480 likes | 655 Views
Crystals and Symmetry. Why Is Symmetry Important?. Identification of Materials Prediction of Atomic Structure Relation to Physical Properties Optical Mechanical Electrical and Magnetic. Repeating Atoms in a Mineral. Unit Cell. Unit Cells.
E N D
Why Is Symmetry Important? • Identification of Materials • Prediction of Atomic Structure • Relation to Physical Properties • Optical • Mechanical • Electrical and Magnetic
Unit Cells All repeating patterns can be described in terms of repeating boxes
The problem in Crystallography is to reason from the outward shape to the unit cell
Rotational Symmetry May or May Not be Combined With Mirror Symmetry
The symmetries possible around a point are called point groups
What’s a Group? • Objects plus operations New Objects • Closure: New Objects are part of the Set • Objects: Points on a Star • Operation: Rotation by 72 Degrees • Point Group: One Point Always Fixed
Symmetry in Repeating Patterns • 2 Cos 360/n = Integer = -2, -1, 0, 1, 2 • Cos 360/n = -1, -1/2, 0, ½, 1 • 360/n = 180, 120, 90, 60, 360 • Therefore n = 2, 3, 4, 6, or 1 • Crystals can only have 1, 2, 3, 4 or 6-Fold Symmetry
No. The Stars Have 5-Fold Symmetry, But Not the Overall Pattern
Translation • p p p p p p p p p p p p p • pq pq pq pq pq pq pq pq pq pq • pd pd pd pd pd pd pd pd pd pd • p p p p p p p p p p p p pb b b b b b b b b b b b b • pd pd pd pd pd pd pd pd pd pdbq bq bq bq bq bq bq bq bq bq • pd bq pd bq pd bq pd bq pd bq pd bq pd bq • p b p b p b p b p b p b p b
Space Symmetry • Rotation + Translation = Space Group • Rotation • Reflection • Translation • Glide (Translate, then Reflect) • Screw Axis (3d: Translate, then Rotate) • Inversion (3d) • Roto-Inversion (3d: Rotate, then Invert)
There are 17 possible repeating patterns in a plane. These are called the 17 Plane Space Groups
Why Is Symmetry Important? • Identification of Materials • Prediction of Atomic Structure • Relation to Physical Properties • Optical • Mechanical • Electrical and Magnetic