Degree Of Charge Localization Research Articles

Understanding Chemical Expansion in Pr-Based Mixed Conducting Perovskites PrGa0.9Mg0.1O3 and BaPr0.9Y0.1O3

Chemical expansion is a strain induced by a change in stoichiometry, such as oxygen loss, and it can have a significant impact on device performance and lifetime. While large coefficients of chemical expansion (CCE) are needed for high displacements in an actuator, the same, large CCE can be deleterious to device longevity in a fuel cell where large chemical potential gradients exist across very small thicknesses. The breadth of CCE values needed in various devices calls for the development of design rules to tailor CCE for optimal material response, so our work targets the establishment of such structure-property insights.Oxygen-loss-induced, stoichiometric chemical expansion in oxides involves the formation of an oxygen vacancy; when oxygen leaves the lattice, charge compensating electrons are left behind and can localize on nearby multivalent cations. As cations are reduced, their atomic radii and the surrounding lattice expand. An empirical formula describing the pseudo-cubic lattice constant of perovskite materials has been developed [1] which relates the lattice parameter to the ionic radii of cation and anion components. This equation predicts that changes in the B-site cation size will have a larger effect on the lattice parameter than an equal change at the A-site. If the multivalent cation is the only one changing size during redox processes, this equation suggests that its placement on the A or B site will have a significant effect on the magnitude of the overall lattice strain during oxygen loss or gain.In an effort to develop and understand design rules to tailor CCE, two compositions, PrGa0.9Mg0.1O3 (PGM) and BaPr0.9Y0.1O3 (BPY), have been synthesized. These compositions allowed for a comparison between A and B-site multivalent Pr (nominally 3+/4+); however, we found that the empirical model did not adequately predict the differences in CCEs on this basis. Other factors including crystal symmetry, charge localization, and location of charge (anion or cation) were instead found to be significantly impactful for both compositions [2]. Values of CCE have been determined by characterizing isothermal changes in stoichiometry with thermogravimetric analysis (TGA) and corresponding changes in strain with dilatometry and in situ, high temperature XRD (HTXRD) as a function of oxygen partial pressure (pO2). The degree of charge localization has been interpreted from impedance measurements of the temperature dependence of conductivity, and the experimental results have been compared to density functional theory (DFT+U) calculations. Over the pO2 and temperature range studied, PGM and BPY have low CCEs, therefore making them of potential interest for fuel/electrolysis cell electrodes. The effects of the abovementioned design rules are discussed to provide insights into rational material design for tailored CCE.[1] Marrocchelli, D., Perry, N. H., & Bishop, S. R. (2015). Understanding chemical expansion in perovskite-structured oxides. Physical Chemistry Chemical Physics, 17(15), 10028-10039.[2] Ricote, S., Hudish, G., O’Brien, J. R. & Perry, N. H. Non stoichiometry and lattice expansion of BaZr0.9Dy0.1O3-δ in oxidizing atmospheres. Solid State Ionics (2019) doi:10.1016/j.ssi.2018.12.006.

Ultrafast structural pathway of charge transfer in n,n,n',n'-tetramethylethylenediamine.

We have explored the ultrafast molecular structural dynamics associated with charge transfer in N,N,N',N'-tetramethylethylenediamine using Rydberg fingerprint spectroscopy in conjunction with self-interaction corrected density functional theory. Excitation at 239 nm prepares the molecule in the Franck-Condon region of the 3s state with the charge localized on one of the two amine groups. As seen from the time-dependent Rydberg electron binding energies, the pathway of the rapidly ensuing dynamics leads through several structurally distinct conformers with various degrees of charge localization before reaching the fully charge-delocalized structure on a picosecond time scale. At several steps along the reaction path, the transient structures are identified through a comparison of the spectroscopically observed binding energies with computed values. The molecular structure is seen to evolve dynamically from an initially folded conformer to the stretched form that supports charge delocalization before an equilibrium sets in with forward and backward time constants of 1.19 (0.14) and 2.61 (0.31) ps, respectively. A coherent wavepacket motion in the charge-localized state with a period of 270 (17) fs and damping of 430 (260) fs is observed and tentatively assigned to the nitrogen umbrella motion. The damping time constant indicates the rate of the energy flow into other vibrations that are not activated by the optical excitation.

