Site Preference Of Cations Research Articles

Persona of Transition Metal Ions in Solids: A Statistical Learning on Local Structures of Transition Metal Oxides.

The local structure of a transition metal (TM) ion is a function of cation elements and valence states. More than that, in this work, by employing a trove of first‐principles data of TM oxides, the local structures of TM cations are statistically analyzed to extract detailed information about cation site preference, bond length, site structural distortion, and cation magnetization. It is found that cation radius alone poorly describes the local structure of a transition metal oxide, while the statistics of coordination number as well as the TM—O bond length distribution, especially that of the 3d TMs, can provide comprehensive knowledge for understanding the behavior of TM elements. Based on these statistics, the interplay of site distortion due to the Jahn–Teller effect, cation site similarity, and a new set of ionic radii are all obtained to chart the “persona” of transition metal ions in solids.

Mixed (Ag+, Ca2+)-LTA zeolite with suitable pore feature for effective separation of C3H6/C3H8

Separating C3H6/C3H8 is a challenging task owing to their similar sizes and boiling points. In this study, we report a sequential and partial Ag+/Ca2+ ion-exchanged zeolite that exhibits ideal C3H6/C3H8 molecular sieving. Moreover, the zeolite exhibited superior C3H6 uptake of 2.37 mmol/g and molecular packing density (∼77% packing of liquid C3H6) at an extremely high C3H6: Ag+ ratio of 21.5 with Ag content as low as 1.18 wt%. The (Ag+, Ca2+) cations were sequentially exchanged into LTAs by comprehensively considering the extra-framework cation distribution and site preferences for different cations. By exchanging Ag+ with Na+ in the extra-framework of LTA, C3H6 can be admitted even at the average LTA apertures of sizes < 4 Å. Meanwhile, exchanged Ag+ also increased the adsorption kinetics of C3H6 by an order of magnitude. Hydrophobic modification on Ca-Ag-LTA was successfully achieved to significantly suppress the competition from water while retaining excellent regenerability in multiple cycles. The strategy of binary ion exchange to delicately tune the pore channels and chemistry and selectively admit molecules with beneficial kinetics might potentially be broadened to other industrial separation processes.

Charged Domain Wall and Polar Vortex Topologies in a Room-Temperature Magnetoelectric Multiferroic Thin Film.

Multiferroic topologies are an emerging solution for future low-power magnetic nanoelectronics due to their combined tuneable functionality and mobility. Here, we show that in addition to being magnetoelectric multiferroic at room temperature, thin-film Aurivillius phase Bi6TixFeyMnzO18 is an ideal material platform for both domain wall and vortex topology-based nanoelectronic devices. Utilizing atomic-resolution electron microscopy, we reveal the presence and structure of 180°-type charged head-to-head and tail-to-tail domain walls passing throughout the thin film. Theoretical calculations confirm the subunit cell cation site preference and charged domain wall energetics for Bi6TixFeyMnzO18. Finally, we show that polar vortex-type topologies also form at out-of-phase boundaries of stacking faults when internal strain and electrostatic energy gradients are altered. This study could pave the way for controlled polar vortex topology formation via strain engineering in other multiferroic thin films. Moreover, these results confirm that the subunit cell topological features play an important role in controlling the charge and spin state of Aurivillius phase films and other multiferroic heterostructures.

High Oxide-Ion Conductivity in a Hexagonal Perovskite-Related Oxide Ba7 Ta3.7 Mo1.3 O20.15 with Cation Site Preference and Interstitial Oxide Ions.

