Cation Site Occupancy Research Articles

Nanocrystalline oxide zinc materials studied by the Mössbauer effect, μSR and neutron diffraction

ZnO and ZnFe2O4 were investigated in nanocrystalline form using various hyperfine techniques in conjunction with neutron diffraction and bulk magnetic measurements. For the zinc oxide, the main interest was the influence of the nanocrystalline state on lattice dynamics and on the local symmetry of the Zn atom as reflected in its quadrupolar coupling, employing the high-resolution Mossbauer resonance of 67Zn. Measurements were carried out on nanocrystalline materials with particle sizes between 4 and 25 nm. It was found that the asymmetry parameter increases drastically from η=0 in ZnO single crystals to η≈0.5 in nanostructured material, indicating an off-axis displacement of either Zn or O atoms in finite size grains. The dependences of the Lamb-Mossbauer factor on grain size and temperature are well described by a two-component model in which small crystallites are surrounded by a network of grain boundaries. For the zinc ferrite the question of changes in the magnetic properties stood in the foreground. Using the combination of techniques mentioned, the magnetic behavior of nanocrystalline ZnFe2O4, which had an average particle size of about 9 nm was compared to differently prepared crystalline samples. It was seen that the role of cation-site occupation, which changes substantially with nanocrystallinity, plays a decisive role for the magnetic properties.

New Force Field for Na Cations in Faujasite-Type Zeolites

We have developed and validated a new force field for cations in zeolites, which explicitly distinguishes Si and Al atoms, as well as different types of oxygens in the framework. Our new force field gives excellent agreement with experimental data on cation positions, site occupancies and vibrational frequencies. Energy minimizations show that Na cations in site I are not at the centers of hexagonal prisms, but rather are in one of two symmetric SI sites displaced by about 0.6 Å along the [111] direction. Molecular dynamics (MD) simulations show that most cations are immobile in Na−X and Na−Y on the MD time scale, even at 1000 K. Only Na−X cations in site III‘ exhibit diffusive motion at 1000 K, with a self-diffusivity from MD of 3.6 × 10-10 m2 s-1. The MD simulations also show that cation movement is highly correlated, composed of jumps involving at least 4 cations or more.

Multiple cation sites in dehydrated brewsterite. An in-situ X-ray synchrotron powder diffraction study

The dehydration process of the zeolite brewsterite, Sr 0.67Ba 0.33Al 2Si 6O 16·5H 2O, has been studied in situ by X-ray synchrotron radiation powder diffraction. Sixty-one consecutive powder patterns were collected between 310 and 684 K, in steps of 6 K, at 5 min intervals, and the crystal structures were refined using the Rietveld method. At 310 K, the original cation and four water sites are fully occupied. As water is expelled, three alternative cation sites successively appear. These cation sites generally have more framework oxygens in their coordination spheres, and/or show an increasing ability to share water molecules between themselves. Most aspects of the brewsterite dehydration process can be related to the variations in the cation site occupancies. The total water content during the dehydration can be fully modelled, based on the occupancies of two of the cation sites. Also, the maxima in the dehydration rate and the reduction in unit cell volume are strongly correlated to cation site occupancies. The connection between the tetrahedral sheets building the brewsterite framework, the T4O9T4 angle, is reduced by 25° between 550 and 610 K. The strain at this ‘weak link’ is believed to be the direct cause of the destruction of crystallinity, whereas the clustering of cations inherent to an effective water sharing is considered as the cause of this framework strain.

Electrochemical Cycling‐Induced Spinel Formation in High‐Charge‐Capacity Orthorhombic LiMnO2

spinel normally undergoes a transformation from its cubic to tetragonal phase when x exceeds 1 due to a collective Jahn‐Teller distortion, resulting in poor cyclability when both the 4 and 3 V intercalation plateaus are utilized. In this study, we show that this transformation is suppressed in spinels of composition up to x ≈ 2 obtained through the electrochemical cycling of orthorhombic . X‐ray diffraction, transmission electron microscopy, and high‐resolution electron microscopy studies together show that cycling produces a cubic spinel containing partial tetrahedral cation site occupancy and a nanodomain structure (20 to 50 nm size) within parent single‐crystalline oxide particles. This structure is responsible for the cycling stability of electrochemically produced spinel. The reversible capacity (272 mAh/g) and energy density (853 Wh/kg) achieved at a low charge‐discharge rate (3.33 mA/g) in the present samples are the highest among crystalline materials reported to date. © 1999 The Electrochemical Society. All rights reserved.

