Ionic Crystal

1
Ionic Crystal

• Calcium fluoride
• CaF2

2
Ionic Crystal

• Consist of a large number of regularly packed
  ions to form a 3D structure
• Ions are held by strong electrostatic forces
• Attractions between oppositely charged ions
• Repulsion between ions of same charge

3
Definition of terms

• Unit cell
• The smallest portion of the crystal which
  contains all the details of the crystal without
  repetition
• Coordination number
• Number of adjacent atoms or ions in direct
  contact with a given atom or ion

4
Structure of CaF2

• A body-centred cubic
• Calcium ions occupy the lattice vertices and at
  the centre of each face
• Fluoride ions are completely within the cubic

5
Structure of CaF2

• Black balls Fluoride ions
• White balls Calcium ions
• By repeating the unit cell, the whole lattice of
  the crystal is produced

6
Co-ordination number

• Co-ordination number of calcium ion 8

7
Co-ordination number

• Co-ordination number of fluoride ion 4

8
Content of CaF2 unit cellnumber of fluoride ions

• Particles completely within the unit cell are not
  shared with others and hence are counted 1 for
  each
• 8 fluoride ions are completely within the cell
• Number of fluoride ions in a unit cell
• 8X1 8

9
Content of CaF2 unit cellnumber of calcium ions
Calcium ion on the face

• Particles on the face are shared equally between
  two adjacent unit cells and hence counted ½ for
  each
• Particles at a corner are shared equally among
  eight adjacent unit cells and hence are counted
  1/8 for each

Calcium ion at the corner
10
Content of CaF2 unit cellnumber of calcium ions

• 8 calcium ions are at the corners of the unit
  cell
• 6 calcium ions are on the face of the unit cell
• Number of calcium ions in a unit cell
• 8X(1/8)6X(1/2) 4

11
Summary of CaF2 structure

• Body-centred cubic
• Number of fluoride ions in a unit cell 8
• Number of calcium ions in a unit cell 4
• Coordination 84

12
Report 10 Making the unit cell of CaF2

• Date 2th February, 2007
• Group members Chan Mio Leung (4)
• Ip Yan (13)