Solid oxide-ion conductors are crucial for enabling clean and efficient energy devices such as solid oxide fuel cells. Hexagonal perovskite-related oxides have been placed at the forefront of high-performance oxide-ion conductors, with Ba7 Nb4- x Mo1+ x O20+ x /2 (x= 0-0.1) being an archetypal example. Herein, high oxide-ion conductivity and stability under reducing conditions in Ba7 Ta3.7 Mo1.3 O20.15 are reported by investigating the solid solutions Ba7 Ta4- x Mo1+ x O20+ x /2 (x= 0.2-0.7). Neutron diffraction indicates a large number of interstitial oxide ions in Ba7 Ta3.7 Mo1.3 O20.15 , leading to a high level of oxide-ion conductivity (e.g., 1.08 × 10-3 S cm-1 at 377 °C). The conductivity of Ba7 Ta3.7 Mo1.3 O20.15 is higher than that of Ba7 Nb4 MoO20 and conventional yttria-stabilized zirconia. In contrast to Ba7 Nb4- x Mo1+ x O20+ x /2 (x= 0-0.1), the oxide-ion conduction in Ba7 Ta3.7 Mo1.3 O20.15 is dominant even in highly reducing atmospheres (e.g., oxygen partial pressure of 1.6 × 10-24 atm at 909°C). From structural analyses of the synchrotron X-ray diffraction data for Ba7 Ta3.7 Mo1.3 O20.15 , contrasting X-ray scattering powers of Ta5+ and Mo6+ allow identification of the preferential occupation of Mo6+ adjacent to the intrinsically oxygen-deficient layers, as supported by DFT calculations. The high conductivity and chemical and electrical stability in Ba7 Ta3.7 Mo1.3 O20.15 provide a strategy for the development of solid electrolytes based on hexagonal perovskite-related oxides.

Evolution of Thermoelectric Properties in the Triple Cation Zintl Phase: Yb13–xCaxBaMgSb11 (x = 1–6)

The band structure of Yb14MgSb11 is tuned by substituting the more earth-abundant cations, Ca and Ba, on the four crystallographically distinct Yb sites (Yb13–xCaxBaMgSb11 (x = 1, 2, 3, 4, 5, 6)). Single crystals of composition Yb9.7(2)Ca3.85(5)Ba0.29(4)Mg1.13(3)Sb11.0(1) were grown from Sn flux revealing the cation site preferences. Magnetic measurements on this crystal show paramagnetic behavior consistent with the presence of ∼0.85 Yb3+. High-purity samples (>98%) with compositions close to nominal of Yb13–xCaxBaMgSb11 (x = 1–6) were prepared by ball milling and spark plasma sintering. The carrier concentration can be rationalized with the presence of Yb3+ for all samples and decreases as a function of x in a systematic fashion at room temperature and increases above ∼600 K for x = 3–6. The temperature dependence of the carrier concentration can be understood considering the electronic structure with a light and heavy band valence band contributing to the properties and suggests the involvement of a localized flat band or impurity state that is active with increasing amounts of Ca. The effect of temperature leads to sustained high Seebeck coefficients with low electrical resistivity arising from the transitioning of the light to heavy band with localization of carriers in the flat band or impurity state for Ca-rich compositions. Speed of sound measurements show that the lattice stiffens with increasing x. Despite the stiffening lattice, the thermal conductivity decreases until x = 3, 4 at which point it increases slightly. The x = 4 sample reaches a peak figure of merit (zT) of 1.32 at 1273 K while being 16% lighter by the molar mass compared to Yb14MnSb11 thereby providing a more power dense material.

Cation site preferences in layered oxide chalcogenides, synthesis, structures and magnetic ordering in Sr3-xCaxFe2O5Cu2Ch2 (Ch = S, Se; x = 1, 2)

Solid solutions between the known compounds Ca2FeO3CuCh and Sr2FeO3CuCh (Ch ​= ​S, Se) in which there are two fairly similar sites (8 and 9 coordinate) for the alkaline earth cations are not attainable under standard high temperature solid state syntheses under thermodynamic control. Instead compounds with greater condensation of FeO5 square pyramids form as these afford one 8-coordinate site and one 12-coordinate site for the alkaline earths which is better suited to the size-mismatched cations in the compounds Sr3-xCaxFe2O5Cu2Ch2 (Ch ​= ​S, Se; x ​= ​1, 2). Sr2CaFe2O5Cu2S2, SrCa2Fe2O5Cu2S2, Sr2CaFe2O5Cu2Se2 and SrCa2Fe2O5Cu2Se2 all crystallise in the tetragonal space group I4/mmm with two formula units in the unit cell with the crystal structure first described for Sr3Fe2O5Cu2S2. Oxide slabs composed of vertex-sharing FeO5 square pyramids are separated by Cu2Ch2 anti-fluorite-type layers. The larger Sr2+ ions have a strong preference for the 12-coordinate site in the oxide slabs, while Ca2+ cations dominate the 8-coordinate sites separating the oxide and chalcogenide slabs. Powder neutron diffraction reveals that all the compounds display antiferromagnetic long range ordering of the Fe3+ moments with ordering temperatures well above room temperature and exceeding 526 ​K in the case of Ca2SrFe2O5Cu2Se2.