Investigations on Zeolite-Based ER Fluids Supported by Experimental Design

The application of experimental design for determining the influence of various parameters on rheological properties of electrorheological (ER) fluids is demonstrated. Such statistical methods allow the quantitative determination of parameter effects even in the presence of considerable property fluctuations of the ER fluid as well as the calculation of interactions between the parameter effects. They thus provide a powerful optimization tool. Investigations have been performed on ER fluids containing particles of zeolite A in silicone oil. The influence of zeolite content, oil viscosity, particle size, cation composition and ER fluid temperature on the viscosity of ER fluids with and without electric field has been studied. In addition to the main effects of the parameters interactions between them also play a considerable role. The ratio of ER fluid viscosities with and without field, respectively, decreases with rising oil viscosity and increases with temperature. The larger value of the ratio for ER fluids with smaller particles is referred to different particle structures. Results of parameter effects on viscosity of the ER fluid in electric fields depend on the shear rate. Cation exchange of sodium to potassium has only a minor influence on ER activity. This result is compared with cation exchange with calcium, by which ER activity is drastically diminished. The loss of activity can be related to the occupation of different cation sites in zeolite A.

Ab initio structural approach on polycrystalline Y2 – x Ca x BaNiO5 (0 ≤ x ≤ 0.33) compounds

Abstract The procedure of ab initio structure solution from conventional X-ray powder diffraction data is successfully applied to a series of Y2–x Ca x BaNiO5 compounds (0 ≤ x ≤ 0.33). The study aimed at understanding if the presence of dopants in polycrystalline Y2BaNiO5 compound can influence the success of the structure determination process and at quantifying the substitutional defect. The new code EXPO has been used to obtain the most accurate set of structure factor moduli and to perform the direct methods procedure. It is demonstrated that the substitution on a crystallographic site, as for the above mentioned compound, does not compromise the performance of direct methods. The structural parameters obtained for samples with different x values are compared with those reported in the literature. It is pointed out that the knowledge of the dependence of lattice parameter and cell volume values on the cation site occupancy factors allows the direct determination of the degree of substitution of calcium on yttrium site of the sample.

Thermal stabilisation of alumina by lanthanide ions

The effect of additions of lanthanide ions (La and Ce) and of the cerium ion concentration on the thermal stability of a γ alumina was studied. Ln//Al2O3 compositions were treated at high temperature (1100°C) for different calcination times (1, 5, and 24 h) and the resultant materials characterised by X-ray diffraction (XRD), BET surface area, X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction (TPR). It was observed that cerium ion stabilisation was dependent on concentration with 6 wt-%Ce being the most efficient. It is suggested that the stabilisation is due to occupation of vacant cation sites in the spinel structure by Ce3+, and that stabilisation breaks down on prolonged exposure at high temperatures because of oxidation of Ce3+ to CeO2 . The same stabilisation mechanism is considered to operate for the lanthanum ion but, unlike Ce3+, it is not subject to oxidation, making La a more effective stabiliser than Ce.

Czochralski growth of RE 3Ga 5SiO 14 (RE=La, Pr, Nd) single crystals for the analysis of the influence of rare earth substitution on piezoelectricity

Pr 3Ga 5SiO 14 and Nd 3Ga 5SiO 14 single crystals with constant diameter of 22 mm and lengths up to 145 mm have been grown by the Czochralski method. The phase identification, site occupancy of cations and axial lattice parameter distribution were determined by X-ray analysis. The transmission spectra within the 340–3200 nm wavelength region were measured. The centre of interest are the piezoelectric properties of (2 1̄ 0) and (0 1 0) plates in comparison with former grown La 3Ga 5SiO 14 crystals in order to find out the influence of the rare earth substitution of La 3+ by Pr 3+ and Nd 3+ on the piezoelectric strain constant d 11. A decrease of | d 11| with increasing atomic number was found giving the hint to the substitution of lanthanum by further elements with larger atomic radii.

