Global Atomic Configurations

In crystals, impurities can take simple configurations. But depending on their concentration, diffusion coefficient, or chemical properties and also on the presence of different kind of impurities or of lattice defects, more complex situations can be found. Apart from indirect information like electrical measurements or X-ray diffraction, methods such as optical spectroscopy under uniaxial stress, electron spin resonance, channelling, positron annihilation or Extended X-ray Absorption Fine Structure (EXAFS) can provide more detailed results on the location and atomic structure of impurities and defects in crystals. Here, we describe the simplest atomic structures more complicated structures are discussed in other chapters. To explain the locations of the impurities and defects whose optical properties are discussed in this book, an account of the most common crystal structures mentioned is given in Appendix B. 

The classical doping of semiconductors shows that a FA can replace an atom of the crystal at a regular lattice site. In covalent or partially covalent crystals, the main parameters which must be considered for the possible location of a FA on a substitutional site are its ability to form chemical bonds with its neighbours and the strengths of these bonds. When a crystal is made 

An interstitial atom in an antibonding (AB) site is bonded to its nearest neighbour lattice atom. This location is often found in H complexes involving a donor atom and results in the relaxation of the local lattice bonding. There also exists a special interstitial structure, the di-interstitial configuration. Incidentally, Fig. 2.5 shows the ternary symmetry of the sphalerite lattice along a 111 direction. This is analogous with the wurtzite structure, where 

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The Born-Oppenheimer approximation decouples the electronic motion from that of the nuclei thus allowing to determine the potential energy for any atomic configuration (we refer to the textbooks and mention especially some new publications ). In other words, a global PES provides the potential energy as a function of the nuclear geometry of the system. For N atoms the potential energy... 

Klepeis, J. L. I. P. Andronlakis M. G. Ierapetritou, et al. Predicting Solvated Peptide Configuration via Global Minimization of Energetic Atom-to-Atom Interactions. Comput Chem Eng 22 765-788 (1998). 

The concept of valence developed in the preceding section is the basis of the first correlations aiming at a global theory of the actinide metallic bond. These correlations were established between the atomic volumes of actinide elemental metals, and the electronic configuration of the actinide atoms Their aim was to provide a general theory of actinides (i.e. to give an answer to the questions i. and ii. of Sect. A.I.l.) within the framework of a simple model of the metallic bond. 

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