Anion Defects Research Articles

Organotin Chalcogenide Salts: Synthesis, Characterization, and Extended Crystal Structures

Reactions of Cl(3)SnR(1)H with Na(2)E (E = Se, Te; R(1) = C(2)H(4)COO) in liquid ammonia afforded the salts of an anionic defect heterocubane, [(RSn)(3)Se(4)](-) (R = R(1) and R(1)H) or an (infinity)(1)[(R(1)Sn)(2)Te(3)(2-)] anionic polymer, respectively. Although a reaction using a 1:1.5 ratio of Cl(3)SnR(1)H:Na(2)S.(9)H(2)O in acetone/water produced a compound containing [(R(1)Sn)(3)S(4)](2-), a 1:2 ratio leads to the formation of (infinity)(1)[(R(1)Sn)(2)S(3)(2-)], isostructural to the homologous Te species.

Determination of the Diffusivity of Point Defects in Passive Films on NiTi and NiTiAl Alloys

A point defect model based on the movement of cation and anion defects in an electrostatic field was carried out to explain the growth and dissolution behavior of a passivation layer on NiTi and NiTiAl thin films. The calculated value of diffusivity was in range of 10−16 to 10−17 cm2/s. The defect of oxygen vacancy revealed that the passive film was an n-type semiconductor. Mott-Schottky analysis showed that the doping level within a passive film was rather large and in the order of 1020 to 1021 cm−3 film, which was considered to be a highly doped structure. The high-resolution transmission electron microscopy (HRTEM) images showed that the highly doped structure consisted of amorphous and crystalline structures of TiO2 and Al2O3, respectively. A thermodynamic evaluation for the difference between crystalline and the fully amorphous oxides was calculated to be 57.57 and 96.87 kJ/mol, respectively. In the amorphous region, the electronic level arises from the presence of an energy band gap in the ideal crystalline structure. Therefore, the smaller donor density and the lower diffusion coefficient retarded the defect movement in the passivation layer, and improved the stability of the passive film during corrosion.

Interstitial Oxide Ion Order and Conductivity in La1.64Ca0.36Ga3O7.32 Melilite

Solid oxide fuel cells (SOFCs) are a major candidate technology for clean energy conversion because of their high efficiency and fuel flexibility.1 The development of intermediate-temperature (500–750 °C) SOFCs requires electrolytes with high oxide ion conductivity (exceeding 10−2 S cm−1 assuming an electrolyte thickness of 15 μm1). This conductivity, in turn, necessitates enhanced understanding of the mechanisms of oxide ion charge carrier creation and mobility at an atomic level. The charge carriers are most commonly oxygen vacancies in fluorites2, 3 and perovskites.3, 4 There are fewer examples of interstitial-oxygen-based conductors such as the apatites5, 6 and La2Mo2O9-based materials,7–9 so information on how these excess anion defects are accommodated and the factors controlling their mobility is important.

Coexistence of anion and cation vacancy defects in vacuum-annealed In 2O 3 thin films

We report on the defect structure and composition of vacuum-annealed In 2 O 3 (VA-In 2 O 3 ) ferromagnetic thin films. VA-In 2 O 3 is highly (∼7 at.%) oxygen deficient. High-resolution microscopy reveals disordered glassy surface layers and crystalline defect states, O and In vacancies, and In–In clustering, predominantly in the vicinity of the surface. These defects are not observed in as-deposited In 2 O 3 films. These structural defects are important for understanding many of the novel properties found in non-stoichiometric In 2 O 3 , including high conductivity and room-temperature ferromagnetism.

