Ionic Size Difference Research Articles

Observation of Predominant Long-Range Ordering by Overcoming the Magnetic Frustration in Cu-Doped La2MnCoO6

We have investigated the effect of copper substitution on the structure, cation valence, and magnetic properties of the double perovskite La2MnCoO6. La2Mn1–xCuxCoO6 (0 ≤ x ≤ 0.4) were synthesized by the sol–gel techniques and characterized by PXRD, FE-SEM, EDX, XPS, and both the dc- and ac-magnetization. There is a structural transition for copper substitution, where the monoclinic structure of the parent La2MnCoO6 transformed to the rhombohedral structure. The ionic size difference between Mn4+ (0.53 Å) and Cu2+ (0.73 Å) may be ascribed to be the driving force behind the structural transition with an increase in Cu doping. EDX analysis confirms the nominal cationic compositions and chemical homogeneity of the samples. The copper substitution has a large impact on the valency of both the Mn and Co cations by following the charge neutrality condition: Mn4+ + Co2+ → (1 – x)Mn4+ + xCu2+ + (1 – 2x)Co2+ + 2xCo3+, where x is the doping concentration. There is also the Co2+ + Mn4+ = Co3+ + Mn3+ interaction. The presence of multivalent cations is evidenced from the XPS analysis. This largely influences the cationic ordering as well as the magnetic interactions. The monoclinic structure of the x = 0 and 0.1 samples show long-range ferromagnetic ordering. This is associated with the Mn4+—O—Co2+ ferromagnetic superexchange interaction in the ordered state of Mn4+ and Co2+. For small Cu doping (x = 0.1), this interaction is marginally disturbed. However, in the rhombohedral phase for x ≥ 0.2, the ferromagnetic interaction is largely suppressed with the evolution of competing antiferromagnetic interactions between the multivalent cations. Most importantly, in the panorama of multivalent mixed cations with randomness and competing exchange interactions, which are the fundamental ingredients of magnetic frustration, the antisite-disordered induced magnetic frustration for x = 0 is practically suppressed for x ≥ 0.2. The antiferromagnetic ordering over magnetic frustration can be attributed to the local lattice distortion induced by Jahn–Teller-active Cu2+ and Mn3+ and intermediate-spin-state Co3+ ions.

Structure, magnetism, and electronic properties in 3d−5d based double perovskite ( Sr1−xCax)2FeIrO6 ( 0≤x≤1 )

The $3d\text{\ensuremath{-}}5d$ based double perovskites offer an ideal playground to study the interplay between electron correlation ($U$) and spin-orbit coupling (SOC) effect, showing exotic physics. The ${\mathrm{Sr}}_{2}{\mathrm{FeIrO}}_{6}$ is an interesting member in this family with ionic distribution of ${\mathrm{Fe}}^{3+}$ ($3{d}^{5}$) and ${\mathrm{Ir}}^{5+}$ ($5{d}^{4}$) where the later is believed to be nonmagnetic under the picture of strong SOC. Here we report a detailed investigation of structural, magnetic, and electronic transport properties along with electronic structure calculations in ${({\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x})}_{2}{\mathrm{FeIrO}}_{6}$ series with $x$ from 0 to 1. While the basic interactions such as $U$ and SOC are unlikely to be modified, a structural modification is expected due to ionic size difference between ${\mathrm{Sr}}^{2+}$ and ${\mathrm{Ca}}^{2+}$ which would influence other properties such as crystal field effect and bandwidths. While nonmonotonic changes in lattice parameters are observed across the series, the spectroscopic investigations reveal that $3+/5+$ charge state of Fe/Ir continue till the end of the series. An analysis of magnetic data suggests similar nonmonotonic evolution of magnetic parameters with doping. Temperature dependent crystal structure as well as low temperature (5 K) magnetic structure have been determined from neutron powder diffraction measurements which further indicate site ordered moments for both Fe and Ir. The whole series shows insulating behavior with a nonmonotonic variation in resistivity where the charge transport follows the three-dimensional variable range hopping model. The electronic structure calculations show, SOC enhanced, a noncollinear antiferromagnetic and a Mott-type insulating state is the stable ground state for the present series with a substantial amount of orbital moment, but less than the spin magnetic moment, at the Ir site and the magnetocrystalline anisotropy. The calculations further show the evolution of the spin and orbital magnetic moment components across the series along with the magnetization density. The obtained results imply that the local structural modification with introduction of lower size ${\mathrm{Ca}}^{2+}$ has a large influence on the magnetic and transport properties, further showing a large agreement between experimental results as well as theoretical calculations.

