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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 review the crystal structures of 703 tellurium oxysalts for which good refinements exist, including 55 that are known to occur as minerals.
Abstract: Relative to its extremely low abundance in the Earth's crust, tellurium is the most mineralogically diverse chemical element, with over 160 mineral species known that contain essential Te, many of them with unique crystal structures. We review the crystal structures of 703 tellurium oxysalts for which good refinements exist, including 55 that are known to occur as minerals. The dataset is restricted to compounds where oxygen is the only ligand that is strongly bound to Te, but most of the Periodic Table is represented in the compounds that are reviewed. The dataset contains 375 structures that contain only Te cations and 302 with only Te, with 26 of the compounds containing Te in both valence states. Te was almost exclusively in rather regular octahedral coordination by oxygen ligands, with only two instances each of 4- and 5-coordination. Conversely, the lone-pair cation Te displayed irregular coordination, with a broad range of coordination numbers and bond distances. A threshold was applied for Te-O links of ~2.45 A or 0.3 valence units with some flexibility, as a criterion to define strongly bound Te-O polymers and larger structural units. Using this criterion, Te cations display one-sided 3-, 4- or 5-coordination by oxygen (with rare examples of coordination numbers 2 and 6). For both valence states of Te, examples are known of TeO complexes which are monomeric (m = 1; neso), noncyclic finite oligomers (soro), rings (cyclo), infinite chains (ino), layers (phyllo) and frameworks (tecto tellurates). There is a clear analogy to the polymerization classes that are known for silicate anions, but the behaviour of Te is much richer than that of Si for several reasons: (1) the existence of two cationic valence states for Te; (2) the occurrence of multiple coordination numbers; (3) the possibility of edge-sharing by TeO polyhedra; (4) the possibility for oxygen ligands to be 3-coordinated by Te; and (5) the occurrence of TeO polymers that are cationic, as well as neutral or anionic. While most compounds contain only one or two symmetrically distinct types of Te atom, Pauling's Fifth Rule is frequently violated, and stoichiometrically simple compounds such as CaTeO can have polymorphs with up to 18 distinct Te sites. There is a tendency for local symmetry features such as the threefold axis of a TeO octahedron or the acentric symmetry of a TeO polyhedron to be inherited by the host structure; the latter in particular can lead to useful physical properties such as nonlinear optical behaviour. We develop for the first time a hierarchical taxonomy of Te-oxysalt structures, based upon (1) valence state of Te; (2) polymerization state of TeO complexes; (3) polymerization state of larger strongly-bound structural units that include non-Te cations. Structures are readily located and compared within this classification.

138 citations

Journal ArticleDOI
TL;DR: A covalent to metallic bonding between the Cu3N host crystal and the interstitial Pd atoms is found, which is mainly caused by Pd 5s and 5p states hybridizing to Cu 3d states.
Abstract: The electronic structure and the chemical-bonding mechanism of ${\mathrm{Cu}}_{3}$N, ${\mathrm{Cu}}_{3}$NPd and related Cu(I) compounds, such as ${\mathrm{Cu}}_{2}$O, are studied on the basis of band-structure calculations, using both the linearized augmented plane wave and linear combination of atomic orbitals (LCAO) methods. In accordance with experimental observations, ${\mathrm{Cu}}_{3}$N is found to be a semiconductor, while ${\mathrm{Cu}}_{3}$NPd should exhibit a semimetallic conductivity. The chemical bonding is investigated using various methods, among them are the valence charge partitioning scheme of Bader and a basis set reduction technique built on the LCAO method. A partly ionic, partly covalent bonding is found. The admixture of the Cu (4s, 4p) states to the Cu 3d--N 2p bands resulted to be essential for the covalent bonding effect, since pure 3d-2p bands, with bonding and antibonding states fully occupied, do not lead to a covalent energy gain. This specific hybridization appears to be the origin of the twofold dumbbell like Cu(I) coordination observed in ${\mathrm{Cu}}_{3}$N and other Cu(I) compounds. In ${\mathrm{Cu}}_{3}$NPd, a covalent to metallic bonding between the ${\mathrm{Cu}}_{3}$N host crystal and the interstitial Pd atoms is found, which is mainly caused by Pd 5s and 5p states hybridizing to Cu 3d states. \textcopyright{} 1996 The American Physical Society.

