Cationic Charge States Research Articles

Study of the synergistic role of multivalence states of 3d cations on the crystal lattice distortion and magnetic behavior of Sr2FeCoO6-δ: A spectroscopic study

In perovskite oxides, the multiple charge states of cations influence their local structure and correlated physical properties. In this report, we studied the role of multivalence cations on the local structure and magnetic behavior of the oxygen-deficient system Sr2FeCoO6-δ (SFCO), prepared by the solid-state synthesis technique. A detailed structural and geometrical analysis confirmed the coexistence of orthorhombic (Brownmillerite) and cubic crystal symmetries and a gesture of the Jahn-Teller (JT) distortion. The intense Raman modes are allowed to be indexed for BO-polyhedra and JT-distortion. Experimental and theoretical analysis of the X-ray photoelectron spectroscopy (XPS) spectra supported the mixed oxidation state of Fe (Fe3+/Fe4+) and Co (Co2+/Co3+), further confirmed by theoretically simulated X-ray absorption spectroscopy (XAS) spectra. The presence of the multivalence states leads to AFM/FM exchange interaction in the sample which further leads to glassy magnetic behavior.

Physical properties in nano-crystalline Ho2CoMnO6

3d based double perovskite materials have received much attention in recent years due to their exotic magnetic structure and magneto-electric coupling. In this work we have prepared and studied the nano-crystalline sample of Ho2CoMnO6. Structural, magnetic, Raman and dielectric properties have been studied in detail. The structural analysis shows that the sample crystallize in monoclanic crystal structure with P21/n phase group. The X-ray photoelectron spectroscopy have been employed to confirms the charge state of cations presents in the material. Magnetic study shows that the sample undergoes a paramagnetic to ferromagnetic phase transition around Tc ~85 K. The isothermal magnetization measurements shows hysteresis curve hence confirm ferromagnetic behavior at low temperature. Temperature dependent Raman study reveals that there is spin phonon coupling in the sample marked by deviation in phonon mode from anharmonic behavior. Dielectric response of Ho2CoMnO6 shows the large dispersion and large dielectric constant. Impedance spectroscopy and electrical modulus study reveal that system shows deviation from ideal Debye model. AC conductivity have been studied as a function of both temperature and frequency. We found that the conduction mechanism is obeyed by Jonscher's model. The exponent factor n is suggest that the material deviates from ideal Debye model.

Charge States of Cations in Lithium–Nickel Phosphates LiNiPO4 Doped with Manganese and Cobalt

Using Ni, Mn, and Co L2,3-edge soft X-ray absorption spectra supported by calculations of atomic multiplets with allowance for the crystal field and charge transfer, the charge states of cations in doped phosphates LiNiPO4, LiNi0.9Co0.1PO4, and LiNi0.9Mn0.1PO4 are determined. It is established that nickel, manganese, and cobalt ions are in the divalent high-spin state and have an octahedral oxygen environment. Oxygen K-edge X-ray absorption spectra showed the absence of impurity states in the energy gap of manganese- or cobalt-doped lithium nickel phosphates.

Computational Studies on Structural, Excitation, and Charge-Transfer Properties of Ureidopeptidomimetics.

Peptides with ureido group enclosing backbones are considered peptidomimetics and are known for their higher stabilities, biocompatibilities, antibiotic, inhibitor, and charge-transduction activities. These peptidomimetics have some unique applications, which are quite different from those of natural peptides. Hence, it is imperative to appreciate their properties at a microscopic level. In this regard, this work outlines, in detail, the charge transfer (CT) properties, hole-migration dynamics, and electronic structures of various experimentally comprehended ureidopeptidomimetic models using density functional theory (DFT). Time-dependent DFT and complete active space self-consistent field computations on basic models provide the necessary evidence for the viability of CT from the end enfolding the ureido group to the other end with a carboxylate entity. This donor-to-acceptor CT has been reflected in excitation studies, in which the higher intensity band corresponds to CT from the π orbital of the ureido group to the π* orbital of the carboxylate entity. Further, hole-migration studies have shown that charge can evolve from the ureido end, whereas the hole generated at the carboxylate end does not migrate. However, hole migration has been reported to occur from both ends (amino and carboxylate ends) in glycine oligopeptides, and our studies show that the ability to transfer and migrate charge can be tuned by modifying the donor and acceptor functional groups in both the neutral and cationic charge states. We have analyzed the possibility of hole migration following ionization using DFT-based wave-packet propagation and found its occurrence on a ∼2-5 fs time scale, which reflects the charge-transduction ability of peptidomimetics.

Spectral sensitization with dyes of core–silver halide shell microsystems

We have studied spectral sensitization with anionic dyes of core–silver halide shell microsystems cores of which can be either nonsilver or silver halide compounds. Conditions under which dye sensitizers, being adsorbed on cores, remain under silver halide shells after their growing are considered. Comparison of results of sensitometric and low-temperature (T = 77 K) luminescent measurements have shown that these conditions are determined by the charge state of cations of microsystem cores. If the shell contains the same univalent component in its composition as the core does, as in the case in which the core is a silver halide compound, the anionic dye is displaced to the outer surface of the shell. If the core contains a divalent cationic component, as in the case in which the core is a nonsilver compound, the dye remains under the silver halide shell; i.e., it is overgrown by the shell. We have shown that the charge state of core cations affects the character of the core interaction with anionic dyes, which ensures differences in the spectral sensitization of core–silver halide shell microsystems, as well as differences in the dye photoexcitation relaxation in them.