Magnetic Structures of Mn11Ta4O21 and Interpretation as an Hexagonal A-site Manganite.

Mn11Ta4O21 is presented as the first hexagonal A-site manganite. Based on simple rules, the structure is compatible with a 14H-layer (cchchch)2 stacking sequence that is related to BaVO3 and BaCrO3 high-pressure polymorphs. The A-site overstoichiometry is explained through difference in ionic radii sizes between Ba and Mn. Magnetic properties show two transitions at TN1 = 88 K and TN2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below TN1 and TN2, respectively. A complementary description with 14H-(hhccccc)2 sequence of only Mn octahedra provides a direct comparison with BaMnO3-δ hexagonal perovskites and naturally explains the AFM order. Below TN2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed.

Can a shape factor in bulk ceramics mitigate unwanted phase transformations?

The present work introduces the concept of ‘shape strain’ to illustrate its beneficial effect to control/avoid any undesired phase transition via consolidation/densification of ceramics; low (4 mole%)-doped ZrO2 ceramics with various dopants have been used for investigation. With ionic radii size difference (of dopant ion and Zr4+) above 30%, CaO-doped zirconia ceramics showed the effect most prominently. The effect was also dependent on the grain boundary mobility of dopants. Controlling final phase evolutions in ceramics is difficult and this effect may serve as remedy if tried with different size fractions of starting powders.

Macrocyclic Ligands with an Unprecedented Size-Selectivity Pattern for the Lanthanide Ions.

Lanthanides (Ln3+) are critical materials used for many important applications, often in the form of coordination compounds. Tuning the thermodynamic stability of these compounds is a general concern, which is not readily achieved due to the similar coordination chemistry of lanthanides. Herein, we report two 18-membered macrocyclic ligands called macrodipa and macrotripa that show for the first time a dual selectivity toward both the light, large Ln3+ ions and the heavy, small Ln3+ ions, as determined by potentiometric titrations. The lanthanide complexes of these ligands were investigated by NMR spectroscopy and X-ray crystallography, which revealed the occurrence of a significant conformational toggle between a 10-coordinate Conformation A and an 8-coordinate Conformation B that accommodates Ln3+ ions of different sizes. The origin of this selectivity pattern was further supported by density functional theory (DFT) calculations, which show the complementary effects of ligand strain energy and metal-ligand binding energy that contribute to this conformational switch. This work demonstrates how novel ligand design strategies can be applied to tune the selectivity pattern for the Ln3+ ions.

Dopant segregation in YAG single crystal fibers grown by the laser heated pedestal growth technique

In this study, we report self-segregation of dopants in a crystal matrix within a single crystal (SC) fiber. Neodymium and holmium-doped yttrium aluminum garnet (YAG) fibers were grown using the Laser Heated Pedestal Growth (LHPG) technique and cross-sectional dopant concentration was measured using electron-probe micro-analysis. It was observed that the degree of auto-segregation of the rare-earth dopant depended on the difference in ionic size of the dopant ion and the Y3+ ion in the YAG matrix. While holmium showed little tendency to self-segregate, the concentration of neodymium ions varied as much as 25% across the cross-section of the fiber. Strong correlation between the dopant concentration profile and fiber draw speed was also demonstrated. Since the local refractive index depends on the concentration of dopants, a refractive index profile can be achieved by a dopant profile across the fiber cross-section. Engineered index profiles can be realized by varying growth conditions, dopants, crystal matrix, etc. Such an approach is promising in applications such as the development of monolithic SC fibers with graded-index profiles.

