Caesium iodide or cesium iodide (chemical formula CsI) is the ionic compound of caesium and iodine. It is often used as the input phosphor of an X-ray image intensifier tube found in fluoroscopy equipment. Caesium iodide photocathodes are highly efficient at extreme ultraviolet wavelengths.
Scintillating CsI crystal
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||259.809 g/mol|
|Appearance||white crystalline solid|
|Melting point||632 °C (1,170 °F; 905 K)|
|Boiling point||1,280 °C (2,340 °F; 1,550 K)|
|848 g/L (25 °C)|
Refractive index (nD)
|1.9790 (0.3 µm)|
1.7873 (0.59 µm)
1.7694 (0.75 µm)
1.7576 (1 µm)
1.7428 (5 µm)
1.7280 (20 µm)
|Pm3m, No. 221|
a = 0.4503 nm
Lattice volume (V)
Formula units (Z)
Heat capacity (C)
Std enthalpy of
Gibbs free energy (ΔfG˚)
|H315, H317, H319, H335|
|P201, P202, P261, P264, P270, P271, P272, P273, P280, P281, P301+P312, P302+P352, P304+P340, P305+P351+P338, P308+P313, P312, P321, P330, P332+P313, P333+P313, P337+P313, P362, P363, P391, P403+P233, P405, P501|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|2386 mg/kg (oral, rat)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
(what is ?)
Bulk caesium iodide crystals have the cubic CsCl crystal structure, but the structure type of nanometer-thin CsI films depends on the substrate material – it is CsCl for mica and NaCl for LiF, NaBr and NaCl substrates.
Caesium iodide atomic chains can be grown inside double-wall carbon nanotubes. In such chains I atoms appear brighter than Cs atoms in electron micrographs despite having a smaller mass. This difference was explained by the charge difference between Cs atoms (positive), inner nanotube walls (negative) and I atoms (negative). As a result, Cs atoms are attracted to the walls and vibrate more strongly than I atoms, which are pushed toward the nanotube axis.
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 low light yield that increases significantly with cooling. 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 is used as a beamsplitter in Fourier transform infrared (FTIR) spectrometers. It has a wider transmission range than the more common potassium bromide beamsplitters, working range into the far infrared. However, optical-quality CsI crystals are very soft and hard to cleave or polish. They should also be coated (typically with germanium) and stored in a desiccator, to minimize interaction with atmospheric water vapors.
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