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.

Band Gap Tuning in ZnO Through Ni Doping via Spray Pyrolysis

Abstract: The tuning of semiconductor band gaps can often provide significant performance increases and new applications for electronic, optoelectronic, and photocatalytic devices. Here, we study the band gaps of pure and nickel-doped zinc oxide thin films synthesized using the low-cost spray pyrolysis deposition method. Nickel concentration is varied from 0 to 15%, and the effects that this doping has on the electronic structure are analyzed. Using optical and synchrotron X-ray techniques, two regimes of band gap reduction via Ni doping are uncovered. For doping up to 4% Ni, there is a strong reduction in the gap, while continued doping up to 15% further reduces the gap, but to a lesser extent. The results are explained using X-ray spectroscopy and an Anderson impurity model. These tools show that the low doping case is driven by the interaction of the Ni 3d and O 2p states in both the valence and conduction bands. At high doping, the removal of Zn 3d states from the valence band and the change in Ni coordination ...

Theory of the Optical Properties of Quartz in the Infrared

Abstract: A theoretical study has been carried out of the eight non-degenerate optical vibrations of $\ensuremath{\alpha}$ quartz including the problem of their infrared and Raman intensities. The investigation consists of three parts: First, the atomic motions and frequencies are calculated on the basis of a valence force model. It is shown that the 207 ${\mathrm{cm}}^{\ensuremath{-}1}$ vibration involves atomic motions very similar to those of the $\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta}$ transformation. Secondly, a general discussion is given of infrared absorption in complex crystals, which shows that in quartz the intensities are determined by the atomic motions through 12 effective charge parameters. The intensities are calculated in good agreement with experiment on the assumption of a 2-charge model suggested by the valence nature of quartz. It is shown that a 1-charge model, the usual model for an ionic crystal, cannot account for the observed intensities. Finally, the relative Raman intensities are calculated with no adjustable parameters in good agreement with experiment on the basis of a simple assumption about the atomic polarizabilities. The calculation accounts for the surprising weakness of the 1082-${\mathrm{cm}}^{\ensuremath{-}1}$ stretching vibration in the Raman effect. The three parts of the investigation are mutually dependent, since the infrared and Raman intensities depend in an essential way upon the atomic motions corresponding to each frequency. It is shown that a consideration of the Raman intensities as well as the usual comparison of frequencies is required to determine the bending constants of the valence force model. It is inferred from the success of the calculations that the three principal assumptions of the present work, namely the valence force model for the vibrations, the 2-charge model for the infrared intensities, and the simple Raman model, are all applicable for quartz.

X-ray Photoemission Spectroscopy Study of Cationic and Anionic Redox Processes in High-Capacity Li-Ion Battery Layered-Oxide Electrodes

Abstract: Electrode materials based on Li-rich layered oxides are of growing interest for high-energy Li-ion battery applications because of their staggering capacities associated with the emergence of a novel, reversible anionic process. However, the fundamental science at work behind this new process needs to be well understood for further optimization. Here we report on the redox mechanisms in high-capacity Li-rich materials Li2Ru1–xMxO3 and Li2Ir1–xMxO3, by combining X-ray photoemission spectroscopy (XPS) core peaks and valence intensity analyses. We fully confirm that these materials electrochemically react with Li via cumulative reversible cationic/anionic redox processes, but more importantly we reveal that, depending on the nature of the metal (Ru or Ir), there is a delicate balance between metal and oxygen contributions. For instance, we show a greater implication of oxide ions for Ir-based electrodes, consistent with the higher covalent character of Ir–O bonds compared to Ru–O bonds. We equally provide ev...

Site-preference and valency for rare-earth sites in (R-Ce)2Fe14B magnets

Abstract: Rare-earth (R) permanent magnets of R2Fe14B have technological importance due to their high energy products, and they have two R-sites (Wyckoff 4f and 4g, with four-fold multiplicity) that affect chemistry and valence. Designing magnetic behavior and stability via alloying is technologically relevant to reduce critical (expensive) R-content while retaining key properties; cerium, an abundant (cheap) R-element, offers this potential. We calculate magnetic properties and Ce site preference in (R1−xCex) 2Fe14B [R = La,Nd] using density functional theory (DFT) methods—including a DFT + U scheme to treat localized 4f-electrons. Fe moments compare well with neutron data—almost unaffected by Hubbard U, and weakly affected by spin-orbit coupling. In La2Fe14B, Ce alloys for 0≤x≤1 and prefers smaller R(4f) sites, as observed, a trend we find unaffected by valence. Whereas, in Nd2Fe14B, Ce is predicted to have limited alloying (x≤0.3) with a preference for larger R(4g) sites, resulting in weak partial ordering and s...