Chemical Expansion of Mixed Ionic and Electronic Conducting Perovskites

Mixed ionic and electronic conducting oxides, employed in, e.g., catalysts and solid oxide fuel cells (SOFCs), often store and release large quantities of oxygen during operation. Upon oxygen release (i.e., during exposure to lower oxygen pressure or higher temperature), multivalent cations reduce their oxidation state, resulting in an expansion that typically outweighs the small lattice contraction around the oxygen vacancies that form [1]. This chemo-mechanical coupling between lattice expansion and change in chemical composition can be described quantitatively by the chemical coefficient of expansion (CCE). When materials with high CCEs are incorporated into operating devices, such as SOFCs, the large stresses accompanying stoichiometry changes can lead to mechanical failure and shortened device lifetimes. Therefore, CCEs of new materials need to be characterized, and factors controlling the chemical expansion need to be investigated in order to engineer materials with low CCEs. In this work the CCEs of two candidate SOFC cathodes, La0.9Sr0.1Ga1-xNixO3-δ (LSGN; 0 ≤ x ≤ 0.5) and SrTi0.65Fe0.35O3-α (STF), were determined as part of an ongoing effort to understand factors governing chemical expansion in perovskites. Prior computational simulations from our group identified that the degree of charge localization on the multivalent cations can play a significant role in determining the CCE, with delocalized charges (i.e., metallic behavior) resulting in decreased changes in cation radii upon reduction [2]. In this presentation, experimental evidence for the effect of charge localization on chemical expansion in LSGN, recently measured for the first time, will be discussed. Bulk samples prepared by the Pechini method were measured from 600 – 900 °C in N2/O2 mixtures by thermogravimetric analysis (TGA), to determine oxygen non-stoichiometry and defect equilibria, and by dilatometry and in situ X-ray diffraction, to determine the corresponding lattice expansion. For LSGN, increasing charge delocalization was achieved by increasing the Ni content, as demonstrated through electrical conductivity measurements [3] and subsequent analysis of electronic mobilities using carrier concentrations derived from TGA data with defect modeling. Correspondingly, the measured CCE decreased by 13% at 800 °C upon increasing the Ni concentration from 0.1 to 0.5 [4], in agreement with the theoretical prediction of decreasing CCE with increasing charge delocalization. Additionally, recent work on measuring the CCE for STF will be presented. STF displays similar chemical expansion behavior to LSGN, with the CCE at 800 °C being comparable to the highest value measured for LSGN.[1] D. Marrocchelli, S. R. Bishop, H. L. Tuller, and B. Yildiz, Advanced Functional Materials, 22 (9) 1958-1965 (2012).[2] D. Marrocchelli, S. R. Bishop, H. L. Tuller, G. W. Watson, and B. Yildiz, Physical Chemistry Chemical Physics, 14, 12070-12074 (2012).[3] N. J. Long, F.’ Lecarpentier, and H. L. Tuller, Journal of Electroceramics, 3 (4), 399-407 (1999).[4] N. H. Perry, J. E. Thomas, D. Marrocchelli, S. R. Bishop, and H. L. Tuller, ECS Transactions, 57 (1) 1879-1884 (2013).

Chemical Strain and Oxidation-Reduction Kinetics of Epitaxial Thin Films of Mixed Ionic-Electronic Conducting Oxides Determined by X-Ray Diffraction

X-ray diffraction, at high T's and switching between N2/air atmospheres, was used to compare the chemical expansion due oxygen non-stoichiometry variations between epitaxial films of different mixed ionic-electronic conductors: La0.6Sr0.4CoO3−δ(LSC), Ba0.5Sr0.5Co0.8Fe0.2O3−δ(BSCF), LaNiO3−δ(LNO), La2NiO4+δ(L2NO) and GaBaCo2O5.5+δ(GBCO) and La0.7Sr0.3MnO3−δ(LSM). LSC and BSCF show the largest relative change in the cell parameter Δc/c = +0.5%, while L2NO and GBCO show negative Δc/c = −0.2% and −0.1%, respectively. LNO and LSM show either reduced or negligible chemical expansions. In all cases the values correspond to their particular defect equilibrium and degree of charge localization. The oxygen surface exchange kinetics was also evaluated from in-situ time-resolved analyses of the cell parameter variations. LSC, LNO and GBCO films show fast oxygen reduction kinetics, kchem = 5·10−6, 3·10−6, and 2·10−7 cm/s at 700°C, respectively, in relative agreement with reported values, while BSCF films show much slower kinetics than expected, below kchem = 10−7 cm/s at 650°C, related to the degradation process observed in the films.

Intrinsic ferromagnetism in CeO2: dispelling the myth of vacancy site localization mediated superexchange

Intrinsic ferromagnetism in CeO2 is a source of controversy in the literature and has been linked to the excess electronsleft over upon oxygen vacancy formation on Ce sites neighbouring the vacancy.A recent theoretical study (Han et al 2009 Phys. Rev. B 79 100403) concludedthat increased vacancy concentration changes the localization behaviour ofCeO2, resulting in some degree of charge localization in the vacancy site itself, which leads tosuperexchange and polarization effects that enhance the stability of ferromagnetism. Inthis report, we show conclusively that oxygen vacancy concentrations of up to12.5% do not cause localization in the vacancy site, and that this is not responsible for anyenhanced ferromagnetism. Investigation of oxygen vacancies on the (111), (110)and (100) low index surfaces also show no evidence for ferromagnetic preference.

