percentage distribution of each element over the A and B sites and thesedata were combined with microprobe analyses to quantify the tetrahedral occupancy foreach element in each sample. Measured site occupancies and an internally consistent set of(M–O)A and (M–O)B bond lengths were used to calculate unit-cell parameters, which show excellentagreement with measured values, pointing to the reliability of the measured occupancyfactors. The average occupancies determined for the tetrahedral sites in ferritesare (atoms per formula unit) Mg 0.44, Co 0.24, Ni 0.11, Zn 0.76, Al 0.11 andFe3+ 0.92–0.19. The widerange found for Fe3+ is consistent with it playing a relatively passive role by making good any A-site deficit leftby the other competing cations.

The effect of cationic disordering on the electrochemical performances of the layered nitrides LiWN 2 and Li 0.84W 1.16N 2

The layered nitrides LiWN 2 and Li 0.84W 1.16N 2 are investigated related to their electrochemical performances as possible electrodes for rechargeable lithium batteries. In the case of LiWN 2, for which Li and W atoms are ideally ordered in alternating Li and W layers, de-intercalation of lithium proceeds up to 50%. The de-intercalated lithium can be intercalated again subsequently. The reversible reaction is the origin of the high cyclability observed in the corresponding lithium cells. On the other hand, in Li 0.84W 1.16N 2 the Li layers are statistically occupied by tungsten (16%) and lithium (84%). A clear effect of the disordered cationic occupation on the electrochemical behaviour has been found, since for Li 0.84W 1.16N 2 a very low extraction rate is obtained during the first oxidation (0.03 Li per formula unit). In addition, cyclability is rather poor. The very different electrochemical behaviour is explained by the structural differences between both layered nitrides.

Ultrahigh resolution imaging of local structural distortions in intergrowth tungsten bronzes

Details of the local structure of a complex tungsten bronze, K x WO 3 have been determined using focal series exit wave reconstruction. Octahedral rotations in different structural regions of the same crystal have been directly measured from the exit wave phase and correlated with variations in cation occupancy determined from the exit wave modulus.

The F-analogue of schorl from Grasstein, Trentino South Tyrol, Italy: crystal structure and chemistry

The F-analogue of schorl has been identified in samples from a pegmatite at Grasstein, Trentino-South Tyrol, Italy. The crystal chemistry of this tourmaline has been characterized by a combination of single-crystal structure refinement, chemical analysis, and Mossbauer spectroscopy, yielding the structural formula X (Na 0.78 K 0.01 □ 0.2l ) Y (Fe 2+ 1.89 Al 0.58 Fe 3+ 0.13 Mn 2+ 0.13 Ti 4+ 0.02 Mg 0.02 Zn 0.02 □ 0.21 ) Z (Al 5.74 Fe 3+ 0.26 ) T (Si 5.90 Al 0.10 O 18 ) (BO 3 ) 3 V (OH) 3 W [F 0.76 (OH) 0.24 ]; a = 15.997(2), c = 7.179(1) A, V = 1591.0(4) A 3 , R 1( F ) = 1.60 %. This F-rich and Fe 2+ -rich tourmaline, a pneumatolytic phase crystallized in the presence of a F-rich fluid (coexisting with fluorite), is very near the proposed end-member composition of the F-analogue of schorl: NaFe 3 2+ Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 F. The relatively high amount of Fe 2+ at the Y site is consistent with the large distance of 2.056 A. Refinement ofthe F:O occupancy ratio at the W site yields F 0.8 O 0.2 pfu, consistent with the chemical data (F 0.76 apfu). Because ofthe local bonding ofthe W -site anion to three neighbouring Y -site cations and the X -site cation, the charge of the X-site cation should affect the F occupancy at the W site. The cation and anion occupancy of this tourmaline is consistent with observations that tourmalines not dominated by X -site vacancies can have high F concentrations in the W site if F is present in the coexisting fluid phase. It is thus likely that the occurrence of high amounts of F in Fe-rich tourmalines requires a significant percentage of Fe 3+ in the tourmaline structure.

The effects of γ-radiation on LiCoO 2

Planetary explorations of human beings necessitate the research of the influence of γ-radiation on lithium-ion cells. The radiation effects on LiCoO 2 as the key component of a lithium-ion cell were investigated. Iodometric titration, Raman spectroscopy, X-ray diffraction were used to characterize the average valence of Co ions and structures. Galvanostatic cell cycling together with DC impedance measurement and cyclic voltammetry were used to characterize their electrochemical properties. It is found that, depending on the atmosphere, the radiation results in the changes in cobalt valence, the extent of Li–Co cation occupancy exchange, and the decrease in electrochemical capacity.