Question

Cesium Chloride (CsCl) unit cell

1). What are the number of (fractional) atoms per unit cell.

Cations: Anions:

2). Coordination number of Cs:

3). Lattice Parameter of the CsCl unit cell: Diagonal: Edge:

Answer 1

The caesium chloride crystal is composed of equal number of caesium (Cs+) and Chloride Cl-ions.

The radii of two ions (Cs+ = 169 pm and Cl- = 181 pm) led to radius ratio of rCS+ to rCl– as 0.93 which suggest a body centred cubic structure having a cubic hole

RCS+ to RCl– = 169 / 181 = 0.93

Each Cs+ ion is surrounded by 8 Cl- ions and each Cl- ion in surrounded by 8 Cs+ ions.

Thus the Co – ordination number of each ion is eight.

The unit cell of caesium chloride has one Cs+ ion and one Cl- ion as calculated below

      No. of Cl- ion 8(at corners) × 1/8 = 1

      No. of Cs+ ion = 1(at body centre) × 1=1

      Thus, number of CsCl units per unit cell is 1