Multiferroic properties of three-layer Aurivillius compound Bi[formula omitted]TiFeNbO12: A first-principles and experimental study

We present a first-principles and experimental study of the structural, ferroelectric, and magnetic properties of the potentially multiferroic three-layer Aurivillius compound Bi4TiFeNbO12. This system can be realized by inserting a BiFeO3 formula unit into the two-layer Aurivilius Bi3TiNbO9 matrix. The calculations are performed using the PBEsol exchange–correlation functional within the DFT+U framework. First we search for potential cation site preference by comparing the relative stability of different Fe, Nb and Ti arrangements. We find a preference for the Fe3＋ ions to occupy the inner site within the pseudoperovskite block. This configuration exhibits a band gap of 1.2 eV (UFe=4eV) and ferroelectric and magnetic orders. A value of 66 μC/cm2 is obtained for the spontaneous polarization, which is similar to the one obtained for Bi4Ti3O12 (BIT). The magnetic ground state of this system is characterized by a strong antiferromagnetic coupling between the Fe3＋ ions located in the central layer. By mapping to a Heisenberg model, the superexchange antiferromagnetic coupling between nearest-neighbor Fe3＋ cations is estimated to be J=53 meV. Finally, we synthesize Bi3.25La0.75TiFeNbO12 ceramics by the solid-state reaction method. Their structural, electric and magnetic properties are confronted with the theoretical predictions.

Formation energies and site preference of substitutional divalent cations in carbonated apatite

AbstractFirst‐principles calculations were performed to examine defect formation energies and site preference of substitutional divalent cations M2+ (M = Mg, Cu, Zn, Cd, Sr, Pb, and Ba) in hydroxyapatite (HAp, Ca10(PO4)6(OH)2) and carbonated apatite (CAp). All inequivalent substitutional sites of and M2+ were investigated to determine their most preferential sites. For all M2+ studied, their defect formation energies for the most stable substitutional sites were lower in CAp than in hydroxyapatite (HAp), demonstrating that M2+ are preferentially substituted into CAp over HAp. For Ca sites in CAp, correlations between the defect formation energies and Ca‐O bond lengths showed that bigger and smaller M2+ than Ca2+ are preferentially substituted for Ca sites with longer and shorter bond lengths than those in HAp, respectively. In addition, Ca sites with lower coordination numbers than 6 are preferentially substituted by Zn2+ and Cu2+ that originally tend to form 4‐ or 5‐fold coordination in their phosphate crystals. substitution is therefore likely to effectively stabilize substitutional foreign ions by modifying bond lengths and coordination numbers of Ca sites from those in pure HAp. These effects may play an important role in enhancing the M2+ solubility into CAp.

Characterization of spinel-type Cd1−xCoxCr2O4 nanocrystals by a microwave-combustion synthesis

Spinel nanocrystals of Cd1−xCoxCr2O4 (0 ≤ x ≤ 1) solid solution are successfully synthesized by a microwave combustion method and their properties are systematically studied. Single phase spinel structure is determined for all the synthesized compositions by x-ray diffraction (XRD) and the two vibrational bands of metal-oxide bonds are assigned by Fourier transform infrared spectroscopy (FTIR). Energy dispersive x-ray spectroscopy (EDX) indicates almost stoichiometric compositions in the whole range of x of the synthesized Cd1−xCoxCr2O4 nanoparticles. Crystallite sizes of 6 – 10 nm estimated from XRD patterns are in consistence with sizes of cubic nanocrystals observed by transmission electron microscopy (TEM). The lattice constant behavior with x shows a deviation from Vegard’s law with a kink anomaly at x ∼ 0.5 that may be attributed to different site-preferences of divalent cations in the two spinel sublattices. Optical absorbance measurements reveal a wide energy gap of 3.5 eV for CdCr2O4 nanoparticles that is smoothly narrowed by Co-substitution for Cd to become 3 eV in pure CoCr2O4 with particular optical characteristics. Magnetization isotherms measured at 300 K confirm the parmagnetic behavior, while those measured at 2 K show hysteresis evolution from a suppressed antiferromagnetic state to a ferrimagnetic state with maximum coercivity for Cd0.7Co0.3Cr2O4.

