Lecture02-BasicCrystallography

Lecture02-BasicCrystallography - Lecture 2 Basic...

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Unformatted text preview: Lecture 2 Basic Crystallography EMSE 312 DIFFRACTION PRINCIPLES Crystallography C r y s t a l Macroscopic Scale: A Solid of Uniform Chemical Composition which is formed with Plane Faces each making Precise Angles with one another. Microscopic Scale: Atoms, or Groups of Atoms, Repeated Regularly in Three Dimensions. Translation Symmetry [Lattice] Symmetry of the Group of Atoms at each Lattice Point [Motif] Symmetry Motif : the fundamental part of a symmetric design that, when repeated, creates the whole pattern Operation : some act that reproduces the motif to create the pattern Element : an operation located at a particular point in space 2D Symmetry Translations (Lattices) 2-D translations = a net a b Unit cell Unit cell Unit Cell: the basic repeat unit that, Unit Cell: the basic repeat unit that, by translation only by translation only , generates the entire pattern , generates the entire pattern How is the UNIT CELL different from the MOTIF? How is the UNIT CELL different from the MOTIF? Lattice Translations Example: Escher print 1. What is the motif ? 2. Pick any point and label it with a big dark dot 3. Label all equipoints the same 4. Outline the unit cell based on your equipoints 5. What is the unit cell content (Z) ?? Z = the number of motifs per unit cell Is Z always an integer ? Lattice Translations Conventions: Conventions: 1. Cell edges should, 1. Cell edges should, whenever possible, whenever possible, coincide with coincide with symmetry axes or symmetry axes or reflection planes reflection planes 2. If possible, edges 2. If possible, edges should relate to each should relate to each other by lattice other by lattice s s symmetry....
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Lecture02-BasicCrystallography - Lecture 2 Basic...

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