Synthesis and Reactivity of Functionalized Binary and Ternary Thiometallate Complexes [(RT)4S6], [(RSn)3S4]2−, [(RT)2(CuPPh3)6S6], and [(RSn)6(OMe)6Cu2S6]4− (R=C2H4COOH, CMe2CH2COMe; T=Ge, Sn)

A series of compounds comprising functionalized thiometallate cages [(RT)4S6] (R terminated by COO(H) or COR groups), based on adamantane (T=Ge) or double-decker (T=Sn) type structures or [(RSn)3S4]2- anionic defect heterocubanes were synthesized and their reactions with 1) transition-metal compounds and 2) hydrazine derivatives were explored. Hence it was possible to generate functionalized ternary CuSnS or CuGeS clusters and to transfer COR ligands into CR(N-NH2) or CR(N-NHPh) terminal groups, respectively. The report provides the proof-of-principle for a directed functionalization and derivatization of chalcogenidometallate cages with respect to the formation of chalcogenidometallate-organic hybrid compounds containing M/E semiconductor nodes, as an alternative to the so far most prominent M/O combination in metal-organic frameworks. DFT investigations deliver further insight in the peculiarities of Ge/S versus Sn/S precursors and their products.

Thermal Transport in Off‐Stoichiometric Uranium Dioxide by Atomic Level Simulation

The thermal conductivity of hypo‐ and hyperstoichiometric UO2 is calculated as a function of defect concentration and temperature using the direct method in molecular dynamics simulations. Anion defects, the dominant defects in UO2, are shown to significantly influence the thermal conductivity. Lattice dynamics calculations show how this reduction arises from changes in the nature of the lattice vibrations, as characterized by the polarization vectors and participation ratios. In addition, 235U isotopic defects are shown to have a negligible influence on the thermal conductivity.

Recombination luminescence of activated CaI2 crystals

Data are presented on the recombination luminescence of CaI2:Eu2+, CaI2:Gd2+, CaI2:Tl+, CaI2:Pb2+, and CaI2:Mn2+ crystals grown by the Bridgman-Stockbarger method. It follows from their photostimulated luminescence, roentgenoluminescence, and thermostimulated luminescence spectra that the activation of CaI2 with Gd2+, Eu2+, Tl+, Pb2+, and Mn2+ cations, which produce anion defects, leads to the formation of defect complexes which act as electron traps and determine the 90-K photostimulation spectra of the crystals. The observed effect of the nature of the dopant on the photostimulation spectrum indicates that the doped calcium iodide crystals contain near-activator F- and FA-type electron centers. Under x-ray and optical excitation, the trapping levels in the crystals are filled mainly by charge carriers delocalized from hydrogen- and oxygen-containing centers. The activation increases the decay probability of impurity-bound excitons.

Electrochemical studies of capacitance in cerium oxide thin films and its relationship to anionic and electronic defect densities

Small polaron carrier density in epitaxial, doped CeO(2) thin films under low oxygen partial pressure was determined from electrochemically-measured capacitance after accounting for interfacial effects and shown to agree well with bulk values.

Simulation of proton transport in the gramicidin A channel

The free energy profiles for proton transfer along the oriented water file inside the gramicidin A channel were calculated. An original implementation of the rigid-body molecular dynamics method was used for describing the peptide groups of the channel and outer water molecules. The inner water wire was simulated using the PM6 force field parameters, which adequately describe the formation and cleavage of chemical and hydrogen bonds in water molecules. Different mechanisms of proton transfer through the gramicidin A channel were considered, namely, proton H+ translocation, transfer of the anion defect OH−, and reorientation of the water file inside the channel. To facilitate parallel calculations of trajectories, the reaction coordinate was divided into segments, and the results were combined by the weighted histogram analysis method. The first two processes, H+ and OH− transfers, were shown to be barrierless. Only the stage of reorientation of the water file inside the channel has an energy barrier.

Treating Poly(3-hexylthiophene) with Dimethylsulfate Improves Its Photoelectrical Properties

Poly(3-hexylthiophene) is reacted with the methylating agent dimethylsulfate. This increases the hole mobility, almost doubles the exciton diffusion length, and substantially enhances the photochemical stability of the polymer in air. The reaction may involve neutralization of anionic defects via addition of a methyl cation to the polymer backbone.

