Lecture38A - Lecture 38 1 Solids and Surfaces: Structure...

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Unformatted text preview: Lecture 38 1 Solids and Surfaces: Structure (Chapter 31.1-5) When solids form crystalline structures, the atoms and molecules in the solid have translational symmetry along three directions. The entire crystal can be replicated by translating a portion of the crystal; the smallest portion or unit from which the crystal can be replicated is called the unit cell. We like to think in orthogonal 3-D patterns so the cubic unit cells are what we typically visualize. Fig 31.3 Face-centered cubic: Cu Lecture 38 2 Fig 31.4 Body-centered cubic: K Fig 31.5 Primitive (simple) cubic: Po is only known example! However, the unit cell can have a much more generalized form. angle is between sides b & c angle is between sides a & c angle is between sides a & b Fig 31.6 Lecture 38 3 As it turns out there exist 14 (and only 14) unique unit cells, also known as the Bravais lattices. Fig 31.7 P rimitive: a lattice point /unit cell I-body centered: 2 lattice pts /unit cell C-end centered: 2 lattice pts /unit cell F-face centered: 4 lattice pts /unit cell R-rhomohedral: 1 lattice pt /unit cell A lattice point is an atom or molecule. Lecture 38 4 Planes which connect these lattice points contain atoms/molecules that are periodically spaced in 2-D. As you learned in Gen. Chem, x-rays can scatter constructively from these planes and give us information...
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Lecture38A - Lecture 38 1 Solids and Surfaces: Structure...

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