X-ray spectra and charge states of cations in nanostructured manganites La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3

From the analysis of experimental X-ray absorption spectra and the calculation of atomic multiplets it is found that the grinding of powder manganites La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3 in a vibration mill results in the formation of nanocrystallites and a partial transformation of trivalent manganese ions into tetravalent. It is shown that in nanostructured manganite La0.5Ca0.5MnO3 the energy gap between the occupied and unoccupied states is absent. It os found that Mn2+ ions arise in Nd0.5Sr0.5MnO3 due to more intense deformation during the grinding.

Air annealing effects on lattice structure, charge state distribution of cations, and room temperature ferrimagnetism in the ferrite composition Co2.25Fe0.75O4

The ferrite composition Co2.25Fe0.75O4 has been prepared by chemical coprecipitation route. The as-prepared material has been annealed at different temperatures to investigate thermal activated changes in structural phase, charge states of cations, and room temperature magnetic properties. Synchrotron x-ray diffraction (SXRD) patterns have shown splitting of cubic spinel structure into Fe-rich and Co-rich phases for annealing temperature up to 800 °C. Single phase cubic spinel structure has been stabilized at annealing temperature 900 °C. The existence of Fe4+ ions, as confirmed from x-ray absorption near edge structure spectroscopy (XANES), is the new information for spinel ferrite. Raman spectra indicated normal spinel structure. The results of SXRD, XANES and Raman spectra have been used to estimate distribution of Co and Fe ions in spinel structure. The variation in population of Co and Fe ions and phase instability has affected the magnetic properties. The sample annealed at 800 °C shows maximum coercivity (∼567 Oe) and squareness (∼0.38), whereas the single-phased sample showed lowest values of ferrimagnetic parameters. The tuning of magnetic parameters by thermal activated structural phase variation of a hetero-structured magnetic system appears to be a new technique for the development of magnetic materials.

Nonmonotonic structural changes in HTSC Bi-2201 ceramics depending on oxygen nonstoichiometry

Effect of the oxygen content in ceramic high-temperature superconductors Bi2Sr2CuO6 + δ on their phase composition, crystal structure, and charge state of cations has been studied. As the oxygen content changes, the lattice parameters were shown to exhibit nonmonotonic variation; no changes in the phase composition and charge state of cations take place. According to neutron diffraction data, the overstoichiometric oxygen atoms are introduced only into the BiO1 + δ layers. The nonmonotonic variations observed are explained assuming that, at a certain oxygen content, the hole charge carriers induced by introduced oxygen are localized in the BiO1 + δ layers rather than “flow down” into superconducting CuO2 layers.

DFT study of the electronic structure of anthracene derivatives in their neutral, anion and cation forms

Anthracene and its simplest derivatives with electron-accepting (anthraquinone) and electron-donating (diaminoanthracene) properties in their anionic, neutral and cationic charge states are investigated using the density functional theory. The effect of addition and removal of the electron on the bond lengths, atomic charges, frontier orbitals, ionization potential (IP), electron affinity (EA) and reorganization energy is considered. The computed IP, EA and optical gaps are in rather good agreement with available experimental data.

Dehydrogenated polycyclic aromatic hydrocarbons and UV bump

Recent calculations have shown that the UV bump at about 217.5 nm in the extinction curve can be explained by a complex mixture of PAHs in several charge states. Other studies proposed that the carriers are a restricted population made of neutral and singly-ionised dehydrogenated coronene molecules (C24Hn, n less than 3), in line with models of the hydrogenation state of interstellar PAHs predicting that medium-sized species are highly dehydrogenated. To assess the observational consequences of the latter hypothesis we have undertaken a systematic study of the electronic spectra of dehydrogenated PAHs. We use our first results to see whether such spectra show strong general trends upon dehydrogenation. We used state-of-the-art techniques in the framework of the density functional theory (DFT) to obtain the electronic ground-state geometries, and of the time- dependent DFT to evaluate the electronic excited-state properties. We computed the absorption cross-section of the species C24Hn (n=12,10,8,6,4,2,0) in their neutral and cationic charge-states. Similar calculations were performed for other PAHs and their fullydehydrogenated counterparts. pi electron energies are always found to be strongly affected by dehydrogenation. In all cases we examined, progressive dehydrogenation translates into a correspondingly progressive blue shift of the main electronic transitions. In particular, the pi-pi* collective resonance becomes broader and bluer with dehydrogenation. Its calculated energy position is therefore predicted to fall in the gap between the UV bump and the far-UV rise of the extinction curve. Since this effect appears to be systematic, it poses a tight observational limit on the column density of strongly dehydrogenated medium-sized PAHs.