Quantum chemical modeling of photoadsorption properties of the nitrogen‐vacancy point defect in diamond

Quantum chemical calculations of geometric and electronic structure and vertical transition energies for several low-lying excited states of the neutral and negatively charged nitrogen-vacancy point defect in diamond (NV(0) and NV(-)) have been performed employing various theoretical methods and basis sets and using finite model NC(n)H(m) clusters. Unpaired electrons in the ground doublet state of NV(0) and triplet state of NV(-) are found to be localized mainly on three carbon atoms around the vacancy and the electronic density on the nitrogen and rest of C atoms is only weakly disturbed. The lowest excited states involve different electronic distributions on molecular orbitals localized close to the vacancy and their wave functions exhibit a strong multireference character with significant contributions from diffuse functions. CASSCF calculations underestimate excitation energies for the anionic defect and overestimate those for the neutral system. The inclusion of dynamic electronic correlation at the CASPT2 level leads to a reasonable agreement (within 0.25 eV) of the calculated transition energy to the lowest excited state with experiment for both systems. Several excited states for NV(-) are found in the energy range of 2-3 eV, but only for the 1(3)E and 5(3)E states the excitation probabilities from the ground state are significant, with the first absorption band calculated at approximately 1.9 eV and the second lying 0.8-1 eV higher in energy than the first one. For NV(0), we predict the following order of electronic states: 1(2)E (0.0), 1(2)A(2) (approximately 2.4 eV), 2(2)E (2.7-2.8 eV), 1(2)A(1), 3(2)E (approximately 3.2 eV and higher).

On the composition and atomic arrangement of calcium-deficient hydroxyapatite: An ab-initio analysis

A systematic study of defect constellations in calcium-deficient hydroxyapatite is reported. Along this line, we explore different arrangements for charge compensation, including cationic vacancies and substitutional defects. The overall defect constellation is governed by both the different proton affinity of the anions or energy costs related to vacancy formation and minimization of the Coulomb energy which implies small distances of the anionic and cationic defects. Depending on the type of the calcium-deficient site, this gives rise to two specific defect arrangements. Among these, the calcium ions forming triangles which embed the OH − ions of hydroxyapatite are most likely to be deficient. The resulting charge is compensated by protonation of the OH − ion within the deficient calcium-triangle and protonation of a PO 4 3− ion in the nearest neighbourhood of the vacant calcium site. The strong energetic favouring of such constellations indicates that the commonly used chemical formulae Ca 10− x (HPO 4) x (PO 4) 6− x (OH) 2− x (H 2O) x (0< x⩽1) reflect a reasonable approximation of the composition of calcium-deficient hydroxyapatite.

Surface nature of nanoparticle zinc-titanium oxide aerogel catalysts

Nanoparticle zinc-titanium oxide materials were prepared by the aerogel approach. Their structure, surface state and reactivity were investigated. Zinc titanate powders formed at higher zinc loadings possessed a higher surface area and smaller particle size. X-ray photoelectron spectroscopy (XPS) revealed a stronger electronic interaction between Zn and Ti atoms in the mixed oxide structure and showed the formation of oxygen vacancy due to zinc doping into titania or zinc titanate matrices. The 8–45 nm aerogel particles were evaluated as catalysts for methanol oxidation in an ambient flow reactor. Carbon dioxide was favorably produced on the oxides with anion defects. Titanium based oxides exhibited a high selectivity to dimethyl ether, so that a strong Lewis acidic character suggested for the catalysts was associated primarily with the Ti 4+ center. Both methanol conversion and dimethyl ether formation rates increased with increasing the zinc content added to the oxide support. Results demonstrate that cubic zinc titanate phases produce new Lewis acid sites having also a higher reactivity and that the nature of the catalytic surface transforms from Lewis acidic to basic characters due to the presence of reactive oxygen vacancies.