Controlling the Electrochemical Properties of Spinel Intercalation Compounds

Spinel intercalation compounds are well-known to facilitate high rate and high voltage Li-ion batteries. Little is known, however, about spinel based electrodes for Na- and Mg-ion batteries. Here we systematically study six spinel chemistries with first-principles statistical mechanics approaches to determine how the electrochemical properties of spinel compounds are affected by (i) host ionicity, (ii) guest cation radius, and (iii) guest cation oxidation state. As model systems, we consider spinel CoO2 and TiS2 as hosts and Li, Na, and Mg as guest cations. Electrochemical properties of spinel compounds are strongly dependent on guest cation site preferences. Our study shows that large guest cations prefer octahedral coordination, while the electrostatic interactions in ionic hosts favor tetrahedrally coordinated guest cations. The insights of this study suggest simple design principles with which new spinel intercalation compounds can be developed, with our predictions indicating that transition metal oxide spinel compounds containing Co, Ni, and/or Mn could enable high rate Na-ion batteries.

Rietveld, cation distribution and elastic investigations of nanocrystalline Li0.5+0.5xZrxFe2.5-1.5xO4 synthesized via sol-gel route

The present report deals with the structural and elastic behavior of Li+ and Zr4+ substituted spinel ferrites. Nanocrystalline spinel ferrite system of chemical formula Li0.5+0.5xZrxFe2.5-1.5xO4 (where, x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) was synthesized by using sol-gel auto combustion method. The as-prepared powder was annealed at 800 °C for 5 h for better crystallinity. The annealed samples were characterized by X-ray diffraction technique and Fourier transform infrared spectroscopy for structural and elastic studies respectively. By applying the full pattern fitting of Rietveld method the x, y, z coordinates of atoms, unit cell dimensions and occupancy of ions were estimated. The lattice parameter was found to decrease after substitution due to increase in electron affinity. Site preferences of cations were obtained by matching experimental and calculated intensity. The characteristic bands of crystallographic sub-lattice for the cubic structure were observed by FTIR spectra.

Structural and magnetic studies on Mn-doped Ni–Zn ferrite nanoparticles

Ni–Zn ferrite nanoparticles with manganese substitutions, Ni0.4Zn0.6−x Mn x Fe2O4, where x varies from 0.00 to 0.25 in steps of 0.05, were prepared using sol–gel autocombustion method. The resultant nanoparticles were analyzed using X-ray diffractometry, transmission electron microscopy, thermogravimetry and vibrating sample magnetometer measurements. X-ray diffraction patterns confirm single-phase cubic spinel structures with crystallite sizes in the range from 25.43 to 37.23 nm. Particle sizes obtained from transmission electron micrographs were also in good agreement with the crystallite sizes obtained from the X-ray data. Saturation magnetization and coercivity data for all the samples obtained from the corresponding M–H loops suggest a sinusoidal variations with Mn concentration (x) implying that the substitution of Mn in the given nano-sized Ni–Zn ferrite system seems to have brought in a profound influence not only on the site preferences of other cations in the system but also in bringing about modifications in its own site preferences. The results are analyzed in terms of the compositional and structural modifications, and it was found that the observed magnetic variation was governed by the crystallite size and the corresponding change in cationic preferences in these materials.

