Influence of surface atomic structure demonstrated on oxygen incorporation mechanism at a model perovskite oxide

Perovskite oxide surfaces catalyze oxygen exchange reactions that are crucial for fuel cells, electrolyzers, and thermochemical fuel synthesis. Here, by bridging the gap between surface analysis with atomic resolution and oxygen exchange kinetics measurements, we demonstrate how the exact surface atomic structure can determine the reactivity for oxygen exchange reactions on a model perovskite oxide. Two precisely controlled surface reconstructions with (4 × 1) and (2 × 5) symmetry on 0.5 wt.% Nb-doped SrTiO3(110) were subjected to isotopically labeled oxygen exchange at 450 °C. The oxygen incorporation rate is three times higher on the (4 × 1) surface phase compared to the (2 × 5). Common models of surface reactivity based on the availability of oxygen vacancies or on the ease of electron transfer cannot account for this difference. We propose a structure-driven oxygen exchange mechanism, relying on the flexibility of the surface coordination polyhedra that transform upon dissociation of oxygen molecules.Austrian Science Fund (SFB “ Functional Oxide Surfaces and Interfaces ” - FOXSI, Project F 45)European Research Council Advanced Grant (“OxideSurfaces” (Project ERC-2011-ADG_20110209))National Science Foundation (U.S.). Division of Materials Research (CAREER Award Grant No. 1055583

Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface

Segregation and phase separation of aliovalent dopants on perovskite oxide (ABO3) surfaces are detrimental to the performance of energy conversion systems such as solid oxide fuel/electrolysis cells and catalysts for thermochemical H2O and CO2 splitting. One key reason behind the instability of perovskite oxide surfaces is the electrostatic attraction of the negatively charged A-site dopants (for example, ) by the positively charged oxygen vacancies () enriched at the surface. Here we show that reducing the surface concentration improves the oxygen surface exchange kinetics and stability significantly, albeit contrary to the well-established understanding that surface oxygen vacancies facilitate reactions with O2 molecules. We take La0.8Sr0.2CoO3 (LSC) as a model perovskite oxide, and modify its surface with additive cations that are more and less reducible than Co on the B-site of LSC. By using ambient-pressure X-ray absorption and photoelectron spectroscopy, we proved that the dominant role of the less reducible cations is to suppress the enrichment and phase separation of Sr while reducing the concentration of and making the LSC more oxidized at its surface. Consequently, we found that these less reducible cations significantly improve stability, with up to 30 times faster oxygen exchange kinetics after 54 h in air at 530 °C achieved by Hf addition onto LSC. Finally, the results revealed a 'volcano' relation between the oxygen exchange kinetics and the oxygen vacancy formation enthalpy of the binary oxides of the additive cations. This volcano relation highlights the existence of an optimum surface oxygen vacancy concentration that balances the gain in oxygen exchange kinetics and the chemical stability loss

Thermal decomposition of solid cyclotrimethylene trinitramine

In this paper, we show how a traditional solid-state chemistry approach, applied to the essentially molecular problem of the energetic barrier for decomposition, gives qualitatively new results and, in fact, brings very new perspectives in the detonation initiation theory development at large. Quantum-chemical simulations of the thermal decomposition of solid cyclotrimethylene trinitramine (RDX) by means of the Hartree-Fock method combined with the cluster and periodic models are performed. We found that the dissociation of the RDX molecule in the bulk crystal is characterized by the different energetic barriers for cleavage of N-NO2 bonds, unlike the gas-phase molecule, where all three of the energies are equal. It is also shown that a rupture of the N-NO2 chemical bond requires less energy for an isolated molecule than for a molecule placed in the bulk of the solid. The situation changes if the molecule is close to the free surface of the crystal. In this case, less energy is required to break the bond than for a bulk molecule. Mechanisms of solid RDX decomposition, the relevant experimental data, and possible applications of the results obtained are discussed in great detail. We also discuss how the conclusion obtained can serve for the better understanding of the well-known mechanism of pore collapse, of different sensitivities to detonation initiation of porous, solid, perfect, and defective explosives, and other processes that take place in hot spots. The mechanisms of the thermal decomposition of solid energetic materials are discussed with the illustrating example of RDX

Electronic excitations in initiation of chemistry in molecular solids

An ab initio study is performed for the initiation of chemistry in high explosive crystals from a solid-state physics viewpoint. Specifically, we are looking for the relationship between the defect-induced deformation of the electronic structure of solids, electronic excitations, and chemical reactions under shock conditions. Band structure calculations by means of the Hartree-Fock method with correlation corrections were done to model an effect of a strong compression induced by a shock/impact wave on the crystals with and without edge dislocations. Based on the results obtained, an excitonic mechanism of the earliest stages for initiation of high explosive solids is discussed with application to cyclotrimethylene trinitramine (also known as RDX) crystal. Experimental verification of the validity of the proposed model is reported for RDX and heavy metal azides. Thus, the key role of electronic excitations facilitated by edge dislocations in explosive solids is established and analyzed. Practical applications of the suggested mechanisms are discussed

Defects in yttrium aluminium perovskite and garnet crystals: Atomistic study

Native and impurity point defects in both yttrium aluminium perovskite (YAP) and garnet (YAG) crystals are studied in the framework of the pair-potential approximation coupled with the shell model description of the lattice ions. The calculated formation energies for native defects suggest that the antisite disorder is preferred over the Frenkel and Schottky-like disorder in both YAP and YAG. The calculated values of the distortion caused by the antisite YAlx in the lattice turn out to be in an excellent agreement with the EXAFS measurements. In non-stoichiometric compounds, the calculated reaction energies indicate that excess Y2O3 or Al2O3 is most likely to be accommodated by the formation of antisites rather than vacancies or interstitials in the lattice. Enthalpies of the reactions for impurity (Ca2+, Mg2+, Sr2+, Ba2+, Cr3+, Fe3+, Nd3+, Si4+) incorporation into both YAP and YAG lattices are calculated. The relevant experimental data are discussed. © 2000 IOP Publishing Ltd

Ab initio simulation of defects in energetic materials. Part I. molecular vacancy structure in RDX crystal

An attempt to simulate a point defect in energetic molecular solids and to find out its effect on the crystal electronic properties at the ab initio Hartree-Fock level using two different solid-state models as performed. As an example, we have considered a molecular vacancy in RDX crystal, in the periodic and in the molecular cluster model. Low formation energy characterizes this defect in the solid RDX indicating that a significant number of vacancies should be present in the crystal even at low temperatures. Narrowing of fundamental gap (about 1 eV) is caused by the presence of vacancies in the material. The obtained results demonstrate anisotropy of the RDX crystal with respect to the vacancy distribution. Trends received in periodic and molecular cluster models agree well

Atomistic modeling of native point defects in yttrium aluminum garnet crystals

Native point defects in yttrium aluminum garnet (YAG) are studied in the framework of the pair-potential approximation, coupled with the shell-model description of the lattice ions. For the perfect lattice, a new set of potential parameters is obtained; these parameters reproduce the structure, elastic, and dielectric constants of YAG very well. The calculated formulation energies for native defects suggest that antisite disorder is preferred over Frenkel and Schottky-like disorder in YAG. The calculated values of the distortion that is caused by the antisite Y atom that substitutes in the Al site in the lattice are in excellent agreement with the extended X-ray absorption fine-structure (EXAFS) measurements. In nonstoichiometric YAG, the calculated reaction energies indicate that excess Y2O3 or Al2O3 most likely is accommodated by the formation of antisites, rather than vacancies, in the lattice