DENSITY FUNCTIONAL THEORY STUDY OF W5 CLUSTERS

Density functional theory(DFT) calculations are performed to study W 5 clusters in their neutral, anionic and cationic charge states. All the possible stable isomers are examined, and the most stable isomers for all these species are found. They are singlet state with D3h symmetry for W 5, and doublets with C2v symmetry for both [Formula: see text] and [Formula: see text]. Equilibrium geometries, electron affinities and dissociation energies are also de termined. Time-depended DFT is used to calculate the low-lying excited states of W 5. Theoretical assignments for the features in the experimental photoelectron spectra are given. All results obtained are in good agreement with available experimental data.

Valence Change of Cations in Ceria-Zirconia Solid Solution Associated with Redox Reactions Studied with Electron Energy-Loss Spectroscopy

It has been known that the ceria-zirconia solid solutions (Ce2Zr2O7+x, 0 ≤ x ≤ 1) have an excellent ability for oxygen absorption/release. The local charge states of cations in the CeO2-ZrO2 solid solutions have been studied by means of electron energy-loss spectroscopy (EELS) attached to a transmission electron microscope. The relative intensity of cerium-M4,5 white-line peaks from Ce2Zr2O7, Ce2Zr2O7.5 and Ce2Zr2O8 showed a systematic change, which corresponded to the valence change of cerium; Ce3+ in Ce2Zr2O7, Ce4+ in Ce2Zr2O8. By contrast, the valence of the zirconium ions, which are mainly at the nearest neighbor sites of absorbed or emitted oxygen atoms, remains to be quaternary irrespective of oxygen contents. It was found that the energy-loss near edge structure (ELNES) of the cerium-M4,5 edge in Ce2Zr2O7.5 was well reproduced by the sum of the ELNES profiles of Ce2Zr2O7 and Ce2Zr2O8 with an equal weight, the phenomenon of which is known as the ‘valence fluctuation,’ or ‘mixed valence state,’ often observed in rare earth metal compounds.

Theoretical Modeling of Infrared Emission from Neutral and Charged Polycyclic Aromatic Hydrocarbons. II.

The nature of the carriers of the interstellar infrared (IR) emission features between 3.3 and 12.7 μm is complex. We must consider emission from a family of polycyclic aromatic hydrocarbons (PAHs) in a multiplicity of cationic charge states (+1, +2, +3, and so on), along with neutral and anionic PAHs. The adopted intrinsic IR cross sections of the various modes of the PAHs are the key to all models. Here we make a comparison between laboratory-measured cross sections and quantum chemical calculations and find that, overall, the agreement is very good. In this paper, we consider emission from a wide variety of specific PAH molecules, which includes all available data from our laboratory and quantum chemical databases. We incorporate this into our model to produce a theoretical analysis that is more realistic, detailed, and comprehensive than prior studies. PAH molecular structures that we consider include symmetric condensed, symmetric noncondensed, aromatics containing pentagonal rings, linear, and methylated PAHs. The synthesized IR spectra show large variations in peak position for the small PAHs studied, while their spectral profile is uniquely characteristic of each different molecular structure. We also investigate the spectral variations with molecular structure of a PAH population at the surface of the Orion photodissociation region (PDR) and include an example of how the IR spectrum of our PAH population varies dramatically as a function of depth (or radiation field) through the PDR. We make a comparison of these results with Infrared Space Observatory data measured at the surface of the Orion PDR. We conclude that the charge of PAHs in a composite population has a stronger effect on its IR emission spectrum than its molecular structure. However, on the basis of the PAH samples considered in this paper, detailed studies of the interstellar IR emission features can be used effectively to identify molecular characteristics of the interstellar PAH family. In Paper III, we extend the theme of this paper by investigating the effects of hydrogenation on a wide variety of PAHs up to size 54 carbon atoms and compare our results with observational profiles for the Orion PDR.

Atomic Structure of a 36.8 (210) Tilt Grain Boundary in TiO2

An understanding of the atomic structure at internal interfaces is of crucial importance for the electronic and structural properties of most advanced materials. Here, we present a detailed study of the atomic structure at a [001] tilt grain boundary of σ5(210) in Tio2 (rutile). Z‐contrast imaging is used to obtain a 2‐D atomic map of the cation positions at the interface. Details of the charge state of cations and atomic structure around anion sites are then provided using electron energy loss spectroscopy. In particular, the spectroscopic data for oxygen is interpreted using multiple scattering theory to give 3‐D structural information. These combined techniques allow a unique grain boundary structure to be defined.

Magnetic properties of perovskites R(Mn0.5A0.5)O3 (R = Nd, Eu; A = Co, Ni)

The magnetic properties of perovskites R(Mn0.5A0.5)O3 (R = Nd, Eu; A = Co, Ni) are reported. It is shown that these compounds are ferromagnets with relatively high Curie temperature. The magnetic anisotropy of the Ni-containing perovskites is much less than that for the Co-containing ones. Magnetic measurement data are interpreted assuming the charge state of cations to be Co2+(Ni2+) and Mn4+ and the ferromagnetism to be governed by superexchange interactions between the Co(Ni) and the Mn ions.