Caesium Chloride - Physical Properties

Physical Properties

Caesium chloride is colorless in the form of large crystals and white when powdered. It readily dissolves in water with the maximum solubility increasing from 1865 g/L at 20 °C to 2705 g/L at 100 °C. The crystals are very hygroscopic and gradually disintegrate at ambient conditions. Caesium chloride does not form hydrates.

In contrast to sodium chloride and potassium chloride, caesium chloride readily dissolves in concentrated hydrochloric acid. Caesium chloride has also a relatively high solubility in formic acid (1077 g/L at 18 °C) and hydrazine; medium solubility in methanol (31.7 g/L at 25 °C) and low solubility in ethanol (7.6 g/L at 25 °C), sulfur dioxide (2.95 g/L at 25 °C), ammonia (3.8 g/L at 0 °C), acetone (0.004% at 18 °С), acetonitrile (0.083 g/L at 18 °С), ethylacetates and other complex ethers, butanone, acetophenone, pyridine and chlorobenzene.

Despite its wide band gap of about 8.35 eV at 80 K, caesium chloride weakly conducts electricity, and the conductivity is not electronic but ionic. The conductivity has a value of the order 10−7 S/cm at 300 °C. It occurs through nearest-neighbor jumps of lattice vacancies, and the mobility is much higher for the Cl- than Cs+ vacancies. The conductivity increases with temperature up to about 450 °C, with an activation energy changing from 0.6 to 1.3 eV at about 260 °C. It then sharply drops by two orders of magnitude because of the phase transition from the α-CsCl to β-CsCl phase. The conductivity is also suppressed by application of pressure (about 10 times decrease at 0.4 GPa) which reduces the mobility of lattice vacancies.