Charge Transport in Proteins Probed by Resonant Photoemission

The degrees of charge localization in the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of the bacterial surface layer protein of Bacillus sphaericus NCTC 9602 were studied by resonant photoemission. In agreement with a charge transport hopping mechanism that involves torsional motions of the peptide backbone, the lifetime of electrons excited into the LUMO was found to be approximately 100 fs.

Charge Delocalization in the Special-Pair Radical Cation of Mutant Reaction Centers of Rhodobacter sphaeroides from Stark Spectra and Nonadiabatic Spectral Simulations

Stark and absorption spectra for the hole-transfer band of the bacteriochlorophyll special pair in the wild-type and L131LH, M160LH, and L131LH/M160LH mutants of the bacterial reaction center of Rhodobacter sphaeroides are presented, along with extensive analyses based on nonadiabatic spectral simulations. Dramatic changes in the Stark spectra are induced by the mutations, changes that are readily interpreted in terms of the redox-energy asymmetry and degree of charge localization in the special-pair radical cation. The effect of mutagenesis on key properties such as the electronic coupling within the special pair and the reorganization energy associated with intervalence hole transfer are determined for the first time. Results for the L131LH and M160LH/L131LH mutants indicate that these species can be considered as influencing the special pair primarily through modulation of the redox asymmetry, as is usually conceptualized, but M160LH is shown to develop a wide range of effects that can be interpreted in terms of significant mutation-induced structural changes in and around the special pair. The nonadiabatic spectra simulations are performed using both a simple two-state 1-mode and an extensive four-state 70-mode model, which includes the descriptions of additional electronic states and explicitly treats the major vibrational modes involved. Excellent agreement between the two simulation approaches is obtained. The simple model is shown to reproduce key features of the Stark effect of the main intervalence transition, while the extensive model quantitatively reproduces most features of the observed spectra for both the electronic and the phase-phonon regions, thus giving a more comprehensive description of the effect of the mutations on the properties of the special-pair radical cation. These results for a series of closely related mixed-valence complexes show that the Stark spectra provide a sensitive indicator for the properties of the mixed-valence complexes and should serve as an instructive example on the application of nonadiabatic simulations to the study of mixed-valence complexes in general as well as other chemical systems akin to the photosynthetic special pair.

Charge distribution in C 60 crystal doped by electric field

The calculations of the charge distribution in the C 60 -based FET structure are presented. A simple model is proposed to describe the distribution of injected electrons or holes between two-dimensional layers. The calculations show that the relative layer charges are independent on the total amount of injected charges. The charge density is maximal on the surface layer and drops exponentially with the depth increase. The relative portions of injected charge involved in the top layer are 73 and 64 per cent in the case of electron and hole injection, respectively. Thus, the degree of charge localization on the crystal surface turns to be markedly different from the result obtained earlier within the tight-binding model predicting near-complete localization for the charge concentration providing superconductivity.

Charge Migration in DNA: Ion-Gated Transport

Electron hole (radical cation) migration in DNA, where the quantum transport of an injected charge is gated in a correlated manner by the thermal motions of the hydrated counterions, is described here. Classical molecular dynamics simulations in conjunction with large-scale first-principles electronic structure calculations reveal that different counterion configurations lead to formation of states characterized by varying spatial distributions and degrees of charge localization. Stochastic dynamic fluctuations between such ionic configurations can induce correlated changes in the spatial distribution of the hole, with concomitant transport along the DNA double helix. Comparative ultraviolet light-induced cleavage experiments on native B DNA oligomers and on ones modified to contain counterion (Na(+))-starved bridges between damage-susceptible hole-trapping sites called GG steps show in the latter a reduction in damage at the distal step. This reduction indicates a reduced mobility of the hole across the modified bridge as predicted theoretically.

Mobilities of carbon cluster ions: Critical importance of the molecular attractive potential

Mobilities in helium gas for isomers belonging to the major structural families of carbon clusters identified in drift tube studies (chains, monocyclic and bicyclic rings, graphite sheets, and fullerenes and their dimers) have been evaluated by trajectory calculations employing a realistic ion-He interaction potential. For all the species considered, the agreement between the measured and calculated mobilities at room temperature improves by at least a factor of 3 over that obtained with the widely used hard-sphere projection approximation. Furthermore, for a large representative sample of clusters belonging to all the above families, the results of trajectory calculations as a function of temperature over the range of 78–360 K are in a good agreement with the measured mobilities. This shows that the C–He pairwise potential is only weakly dependent on the structure and chemical bonding of a carbon cluster. Thus this study demonstrates the universal suitability of trajectory calculations for the accurate prediction of the gas phase mobilities for polyatomic ions with various shapes and sizes, and the uniform superiority of this method over the previously used approximations. In particular, the trajectory calculations for large (n=120–140) fullerenes show that these cages have near-spherical shapes found by theory, while the projection approximation would erroneously assign them as “buckytubes.” It also appears that the mobility may be substantially affected by the degree of charge localization on a specific atom in the cluster, especially at low temperatures.

