Lesson_2 Behaviour of metals (3)

Lesson_2 Behaviour of metals (3) - 58 CHAPTER 3 Atomic and...

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• will.smith
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is necessary to completely describe the cell (angles of 90 ± are assumed unless otherwise specified). This length is the lattice parameter a (some times designated as a 0 ). The length is often given in nanometers (nm) or Angstrom (A ± ) units, where: 1 nanometer ð nm Þ ¼ 10 ² 9 m ¼ 10 ² 7 cm ¼ 10 A ± 1 angstrom (A ± ) ¼ 0 : 1 nm ¼ 10 ² 10 m ¼ 10 ² 8 cm Several lattice parameters are required to define the size and shape of complex unit cells. For an orthorhombic unit cell, we must specify the dimensions of all three sides of the cell: a 0 , b 0 , and c 0 . Hexagonal unit cells require two dimensions, a 0 and c 0 , and the angle of 120 ± between the a 0 axes. The most complicated cell, the triclinic cell, is de- scribed by three lengths and three angles. Number of Atoms per Unit Cell A specific number of lattice points defines each of the unit cells. For example, the corners of the cells are easily identified, as are the body- centered (center of the cell) and face-centered (centers of the six sides of the cell) posi- tions (Figure 3-5). When counting the number of lattice points belonging to each unit cell, we must recognize that lattice points may be shared by more than one unit cell. A lattice point at a corner of one unit cell is shared by seven adjacent unit cells (thus a total of eight cells); only one-eighth of each corner lattice point belongs to one partic- ular cell. Thus, the number of lattice points from all of the corner positions in one unit cell is: 1 8 lattice point corner ± ² 8 corners cell ± ² ¼ 1 lattice point

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• Spring '14
• Colombo
• Physics, Material Science, Plastics, Manufacturing, Metals, Crystal system, lattice point

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