Comparative Zirconium Bromide Cluster Chemistry. A New Structure in K4Zr6Br18C

Reactions of ZrBr4, Zr,ABr (A=Li–Cs, Ba), andZ(Be, B, C, Mn) mixtures with suitable compositions in sealed Ta containers have revealed 19 additional examples ofAx(Zr6Br12Z) Bry-type phases that span 0≤y≤6. A new triclinic structure is reported for K4Zr6Br18C (P1,Z=1,a=10.114(2),b=10.283(3),c=10.374(3),α=118.54(2),β=99.98(2),γ=104.08(2),R(F)/Rw=5.1/4.7%) and K4Zr6Br18Fe. Nearly octahedral Zr6(C)Br4−18clusters stack in layers with K+in pseudo-tetrahedral bromine cavities on their outward surfaces. The present structure can be converted to a pseudo-C-centered monoclinic version that is related to those of Rb4Zr6Cl18C (C2/m) and, more distantly, Rb5Zr6Cl18B (Pnam) according to cluster size and the need for good bonding sites for the cations. A variety of other new bromide phases that form in seven known zirconium cluster chloride or iodide structure types according to powder pattern or single crystal data is also reported. Some of these demonstrate greater, lesser, or alternative cation site occupancies in the same basic structure types.

Processing and cation redistribution of MnZn ferrites via high-energy ball milling

We have produced MnZn ferrites via high-energy ball milling (HEBM) of elemental oxides MnO, ZnO, and Fe2O3. X-ray diffraction (XRD) indicates a pure phase spinel forms after 21 h of HEBM. Extended x-ray absorption fine structure (EXAFS) analysis shows a nonequilibrium cation distribution, with an unusually high population of Zn cations on the octahedral sublattice. We then used EXAFS modeling to study cation site occupancy in an equilibrium MnZn-ferrite standard subjected to HEBM. We found that HEBM produces an increased preference for octahedral-site occupation among all cations for milling durations up to 300 min. After an initial improvement in magnetic properties, the magnetization diminishes steadily throughout this interval. With further increases in milling duration these structural and magnetic trends reverse, possibly due to annealing effects evidenced by XRD.

Lattice site determination of Cr in low doped lithium niobate single crystals using PIXE/channeling

The lattice sites of Cr have been determined in LiNbO 3:Cr doped with 0.1 mol% Cr 3+ as well as in LiNbO 3:Cr,Mg doped with 0.1 mol% Cr 3+ and 6.0 mol% MgO in melt, using PIXE/channeling. In the case of LiNbO 3:Cr for the first time the occupation of both regular cation sites (60% on Li sites and 40% on Nb sites) has been observed by channeling for a trivalent impurity. For LiNbO 3:Cr,Mg a significantly reduced fraction of Cr (20%) is located on Li sites. The major fraction (80%) occupies an interstitial site shifted 0.1 Å from the regular Nb site towards the center of the octahedron.

X-ray diffraction study on the mixed alkalialkaline earth effect in Ba, Na metaphosphate glasses

In ternary Ba, Na phosphate glasses, the mixed oxide effect was studied along the join of the metaphosphate members BaP 2O 6 and NaPO 3 by means of X-ray diffraction. The oxygen polyhedra of the modifier cations Na + and Ba 2+ are deformed if compared with those in the end members. This behaviour is expected according to the competition for the oxygens by the Ba 2+ and Na + cations. The preference of dissimilar pairs of cations is found to be a small effect. The number of such Na-Ba neighbours at distances of about 0.40 nm does not deviate much from that expected for a random occupation of cationic sites. A discernible feature in the scattering intensities of the single glasses, the prepeak or shoulder in front of the first largest maximum, is not detected in the scattering of the mixed glasses. The origin of this prepeak was previously deduced to be due to the mutual ordering of the phosphate chains (and/or rings) and the cation-centered oxygen polyhedra. However, at this time no explanation for the absence of the prepeak in the X-ray scattering functions of the mixed glasses can be proposed.

Analysis and Structural Determination of Nd-Substituted Zirconolite-4M

The structure of a new polytype of zirconolite, zirconolite-4M, has been determined using X-ray and neutron powder diffraction, high resolution transmission electron microscopy, and selected area electron diffraction. Zirconolite-4M occurs when zirconolite is doped with 0.5–0.8 Nd per formula unit. Its structure consists of four hexagonal tungsten bronze (HTB) type layers interleaved alternately with layers of Ca, Zr polyhedra (as in zirconolite-2M) and Ca, Ti polyhedra (as in pyrochlore). Nd substitutes on the Ca and Zr sites. The compositions of the zirconolites were determined using an extrapolation technique based on an analysis of the impurity lines in the diffraction pattern. Cation site occupancies were determined with composition constraints applied and these were consistent with the expected zirconolite-4M cation site occupancies. Observed zirconolite-4M lattice parameters correlated with expected values for a zirconolite and pyrochlore type stacking sequence.

