Anna V. Kimmel

Trapping, self-trapping and the polaron family

The earliest ideas of the polaron recognized that the coupling of an electron to ionic vibrations would affect its apparent mass and could effectively immobilize the carrier (self-trapping). We discuss how these basic ideas have been generalized to recognize new materials and new phenomena. First, there is an interplay between self-trapping and trapping associated with defects or with fluctuations in an amorphous solid. In high dielectric constant oxides, like HfO2, this leads to oxygen vacancies having as many as five charge states. In colossal magnetoresistance manganites, this interplay makes possible the scanning tunnelling microscopy ( STM) observation of polarons. Second, excitons can self-trap and, by doing so, localize energy in ways that can modify the material properties. Third, new materials introduce new features, with polaron-related ideas emerging for uranium dioxide, gate dielectric oxides, Jahn-Teller systems, semiconducting polymers and biological systems. The phonon modes that initiate self-trapping can be quite different from the longitudinal optic modes usually assumed to dominate. Fourth, there are new phenomena, like possible magnetism in simple oxides, or with the evolution of short-lived polarons, like muons or excitons. The central idea remains that of a particle whose properties are modified by polarizing or deforming its host solid, sometimes profoundly. However, some of the simpler standard assumptions can give a limited, indeed misleading, description of real systems, with qualitative inconsistencies. We discuss representative cases for which theory and experiment can be compared in detail.

Positive and Negative Oxygen Vacancies in Amorphous Silica

We modeled all stable positive and negative charge states of oxygen vacancies originating from the neutral O3�A Si=SiA O3 defect in amorphous SiO2 (a-SiO2) using an embedded cluster method on a distribution of structural sites. For the first time, we predict the geometry, electronic structure and spectroscopic properties of doubly ionized and negatively charged oxygen vacancies in a-SiO2 and demonstrate that negatively charged vacancies serve as deep electron traps. The results demonstrate that oxygen vacancies can be responsible for both the electron and hole trapping in silica. We compare our findings with the previous calculations and with the recent experimental data.

Effect of Molecular and Lattice Structure on Hydrogen Transfer in Molecular Crystals of Diamino-dinitroethylene and Triamino-trinitrobenzene

We have studied the intra- and intermolecular hydrogen transfer in a crystalline 1,1-diamino-2,2-dinitroethylene (DADNE) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) by means of an embedded cluster method and density functional theory (DFT). We found that, even though both of these materials have similar amino- and nitro- functional groups and layered crystalline structures, there are important differences in the mechanisms of hydrogen transfer. In particular, our calculations suggest that the proton migration from an amino-group to a nitro-group of the same molecule is a feasible process in TATB but not in DADNE. At the same time, we have found that no intermolecular hydrogen transfer occurs in either molecular crystal. These results imply that the activation of the decomposition reactions proceeds via different paths in these two materials.

Effect of charged and excited states on the decomposition of 1,1-diamino-2,2-dinitroethylene molecules

The authors have calculated the electronic structure of individual 1,1-diamino-2,2-dinitroethylene molecules (FOX-7) in the gas phase by means of density functional theory with the hybrid B3LYP functional and 6-31+G(d,p) basis set and considered their dissociation pathways. Positively and negatively charged states as well as the lowest excited states of the molecule were simulated. They found that charging and excitation can not only reduce the activation barriers for decomposition reactions but also change the dominating chemistry from endo- to exothermic type. In particular, they found that there are two competing primary initiation mechanisms of FOX-7 decomposition: C-NO2 bond fission and C-NO2 to CONO isomerization. Electronic excitation or charging of FOX-7 disfavors CONO formation and, thus, terminates this channel of decomposition. However, if CONO is formed from the neutral FOX-7 molecule, charge trapping and/or excitation results in spontaneous splitting of an NO group accompanied by the energy release. Intramolecular hydrogen transfer is found to be a rare event in FOX-7 unless free electrons are available in the vicinity of the molecule, in which case HONO formation is a feasible exothermic reaction with a relatively low energy barrier. The effect of charged and excited states on other possible reactions is also studied. Implications of the obtained results to FOX-7 decomposition in condensed state are discussed.

Modelling of electron and hole trapping in oxides

We critically review several examples of successful modelling of electron and hole trapping in metal oxides, which demonstrate a breadth of polaronic behaviour. The examples range from self-trapping in the perfect lattice to trapping by structural defects and impurities and illustrate the important phenomenon of charge localization. We present recent results in four different systems: nanoporous mayenite, amorphous SiO2, crystalline hafnia and MgO surfaces and interfaces. The complex nature of charge trapping and polaronic behaviour in these systems can go beyond traditional cases and illustrate the different challenges involved.

Neutral and Charged Oxygen Vacancies Induce Two-Dimensional Electron Gas Near SiO2/BaTiO3 Interfaces

An atomistic model of the SiO2/BaTiO3 interface was constructed using ab initio molecular dynamics. Analysis of its structure and electronic properties reveals that (i) the band gap at the stoichiometric SiO2/BaTiO3 interface is significantly smaller than those of the bulk BaTiO3 and SiO2, and (ii) the interface contains ∼5.5 nm(-2) oxygen vacancies (V(2+)) in the outermost TiO2 plane of the BaTiO3 and ∼11 nm(-2) Si-O-Ti bonds resulting from breaking Si-O-Si and Ti-O-Ti bonds and subsequent rearrangement of the atoms. This structure gives rise to the interface polar region with positive and negative charges localized in the BaTiO3 and SiO2 parts of the interface, respectively. We propose that high dielectric response, observed experimentally in the SiO2-coated nanoparticles of BaTiO3, is due to the electron gas formed in oxygen-deficient BaTiO3 and localized in the vicinity of the polar interface.

The Structure and Decomposition Chemistry of Isomer Defects in a Crystalline DADNE

We simulated substitutional stereoisomer defects in a crystalline 1,1-diamino-2,2-dinitroethylene (DADNE) and studied their atomic and electronic structure and decomposition chemistry. We found that molecular gas phase trans- and cis-isomers are 4.6–6.9 kcal/mol (0.2–0.3 eV) and 11.5–16.1 kcal/mol (0.5–0.7 eV) less stable than DADNE, respectively, which agrees well with earlier studies. We also established that the substitutional trans-isomer defect in the ideal DADNE crystal significantly affects the thermal stability and chemical and physical properties of the solid DADNE matrix, which may imply the existence of alternative synthesis routes for new materials and gives new insight into initiation of detonation in energetic materials.

AN EFFECT OF CHARGED AND EXCITED STATES ON DECOMPOSITION OF 1,1‐DIAMINO‐2,2‐DINITROETHYLENE

The electronic structure of 1,1‐diamino‐2,2‐dinitroethylene molecules in the gas phase and its possible dissociation pathways were simulated by Density Functional Theory. It was shown that charging and excitation may not only reduce the activation barriers for decomposition reactions, but also dramatically change the dominating chemistry. Two competing primary initiation mechanisms of FOX‐7 decomposition were found: C–NO2 bond fission and C–NO2 to CONO isomerization; this reconciles earlier seemingly contradictory predictions.