Specific Ion Effects on the Colloidal Stability of Layered Double Hydroxide Single-layer Nanosheets

The surface charge properties and aggregation behavior of positively charged Mg-Al-NO3 layered double hydroxide (LDH) single-layer nanosheetsdispersed in water were investigated in the presence of K+salts with different mono-, di-, and trivalent anions, using electrophoresis and dynamic light scattering techniques. An increase in the salt concentration can significantly decrease the effective surface charge density (σeff) of LDHs, leading to the aggregation of nanosheets. The critical coagulation concentration (CCC) or ionic strength (CCIS) of salts for nanosheets significantly decreases with an increase in the valence of anions. Specific ion effects, with a partially reverse Hofmeister series, are observed. Based on the Stern model and the DLVO theory, the relationship of CCC with σeff and the ionic valences of salts (zi) is theoretically analyzed, which can accurately describe the dependence of CCC on the σeff and zi, but cannot explain the origin of specific ion effects. To explore the origin of specific ion effects, a correlation between CCIS and the specific adsorption energy (Esc) of anions within the Stern layer is developed. Especially, an empirical relationship of Esc with the characteristic physical parameters of anions is proposed. Our model can accurately predict the CCISs of at least monovalent anions and divalent anions (CO32- and SO42-), demonstrating that the specific ion effects observed can be attributed to the differences in ionic size, polarizability, and hydration free energy (or the formation capacity of anion-cation pairs) of different anions. This work not only deepens the understanding of specific ion effects on the colloidal stability, but also provides useful information for the potential applications of LDH single-layer nanosheets.

Biosynthesis, structural, photoluminescence and photocatalytic performance of Mn/Mg dual doped ZnO nanostructures using Ocimum tenuiflorum leaf extract

Here, pure ZnO, Zn0.98Mn0.02O and Zn0.96Mn0.02Mg0.02O nanostructures using aqueous leaf extract of Ocimum tenuiflorum were reported. XRD patterns revealed that no secondary phase was noticed and ZnO hexagonal wurtzite structure was not altered by Mn/Mg simultaneous doping. Mg co-doping with Zn0.98Mn0.02O considerably decreased the crystallite size from 46 to 34 nm due to ionic size difference. The superior absorbance and tuning of energy gap by Mn/Mg substitution is useful for opto-electronic applications. The presence of Mn and Mg in Zn-Mn-Mg-O network was confirmed by IR analysis. The effective charge separation, enhanced optical absorbance in the visible range, increased surface area and the generation of more defect related states were responsible for the noticed highest photocatalytic activity in Mn/Mg doped ZnO.

Effect of variable oxidation states of Mn+n ion impregnated TiO2 nanocomposites for superior adsorption and photoactivity under visible light

This paper deals with the importance of oxidation state and ionic size of Mn(II) and Mn(VII) co-catalyst impregnated TiO2 for the observed electrokinetic, adsorption and photocatalytic properties of Mn+n-TiO2 nanocomposites. Due to difference in net electronic charge and ionic size of Mn(II) and Mn(VII) ions, the zeta potential, streaming potential and surface charge demand of Mn(II)–TiO2 and Mn(VII)–TiO2 nanocomposites are greatly varied. The absorption edge of Mn+n-TiO2 nanocatalysts revealed the red shift (515–550 nm) with increased oxidation state (from II to VII) of Mn+n due to 6A1g → 4T1g transition. The photoluminescence (404–530 nm) of Mn(II)–TiO2 is highly quenched relative to Mn(VII)–TiO2 during 340 nm excitation. Further, the HRTEM analysis confirmed the presence of Mn(VII) (∼20–45 nm) and Mn(II) (∼40–60 nm) nanodeposits over TiO2 surface. Binding energies at 639.1 and 640.3 eV showed the presence of Mn in (0) and (II) oxidation state for Mn(II)–TiO2 and 646 eV alongwith a satellite peak for Mn(VII)–TiO2 co-catalyst. As a result, the adsorption and photocatalytic degradation of cationic (Fuchsin blue and methylene blue) and anionic (Salicylic acid and Salicylaldehyde) pollutants by Mn+n-TiO2 nanocomposites are notably improved relative to bare TiO2 depending on their respective oxidation state and surface morphological features.

