Laboratory crystal structures I-1

Laboratory crystal structures I-1 - Laboratory: Crystal...

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Laboratory: Crystal Structures Materials Laboratory I Last updated: 8/25/2010 1 /10 1.1 Objectives Build very simple crystal structures. Build one assigned structure. Discuss the possible applications of the material due to its crystal structure. 1.2 Background and Reading Reading keywords: unit cell, crystal systems, lattice, basis, centering operations, Bravais lattice, Miller indices, symmetry elements, metal structures, ceramic structures, amorphous structures, structures of electronic materials, and any words in this handout which are not familiar to you Reading in Callister, 8 th edition: chapters 2-4, 12.1-12.5 (chapter 3 is the most important for this lab) The structure of a material determines its properties and potential applications. Atoms and molecules of a substance can be randomly arranged without periodicity (amorphous), or they can exhibit periodic translational symmetry (crystalline). The unit cell is the smallest group of atoms which is repeated throughout an ideal crystal that demonstrates the symmetry of the infinite structure. The lattice can be described by the lattice parameters made of three lengths of the unit cell, denoted a, b and c, and the corresponding angles α , β and γ . Table 1 shows the seven crystal systems, while Figure 1 shows all 14 Bravais lattices. Table 1: Seven crystal systems
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Laboratory: Crystal Structures Materials Laboratory I Last updated: 8/25/2010 2 /10 Figure 1: Fourteen Bravais lattices In Figure 1, the lattices are shown with points. Technically, you can say that the lattice is ‘decorated’ with atoms. Table 2 offers some information on simple metal crystal structures and examples. Figure 2: Location of lattice points and the actual crystal structure.
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Laboratory: Crystal Structures Materials Laboratory I Last updated: 8/25/2010 3 /10 Table 2: Common Structures For Metals . Structure Bravais Atoms per unit cell Examples Body-centered cubic BCC 1+8x1/8 = 2 -Fe, V, Cr, Mo, Ta, W Face-centered cubic FCC 6x1/2+8x1/8 = 4 -Fe, Al, Ni, Cu, Ag, Th, Au, Pb, Ir, Pt, Rh Hexagonal close packed HCP 1+4x1/6+4x1/12 = 2 Be, Mg, -Ti, Zn, Zr, Cd, La, Tl, Re, Os, Am, Ru, Hf Crystal identification The common and internationally accepted method for crystal identification involves a description of the symmetry elements contained in the structure along with the unique locations of atoms within the structure. Understanding and working with this method is a semester long course in itself. It is beyond the scope of this course; however, I want you to be aware of the designation. The combined symmetry elements are called space groups, and the unique atomic positions are called Wyckoff positions. In this class we will use an easier identification method that will uniquely identify each structure; however, this method will not describe all of the symmetry contained in the structure. The concepts of lattice and basis descriptions seem challenging at first, but with some practice, it can become routine. Our method will use the Pearson symbol for
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Laboratory crystal structures I-1 - Laboratory: Crystal...

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