Caesium iodide or cesium iodide ( chemical formula CsI) is the

ionic compound In chemistry, an ionic compound is a chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged i ...

caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...

and

iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...

. It is often used as the input

phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or vi ...

of an X-ray image intensifier tube found in fluoroscopy equipment. Caesium iodide photocathodes are highly efficient at extreme ultraviolet wavelengths.

Synthesis and structure

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 Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

and NaCl for LiF, NaBr and NaCl substrates. Caesium iodide atomic chains can be grown inside double-wall

carbon nanotube A scanning tunneling microscopy image of a single-walled carbon nanotube Rotating single-walled zigzag carbon nanotube A carbon nanotube (CNT) is a tube made of carbon with diameters typically measured in nanometers. ''Single-wall carbon na ...

s. 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.

Properties

Applications

An important application of caesium iodide crystals, which are

scintillator A scintillator is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the absorbed ...

s, 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 A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degree ...

and a slight

hygroscopicity Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substa ...

. 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 Potassium bromide ( K Br) is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the US. Its action is due to the bromide ion (sodium bromide is equall ...

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. In addition to image intensifier input phosphors, caesium iodide is often also used in medicine as the scintillating material in flat panel x-ray detectors.

References

Cited sources

* {{Authority control Caesium compounds Iodides Alkali metal iodides Metal halides Caesium chloride crystal structure