Biomimetic Nanocones that Enable High Ion Permselectivity.

Artificial counterparts of conical-shaped transmembrane protein channels are of interest in biomedical sciences for biomolecule detection and selective ion permeation based on ionic size and/or charge differences. However, industrial-scale applications such as seawater desalination, separation of mono- from divalent cations, and treatment of highly-saline industrial waste effluents are still big challenges for such biomimetic channels. A simple monomer seeding experimental approach is used to grow ionically conductive biomimetic charged nanocone pores at the surface of an acid-functionalized membrane. These readily scalable nanocone membranes enable ultra-fast cation permeation (Na+ =8.4× vs. Mg2+ =1.4×) and high ion charge selectivity (Na+ /Mg2+ =6×) compared to the commercial state-of-the-art permselective membrane (CSO, Selemion, Japan) owing to negligible surface resistance and positively charged conical pore walls.

Biomimetic Nanocones that Enable High Ion Permselectivity

AbstractArtificial counterparts of conical‐shaped transmembrane protein channels are of interest in biomedical sciences for biomolecule detection and selective ion permeation based on ionic size and/or charge differences. However, industrial‐scale applications such as seawater desalination, separation of mono‐ from divalent cations, and treatment of highly‐saline industrial waste effluents are still big challenges for such biomimetic channels. A simple monomer seeding experimental approach is used to grow ionically conductive biomimetic charged nanocone pores at the surface of an acid‐functionalized membrane. These readily scalable nanocone membranes enable ultra‐fast cation permeation (Na+=8.4× vs. Mg2+=1.4×) and high ion charge selectivity (Na+/Mg2+=6×) compared to the commercial state‐of‐the‐art permselective membrane (CSO, Selemion, Japan) owing to negligible surface resistance and positively charged conical pore walls.

Structural, morphology and optical properties of NaYF4 thin films doped with trivalent lanthanide ions

NaYF4 and NaYF4:Ln3+ (Ln3+ = Nd3+, Eu3+, Yb3+, Er3+) thin films were prepared by electrodeposition technique. After deposition, all samples were thermally annealed and then fully characterized in order to know their structural, morphology and optical properties. Results have shown the formation of cubic phase NaYF4, i.e. no additional phases were detected even when doping with Ln3+ ions due to the direct substitutional mechanism between Y3+ → Ln3+ ions. Although, when Ln3+ ions enter into the crystal lattice small differences are detected in the lattice parameter due to differences in ionic radius size of the Ln3+ ions. The differences in ionic radius size also affect the particle size and the agglomerated structures size of the Ln3+ doped NaYF4 thin films, which is related to small variations in pH of electrolytic solutions during the electrodeposition process. Being NaYF4 a disordered matrix, when doped with Ln3+ ions; variations on the local crystal field arise for the second nearest neighbors of the Ln3+ ions, imposing thus a slightly different crystal field environment for each subset of Ln3+ ions. Such differences imply an inhomogeneous broadening of their bands corresponding to 4f–4f transitions in the photoluminescence spectra unlike those from ordered crystals that exhibit narrow bands.