Theoretical study of O2 adsorption and CO2 formation in bimetallic dimer clusters Au‐M

AbstractCation, anion, and neutral states in Au‐M dimer clusters have been calculated as a way to study in a small model, the Au‐sites, and M‐sites in a supported nanoparticle (NP). These charged states can be formed during a charge transfer process between certain atoms of the NP with the atoms of the support or simply with cationic or anionic defects. These results can be important to evaluate in a preliminary way, the relation between the activity of the Au‐sites and M‐sites because of its charge and the effectiveness of the adsorption of O2, and the subsequent interaction between adsorbed O2 and CO through a Langmuir‐Rideal unimolecular mechanism. For the O2 adsorption over Au active site, the Au‐Ir, Au‐Au, Au‐Ag, Au‐Pd, and Au‐Cu dimers yielded negative binding energies in all the charge states considered (anion, cation, and neutral); when M active site was considered, Au‐Cu and Au‐Ag dimers also gave negative values of binding energies in all the charged states. Furthermore, there was observed that electron rich Au‐sites are essential in the O2 adsorption. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Apparent bipolarity and Seebeck sign inversion in a layered semiconductor: LaZnOP

The optoelectronic properties of a layered mixed-anion compound, LaZnOP, which is expected to be a wide gap $n$-type semiconductor, are examined. LaZnOP shows electronic conduction with conductivities up to $5.2\phantom{\rule{0.3em}{0ex}}\mathrm{S}∕\mathrm{cm}$ at room temperature by Cu doping. The signs of Seebeck coefficients suggest that undoped LaZnOP is typically an $n$-type semiconductor, but Cu-doped LaZnOP is a $p$-type. However, the Seebeck coefficients are very small compared to those observed for isostructural $p$-type semiconductors, $\mathrm{LaCuO}\mathit{Ch}$ $(\mathit{Ch}=\mathrm{S},\mathrm{Se})$, and show a sign inversion as the temperature is varied in the undoped and Zr-doped LaZnOP. The optical band gap is $1.3--1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, which is much smaller than those of related compounds such as LaCuOS $(3.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and ZnO $(3.3\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$. These properties can be understood by the electronic structure obtained using photoemission spectroscopy (PES) and ab initio band calculations. The results indicate that the synthesized LaZnOP has subgap states that pin the Fermi level, and these subgap states form a defect band, which gives the abnormally small Seebeck coefficients and their inversion with temperature. The band calculations suggest that anion defects may be the origin of the defect band implied by the PES measurements and control the observed electrical characteristics.

Molecular dynamics study of oxygen transport and thermal properties of mixed oxide fuels

Evaluation of thermal and transport properties of actinide mixed oxide fuels is a problem of high importance for the development of new generation reactors. Molecular dynamics provides a powerful tool, that can be used for this goal. In this work molecular dynamic simulations have been performed with the purpose to evaluate thermal and diffusion properties of UO 2, PuO 2 and (Pu 0.3U 0.7)O 2 oxide nuclear fuels. Specific heat, thermal conductivity and thermal expansion of these materials were estimated in the temperature range of 300–2500 K and compared with available recommendations. Migration energies of the anion defects via the interstitial and vacancy mechanisms have been estimated together with oxygen self-diffusion coefficients. The presence of a strong interaction between point defects and Pu atoms in (Pu 0.3U 0.7)O 2 oxide has been revealed and this interaction alters the diffusivity of point defects in comparison with the pure uranium and plutonium dioxides.

Annealing effect on nano-ZnO powder studied from positron lifetime and optical absorption spectroscopy

Mechanical milling and subsequent annealing in air at temperatures between 210 and 1200°C have been carried out on high purity ZnO powder to study the defect generation and recovery in the material. Lowering of average grain size (from 76±1to22±0.5nm) as a result of milling has been estimated from the broadening of x-ray lines. Substantial grain growth in the milled sample occurs above 425°C annealing temperature. Positron annihilation lifetime (PAL) analysis of the samples shows a distinct decrease of the average lifetime of positrons very near the same temperature zone. As indicated from both x-ray diffraction (XRD) and PAL results, high temperature (&amp;gt;700°C) annealed samples have a better crystallinity (or lower defect concentration) than even the nonmilled ZnO. In contrast, the measured optical band gap of the samples (from absorption spectroscopy) does not confirm lowering of defects with high temperature annealing. Thermally generated defects at oxygen sites cause significant modification of the optical absorption; however, they are not efficient traps for positrons. Different thermal stages of generation and recovery of cationic as well as anionic defects in granular ZnO are discussed in the light of XRD, PAL, and optical absorption studies.