138 citations

Journal ArticleDOI
TL;DR: In this paper, water adsorption on a periodic model with both the local density approximation (LDA) and the generalized gradient approximation (GGA) of density-functional theory is studied.
Abstract: Water adsorption on a ${\mathrm{PuO}}_{2}(110)$ surface is studied using a periodic model with both the local-density approximation (LDA) and the generalized gradient approximation (GGA) of density-functional theory. The 60 core electrons of the Pu atom are represented by a relativistic effective core potential, and scalar relativistic effects have been incorporated into the valence orbitals. Both molecular and dissociative configurations of the adsorbate ${\mathrm{H}}_{2}\mathrm{O}$ are considered at one molecular layer coverage. For molecular water adsorption, LDA calculations indicate binding only at the top site, whereas the GGA indicates no binding for any site. Dissociative adsorption is found to be energetically more favorable than molecular adsorption, in agreement with experimental observations. The effects on the geometric and electronic structures influenced by water adsorption are investigated.

138 citations

Journal ArticleDOI
TL;DR: Detailed characterization of Fe( tri)2 and the Fe(tri)2(BF4) x materials via powder X-ray diffraction, Mössbauer spectroscopy, and IR and UV-vis-NIR diffuse reflectance spectroscopies reveals that the high conductivity arises from intervalence charge transfer between mixed-valence low-spin FeII/III centers.
Abstract: Metal–organic frameworks are of interest for use in a variety of electrochemical and electronic applications, although a detailed understanding of their charge transport behavior, which is of critical importance for enhancing electronic conductivities, remains limited. Herein, we report isolation of the mixed-valence framework materials, Fe(tri)2(BF4)x (tri– = 1,2,3-triazolate; x = 0.09, 0.22, and 0.33), obtained from the stoichiometric chemical oxidation of the poorly conductive iron(II) framework Fe(tri)2, and find that the conductivity increases dramatically with iron oxidation level. Notably, the most oxidized variant, Fe(tri)2(BF4)0.33, displays a room-temperature conductivity of 0.3(1) S/cm, which represents an increase of 8 orders of magnitude from that of the parent material and is one of the highest conductivity values reported among three-dimensional metal–organic frameworks. Detailed characterization of Fe(tri)2 and the Fe(tri)2(BF4)x materials via powder X-ray diffraction, Mossbauer spectrosco...

138 citations

Journal ArticleDOI
TL;DR: In this article, the stable structures of neutral (V2O5)n clusters (n = 1−5, 8, 10, and 12) are determined by density functional calculations (BP86 functional with a double-ζ (V)/triple-ε (O) valence basis set augmented by polarization functions).
Abstract: Stable structures of neutral (V2O5)n clusters (n = 1−5, 8, 10, and 12) are determined by density functional calculations (BP86 functional with a double-ζ (V)/triple-ζ (O) valence basis set augmented by polarization functions). Comparison is made with calculations for the periodic structure of solid V2O5. The most stable structure of the smallest cluster is doubly O-bridged, OV−O2−VO2, and by 184 kJ/mol VO2.5 less stable than the periodic bulk structure. From the tetrahedral V4O10 structure on (41 kJ/mol VO2.5 above the crystal energy) polyhedral cage structures are the most stable isomers: trigonal prism (V6O12), cube (V8O20), pentagonal prism (V10O25), 16-hedron (V16O40), dodecahedron (V20O50), and truncated octahedron (V24O60). The polyhedra have vanadyl groups at the apexes and bridging oxygen atoms on the edges. Differently from the crystal structure, vanadium is 4-fold coordinated and 3-fold coordinated oxygen is avoided. The energies relative to the periodic solid are 22.1, 12.4, 9.4, 5.5, 3.3, and...

138 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