Microwave properties of BaFe11Mg2+0.25X2+0.25Ti4+0.25O19 (X2+=Cu, Mn, Zn, Ni and Co) nanoparticles in 0–26.5 GHz range

The structural, magnetic and microwave properties of cation substituted barium hexaferrite nanoparticles were investigated. Samples having composition of BaFe11Mg0.252+X0.252+Ti0.54+O19 (X2+=Cu, Mn, Zn, Ni and Co) were synthesized with solid state reaction route and 1wt% boron was added as a catalyst to initialize the crystal growth. XRD analysis showed that hard hexaferrite phase was successfully obtained. The lattice parameters “a” and “c” are almost the same in all substituted ions except Cu2+, in which elongation in c-axis was detected. The highest saturation magnetization occurred in Ni2+ and the highest coercivity was observed in Cu2+ substituted samples. This was explained by site preferences of cations which has direct influence on strength of Fe–O–Fe superexchange interactions. The both ac (σac) and dc (σdc) conductivities have temperature dependency and vary with substitution of Cu2+, Zn2+, Co2+, Mn2+ and Ni2+ ions. The maximum ac conductivity was observed as 2.39×10−6Scm−1 in Ni2+ substituted sample at 120°C and at 10Hz. The increase in dielectric constants (ε′, ε″) with variation of substituted divalent ions was explained by reduction of electron hopping between Fe2+ and Fe3+ ions. The absorbtion of incident microwave occurs at two different frequencies, around 10 and 20GHz. The reflection loss close to 10GHz was observed in all samples, whereas Mn, Zn and Ni substituted samples have the secondary reflection minima. The mechanism of reflection loss at low frequency is quarter wave cancellation at matching thickness. And, at high frequency, it is due to the natural resonance and dipole relaxation of fillers. The best absorbtion capability was obtained in Mn2+ substituted sample, which has reflection losses of −30 and −29dB at 9.5 and 19.2GHz.

Itinerant antiferromagnetism inFeMnP0.8Si0.2single crystals

Compounds based on the Fe2P structure have continued to attract interest because of the interplay between itinerant and localized magnetism in a non-centrosymmetric crystal structure, and because of the recent developments of these materials for magnetocaloric applications. Here we report the growth and characterization of mm size single crystals of FeMnP0.8Si0.2. Single crystal x-ray diffraction, magnetization, resistivity, Hall and heat capacity data are reported. Surprisingly, the crystals exhibit itinerant antiferromagnetic order below 158 K with no hint of ferromagnetic behavior in the magnetization curves and with the spins ordered primarily in the ab plane. The room temperature resistivity is close to the Ioffe-Regel limit for a metal. Single crystal x-ray diffraction indicates a strong preference for Mn to occupy the larger pyramidal 3g site. The cation site preference in the as-grown crystals and the antiferromagnetism are not changed after high temperature anneals and a rapid quench to room temperature.

Structure evolution in "114" oxides CaBaZn₂Ga(2-x)Al(x)O₇ (x = 0, 1, 2) and layered cationic ordering in tetrahedral sites for CaBaZn₂Al₂O₇.

Rietveld refinements were performed on CaBaZn2Ga(2-x)Al(x)O7 (x = 0, 1, 2) to investigate the site preference of cations in tetrahedral cavities. Tri-valent Ga(3+)/Al(3+) preferred the tetrahedral sites (T1-sites) within the triangular layers. Moreover, a layered type cationic ordering was observed in CaBaZn2Al2O7, where all the T1-sites were occupied by Al(3+). This represents the first example of a cationic ordering in "114" compounds. By substituting Al(3+) into CaBaZn2Ga2O7, the major change in the structure was the shrinkage of the T1O4 tetrahedrons, especially the shortening of the T1-O2 bond distance along the c-axis, which led to an anisotropic shrinkage of the unit cell. In the literature, such an anisotropic change of unit cell would induce a structure distortion, as well as a decrease in the symmetry from P6₃mc to Pbn2₁. All the title compounds crystallized in P6₃mc, which is unexpected. A comparison with the selected "114" magnetic oxides help us to confirm that the strong antiferromagnetic interactions of magnetic lattices would also be beneficial for symmetry lowering. Overall, the factors affecting cationic ordering on T1-sites and symmetry lowering are discussed for the title compounds, which may also be applicable to other "114" oxides.