Rietveld X-ray Powder Profile Analysis and Electrical Conductivity of Fastion Conducting Gd2(Til-ySny)2O7 Solid Solutions

AbstractThe A3+2B4+2O7 pyrochlores have fast-ion conduction properties that make them attractive candidates for applications in fuel cells. Rietveld powder-profile analysis of x-ray diffraction data has been used to determine structural parameters for Gd2(Ti1-ySny)2O7 solid solutions (y = 0.20, 0.40, 0.60, and 0.80) for correlation with composition-dependence of the specimens' electrical conductivity. In accord with Vegard's law, the lattice constants increase linearly with y as the larger Sn ion replaces Ti4+. The structures in the system remain remarkably ordered in view of the fact that it had been suggested that increasing the average radius of the ions that occupy the B4+ site relative to A3+ drives the system toward a disordered defect-fluorite state. A small but significant increase in mixing of the occupancy of the cation sites was found with increasing y to a maximum of [GdB] = 0.05. We could detect no significant disorder in the anion array. The activation energy and pre-exponential term for oxygen ion conduction were found to increase linearly and exponentially with y, respectively, such that at a given temperature, the ionic conductivity changes by less than an order of magnitude across the system. The behavior is in marked contrast to other systems where the substitution of a larger cation in the B site increases conductivity by up to three orders of magnitude as a consequence of substantial disorder that is introduced in both the cation and anion arrays.

Fast-Ion Conducting Y2(ZryTi1-y)2O7 Pyrochlores: Neutron Rietveld Analysis of Disorder Induced by Zr Substitution

The structure of Y2(ZryTi1-y)2O7 solid solutions progressively changes, with increasing y, from an ordered pyrochlore structure A2B2O7□, space group Fd3m, to a deflect-fluorite structure (A, B)4 (O0.875□0.125)8, space group Fm3m, at y = 0.90. The anion array consists of three independent sites O(1), O(2), and O(3), occupying positions 48f, 8a and 8b, respectively, of which 8b is unoccupied in a fully ordered pyrochlore. Rietveld powder-profile analysis of data collected with 1.5453-Å thermal neutrons was used to determine the structural state of four samples with increasing Zr content (y = 0.30, 0.45, 0.60, and 0.90). Refinements that employed only the pyrochlore superstructure intensity data provided weighted profile residuals that ranged 8.06 to 8.67% compared with expected values of 7.13 to 7.87% derived from counting statistics. The onset of disorder at y = 0.30 is marked by filling of the vacant 8b site with oxygen ions displaced from the nearest-neighbor anion shell—i.e., O(1) in 48f. Only for y > 0.45 does O(2) participate in the disorder. Mixing of the occupancy of cation sites A and B begins only with under occupancy of the O(2) site. The eight-coordinated A site, position 16c, is occupied solely by Y for y ≤ 0.45 and is predominantly Y at y = 0.60. The substituted Zr4+ thus replaces Ti4+ in the six-coordinated B site for most of the solid-solution series, Complete mixing of all three cation species occurs abruptly over the narrow compositional range 0.60 < y ≤ 0.90. The disordered structural states cannot be described by a single-order parameter. The O(1) ion is displaced 0.46 Å toward the vacant O(3) site in the ordered pyrochlore at y = 0. The displacement relaxes to the ideal position of the fluorite structure with a quadratic dependence on y. Increase of the anisotropic temperature-factor coefficients with y indicates general softening of the structure with increased disorder. The array of O(1) ions forms a continuous path for migration of anions through the structure. The magnitude and composition dependence of the reported change of ionic conductivity with y may be satisfactorily explained by the variation of the product of charge-carrier concentration and vacancy concentration N(N - 1), where N is taken as the site occupancy of O(1).

Identification of crystalline structures using Mössbauer parameters and artificial neural network

Mossbauer spectroscopy is a useful technique for characterizing the valencies, electronic and magnetic states, coordination symmetries and site occupancies of Fe cations. The Mossbauer parameters of Isomer Shift (I.S.) and Quadrupole Splitting (Q.S.) are useful to distinguish paramagnetic ferrous and ferric ions in several substances, while the internal magnetic field provides information on the crystallinity. A correlation is being sought between Mossbauer parameters and several structure properties of some iron-containing minerals using Artificial Neural Networks (ANN). Distinct regions of crystalline structures are defined when any two parameters are plotted, but in several cases superposition of these regions leads to erroneous conclusions. We have tried to eliminate this difficulty by using convenient axes. These axes form n-dimensional vectors as input to our ANN. In recent years ANN has shown to be a powerful technique to solve problems as pattern recognition, optimization, preview ups and downs in stock market, automatic control and identification of a mineral from a Mossbauer spectrum or Mossbauer data bank. Using ANN we have been successful in identification of crystalline structures from plots of Mossbauer spectral parameters of I.S., Q.S., and structure properties of mean metal-oxygen distance in coordination site. Results using ANN in identification of crystalline structures using Mossbauer parameters of I.S., Q.S., and polyhedral volume of a coordination site are presented.

