Caesium chloride

Caesium chloride
Other names	Cesium chloride
Identifiers
CAS number	[7647-17-8]
EC number	231-600-2
Properties
Chemical formula	CsCl
Molar mass	168.36 g/mol
Appearance	white solid hygroscopic
Density	3.99 g/cm3
Melting point	645 °C
Boiling point	1297 °C (vaporizes)
Solubility in water	162 g/100 mL (1 °C)
Solubility	soluble in ethanol
Structure
Crystal structure	Caesium chloride (see text)
Coordination geometry	simple cubic
Related compounds
Other anions	Caesium fluoride Caesium bromide Caesium iodide
Other cations	Lithium chloride Sodium chloride Potassium chloride Rubidium chloride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

Caesium chloride is the chemical compound with the formula CsCl. This colorless solid is an important source of caesium ions in a variety of applications. CsCl is also well known as a structural type.

Preparation

Caesium chloride can be prepared by the reaction of caesium hydroxide or caesium carbonate with hydrochloric acid: the resulting salt is purified by recrystallization.

Crystal structure

The caesium chloride structure is composed of a primitive cubic lattice with a two atom basis, where both atoms have eight-fold coordination. The chloride atoms lie upon the lattice points at the edges of the cube, while the caesium atoms lie in the holes in the center of the cubes. This structure is shared with CsBr and CsI and many intermetallic compounds. In contrast, the other alkaline halides have the sodium chloride structure. When both ions are similar in size (Cs⁺ ionic radius 174 pm for this coordination number, Cl⁻ 181 pm) the CsCl structure is adopted, when they are different (Na⁺ ionic radius 102pm, Cl⁻ 181 pm) the sodium chloride structure is adopted.

Ball-and-stick model of the unit cell of the CsCl structure	Ball-and-stick model of the cubic coordination of Cs and Cl in CsCl

Uses

Caesium chloride is also widely used in the centrifugation in a technique known as isopycnic centrifugation. Centrifugal and diffusive forces establish a density gradient which allows separation of mixtures on the basis of their molecular density. This technique allows separation of DNA of different densities (e.g. DNA fragments with differing A-T or G-C content).

Radioisotopes of caesium chloride are used in nuclear medicine, including treatment of cancer. In the production of radioactive sources, it is normal to choose a chemical form of the radioisotope which would not be readily dispersed in the environment in the event of an accident. For instance, radiothermal generators (RTGs) often use strontium titanate, which is insoluble in water. For teletherapy sources, however, the radioactive density (Ci in a given volume) needs to be very high, which is not possible with known insoluble caesium compounds. A thimble-shaped container of radioactive caesium chloride provides the active source. In the Goiânia accident, such a source was reptured, leading to several deaths.

Miscellaneous applications

Caesium chloride (non-radioactive) is also promoted as an alternative cancer therapy. Caesium chloride is used in the preparation of electrically conducting glasses.