Point defects computation

Y. Shimomura. Point defects and their clusters in f.c.c. metals studied by computer simulations. Mater Chem Phys 50 139, 1997. 

The validity of Equation (21) requires that the lattice must be treated as a continuum in the evaluation of the angular dependence of electric field gradients. It has also been. shown 97) that the proper factor to use in computing the electric field gradient produced by a point defect, taking into account the effect of polarization of the medium, is (2e + 3)/5 , where e is the dielectric constant of the medium. This result is based on the continuum approximation for the solid. 

Direct observation of point defects in metals has been possible by field ion microscopy. Impurity point defects may be usefully investigated by electron microscopy in combination with electron diffraction and electron spectroscopy. Direct observation of the dopant environment in fluorites has been attempted by Catlow et al. (1984) by employing EXAFS in conjunction with computer simulation. 

To test the hypotheses (7.4.17) and (7.4.18), the kinetics of accumulation was simulated on a computer by the method described in [110]. For each of the values vp = 10,16,24, and 50, the process of accumulation was performed independently 200 times until the stage of steady-state values of no was reached. The relationships n(N), N = pt, and a(n) were constructed from the mean values obtained in this series. It was shown that within the limits of error of computer experiment ( 5%), the slowly varying function a(n) can be well approximated by the linear dependence of (7.4.18), which confirms the suitability of this approach for describing the accumulation of point defects in the discrete model. Analogous results are obtained for vp = 16 and 50 for which the values were found respectively, of 1.092 and 1.625 for n0 and 0.463 and 0.478 for f3(oo) = a(oo)vono. 

The first of such procedures was the method of terminal atoms saturating the broken outer bonds of a cluster. It was initially used in cluster computations of point defects in homo-atomic crystals of diamond and silicon. The hydrogen atoms were used as saturating atoms. In Hayns (29) this approach was successfully extended to calculate the chemisorption of hydrogen on a graphite surface. 

Catalytic applications of ceria and ceria-based mixed oxides depend primarily upon the nature and concentration of the defects present in the material. Although experimental techniques are available for the study of these defects, the characterization of their physical properties at the atomic level is often very difficult. The most important point defects in ceria are oxygen vacancies, reduced Ce centers and dopant impurities. The formation energy of such defects and the energetics of their mutual interactions within the bulk oxide have been the subject of several computational studies. 

In Chapter 4, Professor Donald W. Brenner and his co-workers Olga A. Shenderova and Denis A. Areshkin explore density functional theory and quantum-based analytic interatomic forces as they pertain to simulations of materials. The study of interfaces, fracture, point defects, and the new area of nanotechnology can be aided by atomistic simulations. Atom-level simulations require the use of an appropriate force field model because quantum mechanical calculations, although useful, are too compute-intensive for handling large systems or long simulation times. For these cases, analytic potential energy functions can be used to provide detailed information. Use of reliable quantum mechanical models to derive the functions is explained in this chapter. 

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