Luminescent Properties of RE-doped Mullite (Al6Si2O13) Phosphors(RE = Ce3+, Eu2+, Ce3+-Tb3+, and Eu3+)

The luminescent properties of Ce3+, Eu2+, Ce3+-Tb3+, and Eu3+ in the mullite (Al6Si2O13) host were investigated for realization of full colors phosphors with a single type host. Ce3+ and Eu2+ in mullite showed blue photoluminescence (PL), whereas green and red emission were obtained in the Ce3+-Tb3+ and Eu3+ doped mullite. The prepared phosphors had the excitation band around 300 nm. The PL intensity at 200 °C kept 70 % of the initial intensity at room temperature.The X-ray diffraction study indicated the incorporation of the rare-earth (RE) ions in the mullite lattice without forming secondary phases, although it was not probable that the RE ions substituted for Al3+ and Si4+ because of the significant difference in ionic size. The bond-valence-sum and the differential electron density map analysis indicated that the doped RE ions occupied the stoichiometrically-introduced vacant oxygen site, which was the site for oxygen originally. The amphoteric nature of the oxygen site in mullite was proposed.

Diffuse double-layer structure in mixed electrolytes considering ions as dielectric spheres.

The structure of the diffuse part of the electric double layer at solid-electrolyte solution interfaces is examined using a theoretical model that takes into account the finite ion size by modeling the solution as a suspension of polarizable insulating spheres in water. This formalism is applied to mixed electrolyte solutions using the "Boublik-Mansoori-Carnahan-Starling-Leland" (BMCSL) theory for the steric interactions among ions. It is shown that the ionic size differences have a strong bearing on the diffuse part of the electric double-layer structure of these systems. Moreover, for strong potential values, the different size-related effects become important even for binary electrolyte solutions due to the presence of H+ and OH- ions that are substantially smaller than hydrated ions originated from salt dissociation. The obtained results display some of the qualitative features observed in experiments on aqueous systems that are generally interpreted in terms of totally different mechanisms.

Existence of internal domains in (Bi0.5La0.5FeO3)(1-x)–(Ba0.7Sr0.3TiO3)x solid solution and their effects on magnetic and dielectric behaviour

A significant experimental effort has been undertaken to evaluate the doping effect of La3+at Bi-site in solid solutions of BiFeO3 (BFO) and Ba0.7Sr0.3TiO3 (BST) through various techniques like x-ray diffraction, magnetic measurements, and frequency and/or temperature dependent impedance spectroscopy. The magnetic isotherm at room temperature in La doped BFO (BLFO)-BST solid solution indicates weak ferromagnetism due to improvement in the homogeneous canted spin structure. The elusive nano-sized domain structure inside the grains of 40% BST co-doped BLFO was observed by a scanning electron microscopy technique. The domains and domain boundaries are generally formed inside the grain due to strained/unstrained lattice defects and/or mismatch with the substitution of ions of different sizes at the A and B sites. An equivalent circuit model (internal barrier layer capacitor), based on domain, domain boundary, and grain boundary relaxations, has been established to justify the enhancement of the dielectric response with BST concentrations.

Study of the effect of stress/strain of mesoporous Al-doped ZnO thin films on thermoelectric properties

In this study, effects of induced stress and strain on the thermoelectric properties of mesoporous ZnO thin films with various Al doping concentrations were investigated. With Al doping in ZnO structure, the hexagonal wurtzite structure of ZnO was distorted owing to an ionic size difference between Al and Zn. With an increase in Al concentration to 4 at%, thermal conductivity unexpectedly decreased from 1.70 to 1.24 W/mK owing to an increase in the tensile strain, and electrical conductivity increased from 4 S/cm to 15 S/cm owing to an increase in the carrier concentration. Based on this study, the relationship between the induced strain owing to lattice distortion and thermoelectric properties was investigated. Thus, 4 at% Al-doped mesoporous ZnO demonstrated best enhanced thermoelectric properties.

