Face center cubic models crystal structure

We now describe a relatively simple MD model of a low-index crystal surface, which was conceived for the purpose of studying the rate of mass transport (8). The effect of temperature on surface transport involves several competing processes. A rough surface structure complicates the trajectories somewhat, and the diffusion of clusters of atoms must be considered. In order to simplify the model as much as possible, but retain the essential dynamics of the mobile atoms, we will consider a model in which the atoms move on a "substrate" represented by an analytic potential energy function that is adjusted to match that of a surface of a (100) face-centered cubic crystal composed of atoms interacting with a Lennard-Jones... 

In the present work, the interaction of the ethylene molecule with the (100) surfaces of platinum, palladium and nickel is studied using the cluster model approach. All these metals have a face centered cubic crystal structure. The three metal surfaces are modelled by a two-layer M9(5,4) cluster of C4V symmetry, as shown in Fig. 6, where the numbers inside brackets indicate the number of metal atoms in the first and second layer respectively. In the three metal clusters, all the metal atoms are described by the large LANL2DZ basis set. This basis set treats the outer 18 electrons of platinum, palladium and nickel atoms with a double zeta basis set and treats all the remainder electrons with the effective core potential of Hay and Wadt... 

The Elements Handbook (Appendix A) contains a table of properties for each group that includes information on the crystal structures of the elements. Most metals crystallize in one of three lattice arrangements body-centered cubic, face-centered cubic, or hexagonal close-packed. Figure 3.2 shows a model of the face-centered cubic lattice for sodium chloride. Use this figure and the information in the Elements Handbook (Appendix A) to answer the following. 

Patrykiejew et al. [329] have also simulated the behavior of 2D L-J fluids onto the (100) face of a face-centered-cubic crystal. Nevertheless, the Knight and Monson [225] work was not mentioned, so that no comparison of results was performed. To model the gas-surfaee potential, Patrykiejew et al. [329] used the first live terms of Steele s Fourier series [Eq. (14)] for a perpendicular reduced distance less than or equal to 2.5. The results show that at low temperature, the structure of the monolayer film depends strongly on the gas-surface potential corrugation, as well as on the size of the adsorbed atoms. Also, the influence of the corrugation on the melting transition is studied, indicating a different structure of the solid phase. Unfortunately, definitive conclusions about the nature and order of the observed phase transitions were not obtained. 

A phenomenological model based on crystal structure, metallic radius, melting point, and enthalpy of sublimation has been used to arrive at the electronic configuration of berkelium metal [140]. An energy difference of 0.92 eV was calculated between the 5f 7s ground state and the 5f 6d 7s first excited state. The enthalpy of sublimation of trivalent Bk metal was calculated to be 2.99 eV (288 kJmol ), reflecting the fact that berkelium metal is more volatile than curium metal. It was also concluded that the metallic valence of the face-centered cubic form of berkelium metal is less than that of the double hexagonal close-packed modification [140]. 

The structural representations shown in Fig. 3.2 are useful for visualizing the slip-planes that may exist in various metal crystal systems. For example, from the face-centered cubic (fee) lattice shown in Fig. 3.2a, it is easy to see how planes of atoms might slide rather easily over one another from the upper left to the lower right. This lattice also has several other such slip planes. While it is not readily apparent from the figure, a three-dimensional model of these crystal systems would show that the body-centered cubic (bcc) lattice offers the least number of slip planes of the three shown, and the hexagonal-close-packed (hep) system falls in between the fee and bcc systems. 

The atomic bonding in this group of materials is metallic and thns nondirectional in natnre. Conseqnently, there are minimal restrictions as to the nnmber and position of nearest-neighbor atoms this leads to relatively large nnmbers of nearest neighbors and dense atomic packings for most metallic crystal structnres. Also, for metals, when we nse the hard-sphere model for the crystal structure, each sphere represents an ion core. Table 3.1 presents the atomic radii for a number of metals. Three relatively simple crystal structures are found for most of the common metals face-centered cubic, body-centered cnbic, and hexagonal close-packed. 

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