Influence of lanthanum carbonate phases of Ni/La0.98Sr0.02O x catalyst over the oxidative transformation of methane

This work combines the advantages of ~2 mol.% Sr/La2O3 (i.e. La0.98Sr0.02O x ) for OCM (oxidative coupling of methane) with well-dispersed nickel over metal oxides for POM (partial oxidation of methane) to efficiently transform methane to desirable products. The catalysts were prepared by sequential impregnation and characterized by X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, temperature-programmed reduction, temperature-programmed decomposition of carbonates and BET surface area measurements. The aim of this work is to find out not only the effect of the catalyst compositions over the formation and growth of carbonate phases, but also their influence over the transformation oxidative of methane. The La0.98Sr0.02O x – supported Ni catalyst precursors show a strong nickel oxide–lanthana interaction, involving anionic vacancies or structural defects that induce the formation of island-like structure of La2-x Sr x NiO4-type mixed oxide for 2 mol.% Ni/La0.98Sr0.02O x . Higher nickel composition facilitates the formation of LaNiO3. Surface lanthanum hydroxycarbonate formation is probably associated with a conjugated effect between lanthanum oxide and oxidized nickel phases. For high-nickel amount containing catalysts, carbonate-enriched La2(OH)6-2x (CO3) x as intermediate phase in methane reforming reaction is proposed. For low nickel composition (i.e. 2%), partial oxidation of methane could follow predominantly the pyrolysis mechanism. In which, carbon monoxide is the direct product and carbon dioxide is subsequently formed from carbon monoxide oxidation.

Ab initiostudies of the electronic structure of defects in PbTe

Understanding the detailed electronic structure of deep defect states in narrow band-gap semiconductors has been a challenging problem. Recently, self-consistent ab initio calculations within density functional theory (DFT) using supercell models have been successful in tackling this problem. In this paper, we carry out such calculations in PbTe, a well-known narrow band-gap semiconductor, for a large class of defects: cationic and anionic substitutional impurities of different valence, and cationic and anionic vacancies. For the cationic defects, we study a series of compounds RPb2n-1Te2n, where R is vacancy or monovalent, divalent, or trivalent atom; for the anionic defects, we study compounds MPb2nTe2n-1, where M is vacancy, S, Se or I. We find that the density of states (DOS) near the top of the valence band and the bottom of the conduction band get significantly modified for most of these defects. This suggests that the transport properties of PbTe in the presence of impurities can not be interpreted by simple carrier doping concepts, confirming such ideas developed from qualitative and semi-quantitative arguments.

Defect Structure in (Zn,Mg)O Films Prepared on YSZ Substrate

The defect structure of undoped ZnO and (Zn1-x,Mgx)O solid-solution films were deposited on YSZ substrate with pulsed laser deposition (PLD) to investigate defect equilibria in those films. In particular, the effects of thermal treatment on the structures and prosperities of (Zn1-x,Mgx)O solid-solution films were examined. The films with high MgO concentration (x&gt;0.12) decomposed to the wurtzite-type and rock-salt-type phase after thermal treatment, indicating that the solubility limit of Mg was about x=0.12 and the wurtzite-type (Zn,Mg)O films with x&gt;0.12 were indicated to be non-equilibrium ones. The subsequent analyses of oxygen diffusivity in those films revealed that the films under non-equilibrium state, i.e., wurtzite-type (Zn1-x,Mgx)O with x&gt;0.12, contained significantly high concentration of anion defects.