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Valence (chemistry)

About: Valence (chemistry) is a research topic. Over the lifetime, 24937 publications have been published within this topic receiving 645252 citations. The topic is also known as: valency.


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TL;DR: In this paper, the authors showed that the distribution of the cations within the spinel lattice of the ferrite nanoparticles, and consequently their magnetic properties are strongly affected by the synthesis method used.
Abstract: The Co–ferrite nanoparticles having a relatively uniform size distribution around 8 nm were synthesized by three different methods. A simple co-precipitation from aqueous solutions and a co-precipitation in an environment of microemulsions are low temperature methods (50 °C), whereas a thermal decomposition of organo-metallic complexes was performed at elevated temperature of 290 °C. The X-ray diffractometry (XRD) showed spinel structure, and the high-resolution transmission electron microscopy (HRTEM) a good crystallinity of all the nanoparticles. Energy-dispersive X-ray spectroscopy (EDS) showed the composition close to stoichiometric (~CoFe2O4) for both co-precipitated nanoparticles, whereas the nanoparticles prepared by the thermal decomposition were Co-deficient (~Co0.6Fe2.4O4). The X-ray absorption near-edge structure (XANES) analysis showed Co valence of 2+ in all the samples, Fe valence 3+ in both co-precipitated samples, but average Fe valence of 2.7+ in the sample synthesized by thermal decomposition. The variations in cation distribution within the spinel lattice were observed by structural refinement of X-ray absorption fine structure (EXAFS). Like the bulk CoFe2O4, the nanoparticles synthesized at elevated temperature using thermal decomposition displayed inverse spinel structure with the Co ions occupying predominantly octahedral lattice sites, whereas co-precipitated samples showed considerable proportion of cobalt ions occupying tetrahedral sites (nearly 1/3 for the nanoparticles synthesized by co-precipitation from aqueous solutions and almost 1/4 for the nanoparticles synthesized in microemulsions). Magnetic measurements performed at room temperature and at 10 K were in good agreement with the nanoparticles’ composition and the cation distribution in their structure. The presented study clearly shows that the distribution of the cations within the spinel lattice of the ferrite nanoparticles, and consequently their magnetic properties are strongly affected by the synthesis method used.

114 citations

Journal ArticleDOI
TL;DR: In this article, a quantum model capable of describing isotropic compression of single atoms in a non-reactive neon-like environment was presented, which can assist in the working of chemical intuition at extreme pressure and can act as a guide to both experiments and computational efforts.
Abstract: We present a quantum mechanical model capable of describing isotropic compression of single atoms in a non-reactive neon-like environment. Studies of 93 atoms predict drastic changes to ground-state electronic configurations and electronegativity in the pressure range of 0-300 GPa. This extension of atomic reference data assists in the working of chemical intuition at extreme pressure and can act as a guide to both experiments and computational efforts. For example, we can speculate on the existence of pressure-induced polarity (red-ox) inversions in various alloys. Our study confirms that the filling of energy levels in compressed atoms more closely follows the hydrogenic aufbau principle, where the ordering is determined by the principal quantum number. In contrast, the Madelung energy ordering rule is not predictive for atoms under compression. Magnetism may increase or decrease with pressure, depending on which atom is considered. However, Hund's rule is never violated for single atoms in the considered pressure range. Important (and understandable) electron shifts, s→p, s→d, s→f, and d→f are essential chemical and physical consequences of compression. Among the specific intriguing changes predicted are an increase in the range between the most and least electronegative elements with compression; a rearrangement of electronegativities of the alkali metals with pressure, with Na becoming the most electropositive s1 element (while Li becomes a p group element and K and heavier become transition metals); phase transitions in Ca, Sr, and Ba correlating well with s→d transitions; spin-reduction in all d-block atoms for which the valence d-shell occupation is d n (4 ≤ n ≤ 8); d→f transitions in Ce, Dy, and Cm causing Ce to become the most electropositive element of the f-block; f→d transitions in Ho, Dy, and Tb and a s→f transition in Pu. At high pressure Sc and Ti become the most electropositive elements, while Ne, He, and F remain the most electronegative ones.

114 citations

Journal ArticleDOI
TL;DR: The structures and properties of transition metal oxide (TMO) clusters of the group VIB metals, (MO(3) (M = Cr, Mo, W; n = 1-6), have been studied with density functional theory (DFT) methods and the acid/base properties are expected to play important roles in their catalytic activities.
Abstract: The structures and properties of transition metal oxide (TMO) clusters of the group VIB metals, (MO3)n (M = Cr, Mo, W; n = 1−6), have been studied with density functional theory (DFT) methods. Geometry optimizations and frequency calculations were carried out at the local and nonlocal DFT levels with polarized valence double-ζ quality basis sets, and final energies were calculated at nonlocal DFT levels with polarized valence triple-ζ quality basis sets at the local and nonlocal DFT geometries. Effective core potentials were used to treat the transition metal atoms. Two types of clusters were investigated, the ring and the chain, with the ring being lower in energy. Large ring structures (n > 3) were shown to be fluxional in their out of plane deformations. Long chain structures (n > 3) of (CrO3)n were predicted to be weakly bound complexes of the smaller clusters at the nonlocal DFT levels. For M6O18, two additional isomers were also studied, the cage and the inverted cage. The relative stability of the ...

114 citations

Journal ArticleDOI
TL;DR: In this paper, the transition metal ions exhibit several valence states and their electronic properties arise from electron exchanges between metal ions in different states, which can be used for making electrochromic display devices.
Abstract: Transition metal ions exhibit several valence states. Redox reactions occur during the sol-gel synthesis of transition metal oxides. Mixed valence compounds are obtained. Their electronic properties arise from electron exchanges between metal ions in different valence states. Thermally activated electron hopping leads to semiconducting materials. Optical absorption arising from intervalence transfers gives rise to reversible optical switching. They can be used for making electrochromic display devices. Electron and ion transfers can occur at the oxide-water interface. They seem to be typical of mixed valence oxides having a spinel structure and lead to chemical modifications of both the oxide network and the solution.

114 citations

Journal ArticleDOI
TL;DR: The relationship between main lines and adjacent satellites is systematically examined for 3 d-series insulators and it is demonstrated that the main-line and satellite energies are both dependent on the cation charge environment but with different degrees of sensitivity.
Abstract: Photoelectron spectroscopic measurements of core-electron levels in cations of transition-series insulators commonly show satellite features in the vicinity of the main photoelectron peaks. In this paper, the relationship between main lines and adjacent satellites is systematically examined for 3 d-series insulators. The spectral features are interpreted using a relaxation model that allows for the possibility of different final-state screening conditions of the photoinduced hole. Systematic trends are examined by employing atomic calculations and utilizing transition-state theory to calculate electron removal energies for atoms in appropriately simulated chemical environments. It is demonstrated that the main-line and satellite energies are both dependent on the cation charge environment but with different degrees of sensitivity. The consequence is that sensitivity to chemical change can be monitored in the observed main-line to satellite separation. Model predictions are tested by monitoring the dependence of the satellite separations on the cation valence state and on the electronegativity of both cations and ligated anions. Systematic tests are applied to 3 d-series insulators but more general applicability of the relaxation model is also discussed. 93 references, 19 figures, 1 table.

114 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20231,521
20222,997
2021616
2020611
2019584
2018577