Carbon tetraiodide

Carbon tetraiodide is CI4, a tetrahalomethane. Being bright red, it is a relatively rare example of a highly colored methane derivative. It is only 2% by weight carbon, although other methane derivatives are known with still less carbon. The tetrahedral molecule features C-I distances of 2.12 ± 0.02 Å. The molecule is slightly crowded with short I---I contacts of 3.459 ± 0.03 Å, and possibly for this reason, it is thermally and photochemically unstable. Hexaiodoethane is unknown, probably for the same reason.

Properties, synthesis, uses
CI4 is slightly reactive towards water, giving iodoform and I2. Otherwise it is soluble in nonpolar organic solvents. It decomposes thermally and photochemically to tetraiodoethylene, I2C=CI2. Its synthesis entails AlCl3-catalyzed halide exchange, which is conducted at room temperature:

CCl4 + 4 EtI ? CI4 + 4 EtCl

The product crystallizes from the reaction solution.
CI4 is used as an iodination reagent, often upon reaction with base. Ketones are converted to 1,1-diiodoethenes upon treatment with PPh3 and CI4. Alcohols are converted in and to iodide, by a mechanism similar to the Appel reaction. In an Appel reaction carbon tetrachloride is used to generate the chloride from alcohols.

Safety considerations
Manufacturers recommend that CI4 be stored near 0 °C. As a ready source of iodine, it is an irritant. LD50: 178 mg kg–1. In general perhalogenated organic compounds should be considered toxic.

Calcium fluoride

Calcium fluoride (CaF2) is an insoluble ionic compound of calcium and fluorine. It occurs naturally as the mineral fluorite (also called fluorspar), and it is the source of most of the world's fluorine. It reacts with concentrated sulfuric acid to produce hydrogen fluoride:
CaF2(s) + H2SO4(l) ? CaSO4(s) + 2 HF(g) This is the reaction used to produce hydrogen fluoride in industry.

Applications
Calcium fluoride is commonly used as a window material for both infrared and ultraviolet wavelengths, since it is transparent in these regions (about 0.15 µm to 9 µm) and exhibits extremely weak birefringence. Nevertheless, at wavelengths as low as 157 nm, which are interesting to semiconductor manufacturers, the birefringence of calcium fluoride exceeds tolerable limits. This may be overcome by minimizing birefringence by optimimizing the growth process. It is particularly important as an ultraviolet optical material for integrated circuit lithography. Canon also uses artificially-crystallized calcium fluoride elements in some of its L-series lenses to reduce light dispersion. As an infrared optical material, calcium fluoride is sometimes known by the Eastman Kodak trademarked name Irtran-3, although this designation is long since obsolete.
Uranium-doped calcium fluoride was the second type of solid state laser invented, in the 1960s. Peter Sorokin and Mirek Stevenson at IBM's laboratories in Yorktown Heights, New York, achieved lasing at 2.5 µm shortly after Maiman's ruby laser.