Page 28 - Microsoft PowerPoint - lecture1.ppt

2.1 Basics of Structures

Wyckoff-notation and occupancy-factors

Crystal data

Formula sum Cu 0.8 In 2.4 Se 4
Crystal system tetragonal

Space group I -4 2 m (no. 121)

Unit cell dimensions a = 5.7539(3) Å c = 11.519(1) Å

Z 2

Atomic coordinates

Atom Ox. Wyck. Occ. x y z

Cu1 +1 2a 0.8 0 0 0
In1 +3 4d 1.0 0 1/2 1/4

In2 +3 2b 0.4 0 0 1/2

Se1 -2 8i 1.0 1/4 1/4 1/8

• Occ. factor < 1.0: mixing of atoms and vacancies on the same position

• Calculation of the composition: Cu: 2 0.8; In: 4 1 + 2 0.4; Se: 8 1