Caesium iodide

Caesium iodide
CAS number 7789-17-5 YesY
ChemSpider 23003 YesY
EC-number 232-145-2
Jmol-3D images Image 1
Molecular formula CsI
Molar mass 259.81 g/mol
Appearance white crystalline solid
Density 4.51 g/cm3, solid
Melting point

621 °C

Boiling point

1277±5 °C

Solubility in water 44 g/100 ml (0 °C)
Refractive index (nD) 1.739
Crystal structure CsCl
Space group Pm3m, No. 221
Lattice constant a = 456.67 pm
Cubic (Cs+)
Cubic (I)
EU Index not listed
Flash point non-flammable
Related compounds
Other anions Caesium fluoride
Caesium chloride
Caesium bromide
Other cations Lithium iodide
Sodium iodide
Potassium iodide
Rubidium iodide
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Caesium iodide (American English: Cesium iodide) (CsI) is an ionic compound often used as the input phosphor of an x-ray image intensifier tube found in fluoroscopy equipment.

An important application of caesium iodide crystals, which are scintillators, is electromagnetic calorimetry in experimental particle physics. Pure CsI is a fast and dense scintillating material with relatively high light yield. It shows two main emission components. One in the near ultraviolet region at the wavelength of 310 nm and one at 460 nm. The drawbacks of CsI are a high temperature gradient and a slight hygroscopicity.

Caesium iodide can be used in Fourier Transform Infrared (FT-IR) spectrometers as a beamsplitter. CsI has a wider transmission range than the more common potassium bromide beamsplitters, extending usefulness into the far infrared. A problem with optical-quality CsI crystals is that they are very soft with no cleavage, making it difficult to create a flat polished surface. Also, the CsI optical crystals must be stored in a desiccator to prevent water damage to the surfaces, and coated (typically with germanium) to minimise water damage from short term atmospheric exposure during beamsplitter swapouts.

Optical properties

Physical properties

  • Hardness (Knoop): 137.9 kPa (20 lbf/in2)
  • Young's modulus: 5.3 GPa (769 lbf/in2)
  • Modulus of rupture: 5.6 GPa (810 lbf/in2)
  • Apparent elastic limit: 5.6 GPa (810 lbf/in2)
  • Structure: Cubic, no cleavage
  • color: colorless


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