Heterogeneous electron transfer kinetics for a variety of organic electrode reactions at the mercury-acetonitrile interface using either tetraethylammonium perchlorate or tetraheptylammonium perchlorate electrolyte

The electrode reactions of 26 compounds have been studied by cyclic voltammetry at mercury electrodes using acetonitrile as solvent and either 0.10 M tetraethylammonium perchlorate (TEAP) or 0.10 M tetra-n-heptylammonium perchlorate (THpAP) as supporting electrolyte. The standard potentials of the compounds ranged from −0.07 to −2.90 V vs. the Ag/AgNO 3 reference electrode. In several of the redox couples studied, the charge on the ionic partner was localized on a few atoms which is expected to cause the outer reorganization energy to be high, giving small electron transfer rate constants. However, is some instances it does not appear that the degree of charge localization is great enough to account fully for the small rate constants. It is argued that the inner reorganization energy should not be ignored in such cases. The rate constants observed for THpAP were always smaller than those for TEAP when the values were small enough to measure in at least one electrolyte. This general result was explained by a suppression of the probability of electron transfer by an adsorbed film containing mainly tetra-n-heptylammonium ions. The interpretation was supported by measurement of the activation parameters for nitroethane in the two electrolytes. The activation enthalpies were almost identical but the preexponential factor for THpAP was only 1/44 that for TEAP.

A theoretical study of the site selectivity of activating methylthio and methoxy groups in the synthesis of isoquinolines

The site selectivity of activating methylthio and methoxy groups in the synthesis of isoquinolines has been studied theoretically in terms of the static index approach. The dependence of the relative amounts of ortho- and para-isomer produced upon the nature of the electrophilic group has been explained in terms of the degree of charge localization in the electrophile, together with the calculated electronic structure of the activated benzene ring.

Quantum-chemical simulation of active centres of anionic polymerization of ethylene oxide

Quantum-chemical CNDO/2 and CNDO/BW methods were used to calculate equilibrium geometry, charge distribution and the total energy of CH 3(OCH 2CH 2) n O-anions when n=0, 1, 2, 3 and of their ionic pairs with Na + as models of active centres of anionic polymerization of ethylene oxide. A high (up to 0·83) and practically independent (of n) degree of charge localization on terminal oxygen and significant donor properties of intra-chain oxygen were established. Cyclic auto-solvates of ionic pairs ( n=1, 3) and complexes of dimethoxyethane were examined as models of the monomer unit of polyethylene oxide with H + and Na + and it was shown that they are useful, in comparison with complex-uncombined forms. Results are in agreement with main mechanisms of anionic polymerization of ethylene oxide.

Mass Spectral Fragmentation of Unsymmetrical Diesters of Malonic Acid and Succinic Acids

Abstract The mechanisms of mass spectral break down of unsubstituted malonate and succinate esters have been more completely defined by this study of unsymmetrical diesters. Rate determining rearrangement prior to extrusion of CO in daughter ions of succinate esters was verified by observation of broad metastable peaks. A Hammett plot correlation was interpreted as resulting from a greater degree of charge localization on the ester moiety in the decreasing order of -CO2(i-Pr)≥-CO2(n-Pr)≥-CO2Et≥-CO2CH3. The mass spectral fragmentation of symmetrical diesters of malonic acid1 and methyl esters of other diacids2 have been previously investigated. However, this mass spectral investigation of unsymmetrical diesters of malonic and succinic acids have permitted more complete determination of all the associated fragmentation pathways.

Electron‐impact induced fragmentation of partially fluorinated β‐diketones

AbstractThe ionic fragmentation of twelve partially fluorinated β‐diketones, RCOCH2COCF3 was studied with a medium resolution mass spectrometer. In addition to the anticipated fragmentation by β‐cleavage, a number of interesting rearrangements are observed. [CF3COCH2CO]+., an ion common to all of the compounds investigated, decomposes predominantly by elimination of HF. When R is an alkyl group containing hydrogen γ to the adgacent carbonyl, the McLafferty rearrangement occurs. The [RCOCH2CO]+.ion eliminates neutral RH when R is phenyl, thienyl, or cyclopropyl. An intense metastable peak, absent when R is an alkyl substituent, accompanies the rearrangement in these three compounds. A number of fragments characteristic of the R substituent are also observed in many cases. This is interpreted as indicating a considerable degree of charge localization on the R moiety.