Stability Of Charged States Research Articles

Influence of Phenazine Structure on Polaron Formation in Polyaniline: In Situ Electron Spin Resonance−Ultraviolet/Visible−Near-Infrared Spectroelectrochemical Study

The role of the phenazine structure in the stabilization of charged states in polyaniline was studied by in situ electron spin resonance (ESR)-UV/vis-near-infrared (NIR) spectroelectrochemistry of polyaniline and the copolymers of aniline and a phenazine derivative (3,7-diamino-5-phenylphenazinium chloride, phenosafranine). It is shown that the copolymer can be prepared by electropolymerization, and its structure was confirmed by mass spectrometry and IR spectroscopy. The electrochemistry of polyaniline and its copolymer pointed to preferred stabilization of a polaron pair in the charged states at the initial charge transfer reaction instead of polarons that are formed by equilibrium reaction at higher electrode potentials. A second polaron pair is detected for higher doped states of the polymer films. A mechanism of the formation of charged states in polyaniline and their equilibrium is given. It is shown that in situ ESR-UV/vis-NIR spectroelectrochemistry is the method of choice to differentiate between polarons and polaron pairs in their potential-dependent formation. Thus, by this in situ spectroelectrochemical method the influence of phenazine structure on the formation of polarons in aniline polymers and copolymers can be followed.

Density-functional calculations of defect formation energies using the supercell method: Brillouin-zone sampling

Using the DFT supercell method, the BZ sampling error in the formation energy and atomic structure are investigated for vacancy and interstitial defects in diamond and silicon. We find that the $\mathbf{k}$-point sampling errors in the total energy vary considerably depending on the charge state and defect type without systematic cancellation, even for the same size of supercell. The error in the total energy increases with decreasing electronic perturbation of the defect system relative to the perfect bulk; this effect originates in the localization of electronic states due to the symmetry reduction induced by the presence of a defect. The error in the total energy is directly transferred to the formation energy, and consequently changes the thermodynamic stability of charge states and shifts the ionization levels. In addition, in force calculations and atomic structure determinations, the $\mathbf{k}$-point sampling error is observed to increase as the charge becomes more negative. The $\ensuremath{\Gamma}$-point sampling results in erroneously large relaxation of the four atoms surrounding a vacancy in diamond. We suggest that stronger repulsions between electrons occupying degenerate defect levels at $\ensuremath{\Gamma}$-point compared to those occupying split energy levels at other $\mathbf{k}$ points induces larger atomic movements.

Structure changes in lithium manganese spinel after high-temperature storage

Li1+xMn2-xO4 spinels (x=0.0–0.1) after storage at several temperatures were investigated by the Rietveld analysis of high-resolution neutron powder diffraction data. With increasing storage temperature in an electrolyte, the Mn site is likely to be more deficient. In chemically Li-deintercalated Li1.03Mn1.97O4, which was used as a model system for studying the stability of charged states with different charge depths, phase separation due to variation of the Li occupancy in the structure proceeded in slightly deintercalated spinels after storage at 80 °C.

Charge-State Stability and Optical Transitions of Oxygen Impurities in Barium Fluoride Crystal

ABSTRACTThe linear combination of atomic orbital-molecular orbital embedded-cluster is used to investigate the electronic structure and binding energies of small oxygen impurity clustersin the local density theory. Binding energies for substitutional oxygen ions in BaF2 crystal are studied for its 2–, 1–, and 0 charge states. These are used to assess charge-state stability. The energy eigenvalue spectra and optical transitions are obtained by transition state calculations for the oxygen impurities. These results of optical transition calculations obtained are very good and provide a useful interpretation of the ultraviolet-absorption experiments.

Comments on the stability of charge states and locations for hydrogen and muonium in semiconductors

This paper brings together some current concepts concerning hydrogen states in semiconductors (illustrated with reference to silicon but quite generally applicable) and highlight certain findings of recent quantum electronic structure calculations for defect centres involving hydrogen and its isotopes. As regards muonium, the situation which held in the early μSR literature [1] is entirely reversed: the location and structure of Mu* are now well established [2], whereas the precise location and the nature of the metastability of Mu′ have become open questions! In fact, neither state is stable in the presence of other electrically active impurities, which undoubtedly accounts for the difficulty in detecting paramagnetic protium. The implications for the interpretation of existing data, and for various possible future experiments, are examined.

Charge-state stability of Ni and Cu impurities in MgO.

Total energies for substitutional Ni and Cu ions in MgO are studied for their 1+, 2+, and 3+ charge states. These are used to assess charge-state stability. Specifically, electron loss to the conduction band for ${\mathrm{Ni}}^{+}$ and electron capture from the valence band for ${\mathrm{Ni}}^{3+}$ and ${\mathrm{Cu}}^{3+}$ are considered. The impurity and its near-neighboring ions are analyzed in the unrestricted Hartree-Fock self-consistent-field approximation, as a molecular cluster embedded in a consistently relaxed shell-model lattice. The results indicate that both electron loss for n=1 and electron capture for n=3 should occur, so that disproportionation of the various charge states is intrinsically unstable.

Charge state stability: “Negative-U” centres in ionic crystals

Abstract Many impurity and defect centres can exist in several charge states in a given ionic host lattice. In special cases a particular charge state may be unstable against the reaction: Such “negative U” centres have important consequences for both amorphous and crystalline semiconductors. In ionic crystals, whether or not the reaction is exothermic determines which charge states will be stable and, where appropriate, which capture-luminescence transitions or charge-transfers can occur. We have used the HADES code to make systematic quantitative calculations for a number of systems, notably for transition metal ions in MgO and CaO and for CsI:Na. From these we can assess the important factors both for charge-state stability and for negative U. Indeed, the fact that certain transition metal ions (e.g. Fe+ in MgO) are observed to be stable puts bounds on the electron affinity of the host lattice. These bounds may be used to estimate the stability of other defects in their various charge states. In none o...