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 HfO 2 , 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.

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.

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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.

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.

We modeled all stable positive and negative charge states of oxygen vacancies originating from the neutral O 3 Si=Si O 3 defect in amorphous SiO 2 (a-SiO 2 ) 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-SiO 2 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.

We have developed a shell model force field that reproduces the details of the phase diagram of the Pb(Zr 1−x Ti x )O 3 (PZT) solid solution compound, including the low-and high-temperature phases of PbZrO 3 and PbTiO 3 . The developed force field supports the temperature-induced phase transitions from cubic to low symmetry phases over the whole composition range and additionally reproduces the composition driven phase transitions. Indeed, the increase of Ti content induces a phase change of PZT from rhombohedral to tetragonal symmetry, mediated by a monoclinic phase, all in excellent agreement with the experiments.

The effects of the lattice strain induced by neutral oxygen vacancies in ferroelectric tetragonal BaTiO(3) and KNbO(3) are investigated using ab initio simulations. We propose that an oxygen vacancy can transform from its metastable equatorial configuration to the stable axial configuration via either diffusion or rotation of the polar axis near the vacancy site by 90°. The latter mechanism, predicted to dominate in materials with slow oxygen vacancy diffusion and low formation energy of 90° domain walls, can stimulate the formation of domains with their polar axes pinned by the vacancies.