CsI

Caesium iodide

CsI crystal
Scintillating CsI crystal
Crystal structure
Names
IUPAC name Caesium iodide
Other names Cesium iodide
Identifiers
CAS Number	7789-17-5 Y
3D model (JSmol)	Interactive image
ChemSpider	23003 Y
ECHA InfoCard	100.029.223
EC Number	232-145-2
InChI InChI=1S/Cs.HI/h;1H/q+1;/p-1 Y Key: XQPRBTXUXXVTKB-UHFFFAOYSA-M Y InChI=1/Cs.HI/h;1H/q+1;/p-1 Key: XQPRBTXUXXVTKB-REWHXWOFAA
SMILES [Cs+].[I-]
Properties
Chemical formula	CsI
Molar mass	259.809 g/mol
Appearance	white crystalline solid
Density	4.51 g/cm3
Melting point	632 °C (1,170 °F; 905 K)
Boiling point	1,280 °C (2,340 °F; 1,550 K)
Solubility in water	848 g/L (25 °C)
Magnetic susceptibility (χ)	-82.6·10−6 cm3/mol
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)
Structure
Crystal structure	CsCl, cP2
Space group	Pm3m, No. 221
Lattice constant	a = 0.4503 nm
Lattice volume (V)	0.0913 nm3
Formula units (Z)	1
Coordination geometry	Cubic (Cs+) Cubic (I−)
Thermochemistry
Specific heat capacity (C)	52.8 J/mol·K
Std molar entropy (So298)	123.1 J/mol·K
Std enthalpy of formation (ΔfHo298)	−346.6 kJ/mol
Gibbs free energy (ΔfG˚)	-340.6 kJ/mol
Hazards
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)	2386 mg/kg (oral, rat)
Related compounds
Other anions	Caesium fluoride Caesium chloride Caesium bromide Caesium astatide
Other cations	Lithium iodide Sodium iodide Potassium iodide Rubidium iodide Francium iodide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Y  (what is YN ?)
Infobox references

Caesium 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.

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, extending its working range into the far infrared. However, optical-quality CsI crystals are very soft and a 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.