FP-LAPW investigation on structural, electronic and optical properties of Eu2+-doped MF2 (M = Ca and Ba)

The structural, electronic and optical properties of MF2 (M = Ca and Ba) doped with Eu2+ ion have been studied using full-potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT), as implemented in Wien2k package. Due to the difference in ionic size, the Eu2+ incorporation in MF2 induces a re-accommodation of its nearest neighbors to reach more stable configuration. Along with the generalized gradient approximation (GGA-PBE), the electronic properties also are investigated by Tran-Blaha modified Becke Johnson potenital (mBJ) due to that it gives very accurate band gap of a lot of types of solid. The Eu2+ doping causes a slight increase on band gap of CaF2 and BaF2, and there is found Eu2+-4f state in the forbidden energy region of host. In this work, the spin-orbit coupling (SOC) effect on electronic properties of MF2:Eu2+ is also investigated. Finally, the doping effect on optical absorption property is also studied by deriving the absorption coefficient of all materials on consideration from their calculated complex dielectric function in energy range up to 17 eV.

Multiionic and Permittivity‐Related Effects on the Diffuse Electric Double Layer Structure at Solid‐Electrolyte Solution Interfaces

The structure and differential capacitance of the diffuse part of the electric double layer at solid‐electrolyte solution interfaces are examined using a theoretical model that takes into account the finite ion size by modeling the solution as a suspension of polarizable insulating spheres in water. This formalism is applied to binary and mixed electrolyte solutions using the “Boublik–Mansoori–Carnahan–Starling–Leland” (BMCSL) theory for the steric interactions among ions. It is shown that the ionic size differences have a strong bearing on the diffuse part of the electric double layer structure, as well as on the differential capacitance dependence on the surface potential for mixed electrolytes.

Electric double layer capacitance for ionic liquids in nanoporous electrodes: Effects of pore size and ion composition

The energy density of an electric double layer (EDL) capacitor, a type of supercapacitor, depends on the ion distribution within the micropores of electrodes that are typically made of amorphous carbon. By using coarse-grained models and the classical density functional theory, we investigate the distributions of ionic species among different idealized nanopores in contact with an asymmetric ionic liquid mixture and the effects of the bulk electrolyte composition on capacitive energy storage. We find that a charged pore is always small-ion selective, provided all ions have the same valence and similar non-electrostatic interactions. While small ions enhance both the EDL capacitance and the accessibility of micropores, an ionic mixture containing ions of different sizes may yield a capacitance higher than those corresponding to pure ionic liquids. The increased capacitance may be attributed to more efficient ion packing near the charged surface. At certain conditions, the improvement is on a par with the anomalous capacitance rise for pure ionic liquids in electrodes with ultranarrow pores.

The effects of PbZn1/3Nb2/3O3-doping on structural, thermal, optical, dielectric, and ferroelectric properties of BaTiO3 ceramics

Low-lead (1-x)BT-xPZN (x = 0, 0.025, 0.05, 0.075, 0.10, 0.125, and 0.15) ceramics were successfully synthesized by the spark-plasma-sintering method for the first time. Their phase transition behavior as well as structural, thermal, optical, and electrical properties was investigated. These materials exhibit the structure of perovskite-type solid solutions and undergo a sequence of phase transitions, typical of pure BaTiO3 (BT). The dielectric test results revealed that with the increase in the PbZn1/3Nb2/3O3 (PZN) content, the frequency dispersion of electric permittivity increases, whilst the dielectric/ferroelectric properties tend to deteriorate, which is characteristic of relaxor-type behavior. Therefore, it is reasonable to suppose that these ceramics progressively lack long-range ordering. These effects are due to the competition between lone-pair electrons' induced changes in the A-O band upon Pb2+ addition and ionic size differences. In general, the transition temperatures observed by dielectric analyses are in good agreement with those obtained from X-ray diffraction and differential scanning calorimetry measurements. The BT-PZN system may help to understand why relaxor behavior appears in perovskite-based materials. It appears that these materials can become a good starting point for the development of new low-lead electronic ceramics.