Page 20 - Microsoft PowerPoint - lecture1.ppt

2.1 Basics of Structures

Unit cells and crystal system

• millions of structures but 7 crystal systems

• crystal system = particular restriction concerning the unit cell
• crystal system = unit cell with characteristic symmetry elements (later)

Crystal system Restrictions axes Restrictions angles

Triclinic - -

Monoclinic - °

Orthorhombic - °

Tetragonal a = b °

Trigonal a = b °°

Hexagonal a = b °°

Cubic a = b = c °