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.

Bulk electronic structure of SrTiO3: Experiment and theory

Abstract: Valence electron-energy loss spectroscopy (VEELS) in a dedicated scanning transmission electron microscope, vacuum ultraviolet spectroscopy and spectroscopic ellipsometry, and ab initio band structure calculations in the local density approximation have been used to determine the optical properties and the electronic structure of SrTiO3 Assignments of the interband transitions in the electronic structure of bulk SrTiO3 have been determined quantitatively by comparison of VEELS spectra with vacuum ultraviolet spectra and with the ab initio calculated densities of states The experimentally determined indirect band gap energy is 325 eV, while the direct band gap energy is 375 eV The conduction bands in SrTiO3 correspond to the bands composed of mainly Ti 3d t2g and eg states, followed at higher energies by the bands of Sr 4d t2g and eg states, and free electron like states dominating at energies above 15 eV The upper valence band (UVB) contains 18 electrons in dominantly O 2p states, hybridized with Ti and Sr states, and has a bandwidth of 5 eV The interband transitions from the UVB to the Ti 3d bands and to the Sr 4d bands give rise to the transitions spanning from the indirect band gap energy of 325 eV up to 15 eV The lower valence band contains 12 electrons in Sr 4p and O 2s states which are separated by 2 eV, while having a bandwidth of 5 eV The interband transitions from the Sr 4p to the Ti 3d and Sr 4d bands give rise to transition energies spanning from 15 to 24 eV Interband transitions from the O 2s band to the conduction bands appear at 26 eV A very narrow band at −33 eV below the top of the valence band is composed of Sr 4s and Ti 3p states and contains eight electrons

The crystal chemistry of hexavalent uranium; polyhedron geometries, bond-valence parameters, and polymerization of polyhedra

Abstract: The geomeffy, bond valences, and polymerization ofhexavalent uranium polyhedra from 105 well-refined structures are analyzed. The Utu cation is almost always present in crystal stnrctures as part of a nearly linear (UOr)z* uranyl ion that is coordinated by four, five or six equatorial anions in an approximately planar arangement perpendicular to the uranyl ion, giving square, pentagonal and hexagonal bipyramids, respectively. The Utu-O7\" bond length (Oy,: uranyl-ion O atom) is independent of the equatorial anions of the polyhedra;-averages of all polyhedra tlat contain uranyl ions ffs; I6lIJ6f-Or. = 1.79(3), mg0.-.9 a,= 1.79(4), and t8lu6+-Our = 1.78(3) A. Not a[ r6lu6+ polyhedra contain uranyl ions; there is a continuous series of coordiaation polyhedr4 from square bipyramidal polyhedra with uranyl ions to holosymmehic octahedral geometry. The mUo* and t8lu6+ polyhedra invariably contaitl a uranyl ion. The equatorial U6.-0 (0: O,-, OH-) bond-lengths of uranyl polyhedra depend upon coordhation number; averages for all polyhedra are t6lu6+-dq = 2,28(5), rlUot-$* = 2.37(9), afi t8tlJ6+-$q2.47 (12) A. Cunently available bond-valence parameters for U& are unsatisfactory for determining bond-valence sums. Coordination-specific bond-valence paxameters have been derived for U6|, together with parameters applicable to all coordination geometries. The parameters give bond-valence sums for Ue of -6 vlr and reasonable bond-valences for Uc,-Ou, bonds. The bond-valence paraneters facilitate the recognition of Ua, U5+ and U6| catiotrs in refined crystal structures. The crystal-chemical consfraints ofpolyhedral polymerization in uranyl phases are discussed.

High-Tc Molecular-Based Magnets: Ferrimagnetic Mixed-Valence Chromium(III)-Chromium(II) Cyanides with Tc at 240 and 190 Kelvin

Abstract: Molecular-based magnets with high magnetic-ordering temperatures, T(c), can be obtained by mild chemistry methods by focusing on the bimetallic and mixed-valence transition metal micro-cyanide of the Prussian blue family. A simple orbital model was used to predict the electronic structure of the metal ions required to achieve a high ordering temperature. The synthesis and magnetic properties of two compounds, [Cr(5)(CN)(12)].10H(2)O and Cs(0.75) [Cr(2.125)(CN)(6)].5H(2)O, which exhibited magnetic-ordering temperatures of 240 and 190 kelvin, respectively, are reported, together with the strategy for further work.

Preparation, structure, and magnetic properties of a dodecanuclear mixed-valence manganese carboxylate

Abstract: This new type of dodecanuclear crystalline complex was obtained by reaction of Mn 2÷ with MnO 4 in acetic and propionic acids. The reddish-black acetate complex has the formula [Mn12(CHaCOO)I6(H20)4OI2 ] .2CH 3COOH.4H20, established by chemical and singlecrystal X-ray diffraction methods. This complex is tetragonal, space group I~,, with a -- 17.319 (9), c = 12.388 (7)/t,, V = 3716 ,/k 3, Z = 2, M r = 2060.3, D c = 1.84, D,,, = 1.83 Mg m -a. The final R and R w were 0.045 and 0.034 for 1172 non-zero reflexions. The crystals are built up of [MnIE(CH3COO)16(H20)4012] molecules, waters of crystallization and disordered acetic acid molecules. In the dodecanuclear molecules, which have 4 ($4) crystallographic symmetry, the Mn atoms are linked by triply bridging oxo O atoms and by carboxylate bridges from acetate anions. The occurrence of a strong Jahn-Teller effect in Mn a+ ions differentiates the Mn 3+ and Mn 4+ ions. The interesting magnetic properties (the magnetic moment increases from 30-9 x 10 -24 J T -1 at 3-3 K to a maximum of 56.5 x 10 -24 J T -~ in the range 17-31 K and then decreases to 33.4 x 10 -24 J T -~ at 280K per Mn atom) may be interpreted in terms of the Mn-Mn distances and superexchange via bridge O atoms.