Theoretical Analysis About The Surface Structure And Energy Anisotropy Of Crystal

Surface plays an important role in the process of preparation and using for materials, because various physical and chemic reactions take place gradually from the surface to the inside of materials. These processes depend on the surface structure and properties of materials. However, the surface structure and properties of solid are different from the inside completely. The reason of surface phenomena is that the arrangement of particles at the surface of materials is different from that of the inside. Every atom inside of the bulk is surrounded by other atoms which bring symmetrical forces to it. So these atoms are located in a symmetrical field of force, and the composition of forces to it equals to zero. Thus the atom inside of the bulk has the lowest energy. However, the surface atoms are contacted with gas (or solid). The force they suffered from the gas molecules can be neglected. So the surface atoms are located in an asymmetric field of force and have a higher energy. This amount of energy of surface atoms higher then the inside atoms is named as surface energy.The research in surface structure and energy is the foundation for the knowledge of materials and then enable us to change some of their properties even to design new type of materials. In this paper, the surface structure and energy about various surfaces of body-centered cubic metals (BCC), closed-packed hexagonal (HCP) metals and B2 type intermetallic compound NiAl are analyzed and calculated with embedded atom method (EAM) combining computer simulation. The research can be divided into three parts:(1) The surface energies for 24 surfaces of all BCC transition metals Fe, Cr, Mo, W, V, Nb and Ta have been calculated by using the second nearest-neighbor modified embedded atom method (2NN MEAM). The results show that, for all BCC transition metals, the order among three low-index surface energies E₁₁₀＜E₁₀₀＜E₁₁₁ is in agreement with experimental results and E₁₁₀ is also the lowest surface energy for various surfaces. So that from surface energy minimization, the (110) texture should be favorable in the BCC films. This is also consistent with experimental results. The surface energy for the other surfaces increases linearly with increasing angle between the surfaces (hkl) and (110). Therefore a deviation of the surface orientation from (110) can be used to estimate the relative values of the surface energy.(2) With the modified embedded atom method (MEAM), the surface energies of three kinds of representative surfaces, (h01), (hh1) and (hk0) belong to [010], [110] and [001] crystal band respectively, have been calculated for 13 HCP metals Co, Dy, Er, Gd, Ho, Mg, Nd, Pr, Re, Sc, Tb, Tl and Zr. For all 13 HCP metals, the basal plane (001) has the minimum surface energy. So from surface energy minimization, the (001) texture should be favored in the HCP films, this is consistent with the experimental results. The fact that the short termination corresponds to much lower surface energy than long one implies the former is more stable for those surfaces having two possible terminations. Such as the prism plane (100), only the short termination was observed in experiment.(3) The surface structure and energies for 22 surfaces of NiAI, an ordered intermetallic compound of B2 structure, have been studied by using embedded atom method. The results show that, for alternating Ni and Al surfaces with odd numbers of the sum of their three Miller indices, the energy difference between the Ni terminated surface and Al terminated surface increase linearly with increasing the interlayer distance. So from surface energy minimization, the Al terminated surface is favorable for each alternating Ni and Al surface. This is in agreement with experimental results. However, the energy of the (110) surface belonged to the other kind of the surfaces consisted of stoichiometric atomic layers and with even numbers of the sum of their three Miller indices, is the lowest in all two kinds of the surfaces. Therefore the (110) texture of NiAl appears mostly in the experiments.