Vacancy defect

Abstract: The movement of defects in solids on the basis of classical absolute rate theory is reexamined with special attention to many-body aspects. The effective frequency in the Arrhenius expression governing these processes is shown to be, in harmonic approximation, the ratio of the product of the N normal frequencies of the entire crystal at the starting point of a transition to the product of the N −1 normal frequencies of the crystal when it is constrained in a saddle point configuration. The influence of the masses of the various atoms on the effective frequency is investigated. It is shown that an effective mass which depends on the direction of the path through the saddle point in configuration space determines this frequency. In the case of chemical diffusion by the vacancy mechanism the effective mass is approximately the same as the mass of the solute atom, and must always lie between the mass of the solute and the mass of the solvent. It is finally shown that the classical rate theory, even with many-body considerations, is unable to explain the recent observations of L azarus and O kkerse on the isotope effect in the diffusion of iron in silver.

Abstract: The temperature dependence of positron lifetimes in both the brittle and plastic phases of trimethylacetic (pivalic) acid 1 has been examined using the positron lifetime technique. In the plastic phase two long-lived components attributable to ortho-positronium (ortho-Ps) decay are observed. The longer of these (≈2.8 ns) increases with temperature and is believed to be characteristic of ortho-Ps trapped at defects, probably mono- and di-vacancies. A shorter lifetime component (≈1 ns) which shows little temperature dependence is also present in the brittle phase. Additionally in the brittle phase and close to the transition region a longer lifetime is detected. This may also be associated with trapping of ortho-Ps in thermally created defects, probably vacancies. In the plastic phase the intensity of a short-lived component associated with para-Ps is confirmed to be approximately one third of the total intensity of the two long-lived components associated with ortho-Ps, but in the brittle phase it higher than one third. On using data for several plastic crystals a relationship is established between the lifetime of ortho-Ps trapped in a vacancy and the vacancy volume.

Abstract: First-principles calculations based on hybrid Hartree-Fock density functionals provide a clear picture of the defect energetics and electronic structure in ZnO. Among the donorlike defects, the oxygen vacancy and hydrogen impurity, which are deep and shallow donors, respectively, are likely to form with a substantial concentration in $n$-type ZnO. The zinc interstitial and zinc antisite, which are both shallow donors, are energetically much less favorable. A strong preference for the oxygen vacancy and hydrogen impurity over the acceptorlike zinc vacancy is found under oxygen-poor conditions, suggesting that the oxygen vacancy contributes to nonstoichiometry and that hydrogen acts as a donor, both of which are without significant compensation by the zinc vacancy. The present results show consistency with the relevant experimental observations.

Abstract: We calculate absolute formation energies of native defects in GaAs. The formation energy and hence the equilibrium concentration of the defects depends strongly on the atomic chemical potentials of As and Ga as well as the electron chemical potential. For example, the Ga vacancy concentration changes by more than ten orders of magnitude as the chemical potentials of As and Ga vary over the thermodynamically allowed range. This result indicates that the rate of self-diffusion depends strongly on the surface-annealing conditions.

Abstract: We study from first principles the magnetism in graphene induced by single carbon atom defects For two types of defects considered in our study, the hydrogen chemisorption defect and the vacancy defect, the itinerant magnetism due to the defect-induced extended states has been observed Calculated magnetic moments are equal to 1µB per hydrogen chemisorption defect and 112–153µB per vacancy defect depending on the defect concentration The coupling between the magnetic moments is either ferromagnetic or antiferromagnetic, depending on whether the defects correspond to the same or to different hexagonal sublattices of the graphene lattice, respectively The relevance of itinerant magnetism in graphene to the high-TC magnetic ordering is discussed