CRYSTAL STRUCTURES

# CRYSTAL STRUCTURES - Crystal Structures Chapter 3 Atomic...

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Crystal Structures Chapter 3

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Atomic Arrangements In gases there is no order In liquids there is short range order In solids there is long range range The order is determined by the type of atomic bonds
Lattices A grid like pattern Composed of unit cells Unit cells are stacked together endlessly to form the lattice (with no empty spaces between cells)

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Scanning Tunneling Microscope Image of Iron in the (110) plane
Unit Cells There are seven basic shapes of the unit cells Defined by lattice parameters The edge lengths Angles 14 Bravais lattices Models are in the classroom

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Cubic Unit Cells Simplest to understand Simple (SC) One atom per unit cell Body Centered (BCC) Two atoms per unit cell Face Centered (FCC) 4 atoms per unit cell
Relationships The problem button in the text, shows the development of the relationships between radius and the lattice parameters for each From these values you can calculate the density, the packing fraction etc.

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Simple Cubic r r a o a o = 2r
Face Centered Cubic (FCC) r a 4 2 0 = a 0 a 0

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Body Centered Cubic (BCC) 0 2 a 0 3 a a 0 r a 4 3 0 =
Z Y X 4r

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Coordination Number The number of touching neighbors SC is 6 BCC is 8 FCC is 12
Density ρ = m/V m = #atoms * Atomic Weight Avogadro’s number V = edge length ^3 We already know how to calculate the edge length

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Packing Factor The fraction of the cell that is filled up with atom (The rest is empty) Packing Factor = Atom volume Cell volume Find the maximum packing factor
Lab on Crystal Lattices Use styrofoam balls to create the 3 cubic lattices Determine density Determine packing fraction Determine Coordination Number

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Hexagonal Packing Hexagonal close packing (ababab) Hexagonal cubic packing (abcabc) Use checkers to see the difference
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## This note was uploaded on 01/22/2012 for the course ME 2733 taught by Professor Meng during the Fall '10 term at LSU.

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CRYSTAL STRUCTURES - Crystal Structures Chapter 3 Atomic...

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