ZnxCu(1−x)Fe2O4 Nanoferrites by Sol–Gel Auto Combustion Route: Cation Distribution and Microwave Absorption Properties

The effect of Zn doping on magnetic properties and site preference of cations of CuFe2O4 has been investigated. Zn substituted Cu ferrite nanoparticles with general formula ZnxCu(1-x)Fe2O4 were syn ...

Study of Y-type Hexaferrite by Means of ^{57}Fe NMR and Electronic Structure Calculations

The electron structure and site preferences of Zn and Fe cations in Y hexaferrite system were calculated. The hyper ne magnetic elds on Fe nuclei were determined using WIEN2k and corrections for hyper ne contact interaction. The calculated elds were compared to Fe nuclear magnetic resonance (NMR) experiment in Ba2Zn2Fe12O22 single crystal with an aim of interpretation of experimental NMR spectrum.

Prediction of Quaternary Spinel Oxides as Li-Battery Cathodes: Cation Site Preference, Metal Mixing, Voltage and Phase Stability

Using first-principles calculations, we explore the quaternary spinel oxides of the 3d transition metal series (Ti, V, Cr, Mn, Fe, Co, Ni) having pairs of transition metals in equal concentrations. We examine several properties of these materials relevant to Li-ion battery cathode applications. By examining cation site preference, delithiation voltage and thermodynamic stability, we predict five new quaternary spinel compounds which show promise as novel cathode materials: LiTiFeO4, LiCrFeO4, LiMnFeO4 and LiFeCoO4 and LiCoNiO4. The predicted compounds have tetrahedral Li/vacancy sublattice for efficient Li-transport, average voltage greater than 4.4 V, and are predicted to be stable ground states on a T = 0 K computed phase diagram. These predictions should be tested experimentally by synthesis and electrochemical measurements of these five compounds. For the lithiated oxides, we find predominant preference for the normal spinel structure (with Li in tetrahedrally coordinated sites), and for most of the delithiated oxides, transition metal cations spontaneously relax to non-spinel octahedral sites. We also find that metal mixing in these spinels is a strong function of charge state: most of the pairs of ternary transition metal oxide spinels tend to mix when lithiated and phase separate when delithiated.

Impact of transition metal doping on Mössbauer, electrical and dielectric parameters of structurally modified lithium ferrite nanomaterials

Zr–Mn doped spinel lithium ferrites Li0.5Fe2.5−2xZrxMnxO4 (0.0 ≤ x ≤ 0.5) are synthesized using the citrate precursor method. The spinel ferrite is formed at a relatively lower annealing temperature (873 K) compared to those synthesized by other conformist methods. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis are carried out to determine the cell parameters, crystallite size and grain growth. Cation distribution and site preferences for the doped ions are determined by Mössbauer spectroscopy at room temperature. The impact of doping of Li0.5Fe2.5O4 with the binary mixtures of transition metals (Mn, Zr) on hyperfine interaction parameters (δ, Δ and Hint), electrical resistivity (ρ), dielectric constant (έ) and dielectric loss tangent (tan δ) over the frequency range of 100 Hz to 3 MHz is discussed in details. Zr–Mn doping enhanced the DC electrical resistivity and decreased the loss tangent value which is considered useful for technological application in microwave and telecommunication devices.

Magnetic properties of Ni–Co doped barium strontium hexaferrite

A series of Ni–Co substituted barium strontium hexaferrite materials, Ba0.5Sr0.5NixCoxFe12–2xO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8 mol%) was synthesized by the sol–gel method. X-ray diffraction analysis has shown that the Ni–Co substitutions maintain in a single hexagonal magnetoplumbite phase. The room temperature magnetic properties and the cation site preferences of Ni–Co substituted ferrite were investigated by VSM. Substitutions led to decrease in coercivity while saturation magnetization remains the almost same. It indicates that the saturation magnetization (52.81–59.8 Am2/kg) and coercivity (69.83–804.97 Oe) of barium strontium hexaferrite samples can be varied over a very wide range by an appropriate amount of Ni–Co doping contents.