Investigation of Fe structural environments in “leucite”-type framework silicates using a combination of Mössbauer and X-ray absorption spectroscopies

Synthetic iron-containing leucite-type phases have been studied using57Fe Mossbauer spectroscopy in combination with X-ray absorption spectroscopy (XAFS and XANES). The Mossbauer and XAS data confirm that both Fe2+ and Fe3+ occupy tetrahedrally coordinated framework sites in the synthetic phases. In addition, information on Fe2+/Fe3+ ratios, cation site occupancies and first shell bond lengths has also been obtained.

Distribution of Iron Cations in Natural Chromites at Different Stages of Oxidatio - A57Fe Mossbauer Investigation

Abstract 57Fe Mossbauer spectroscopic investigation of natural chromites from two chromite deposits of India (Sukinda and Byrapur) documents partly inverse spinel structure arising out of oxidation. The spectral fitting was based on allowing a disordering distribution of Fe2+ and Fe3+ ions at tetrahedral (A) and octahedral (B) sites. Mossbauer investigation of the samples taken from the physico-chemically distinct two horizons of Sukinda viz. brown are and grey ores, and Byrapur area revealed three types of iron ion distribution as: Fe2+(A), Fe3+(A) and Fe2+(B) (GC-group); (b) Fe3+(A1), Fe3+(A2) and Fe2+(B) (BC-2 group), and (c) Fe3+(A1), Fe3+(A2) and Fe3+(B) (BC-1 group). The distribution pattern of iron cations at A and B sites was linked to the degree of oxidation. The stages of oxidation could be modelled from normal to inverse fonn. A model suggesting 'electron localisation' at the B-sites makes the intermediate stage. Iron site occupancy determined by Mossbauer spectroscopy of the presently studied samples indicates that these fall under three groups of oxidation stages. An early stage of oxidation is shown by samples of group GC, intermediate stage by group BC-2 and final stage by BC-1 group of chromite samples. The imprint of progressive oxidation manifested by Fe cation site occupancy has been correlated with the Fe2+/S:Fe ratios, obtained for each group of samples.

The magnetic and structural properties of pulsed laser deposited epitaxial MnZn–ferrite films

The magnetic and structural properties of pulsed laser deposited MnZn–ferrite films have been examined. The results show that the uniaxial anisotropy, ferromagnetic resonance linewidth and coercive force are strongly influenced by the microstructure of the films, and the saturation magnetization and first-order magnetocrystalline anisotropy constant depend on intrinsic properties such as composition and cation site occupation. A comparison of bulk and film magnetic properties shows that the magnetic properties of the films are comparable to the bulk, which makes pulsed laser deposition ferrite films a prime candidate for thin film high-frequency microwave device applications.

Mössbauer spectroscopy and structural analysis of solids

Mossbauer spectroscopy is reviewed as a method of analysis of hyperfine interactions in the solid state. It is sensitive both to the atomic scale and to phase structures. It utilizes the interactions between the hyperfine fields and nuclei in solids measured by a nuclear technique. The importance of various Mossbauer isotopes is discussed, the 57Fe being still the most important. Principles of the qualitative determination of the structure sites and/or phase attachment are explained on the basis of the measurement of hyperfine structure parameters (i.e. the isomer (chemical) shift, the quadrupole and magnetic splittings). The role of the hyperfine field distribution determination is stressed, especially the magnetic hyperfine induction distribution in magnetically ordered solids. Conditions are explained for the feasibility of quantitative estimations of site occupancy and phase abundance. With respect to the predominant role of the magnetic hyperfine structure predestinating Mossbauer spectroscopy to be considered simultaneously as a special magnetic measuring technique, examples are chosen from the field of new magnetic materials. For the substituted hexagonal (M-type) ferrites (aimed, e.g., for the perpendicular magnetic recording), Mossbauer determination of the cation site occupancy is discussed. Structural changes in ion implanted Fe-B-based amorphous alloys detected by the hyperfine field distribution are shown. For the magnetically extremely soft FeCuNbSiB alloys, produced by the controlled crystallization of an amorphous ribbon, the estimation of their rather complicated phase composition by the Mossbauer phase analysis is demonstrated.