About Phosphides

solid oxide fuel cell cathode and electrolyte cross section by SEM

Phosphides are compounds of phosphorus with one or more less electronegative elements. Most elements less-electronegative than phosphorus form at least one binary phosphide, and most form a number of binary phosphides with different stoichiometries. Notably, potassium is capable of forming nine different binary phosphides: K3P, K4P3, K5P4, KP, K4P6, K3P7, K3P11, KP10.3, KP15. Phosphides may be principally ionic compounds with P3- ions, ionic compounds with polymeric phosphorus ions such as P24- or P113-, or compounds with individual phosphorus atoms incorporated into a metal lattice which may exhibit metal-like or semiconductor-like properties.

Although there are a number of ways to prepare phosphides, the most general method is to heat stoichiometric amounts of the metal and red phosphorus to high temperature in an inert atmosphere or a vacuum. Alkali and alkaline earth metals transition-metal phosphides are inert metallic-looking solids with high melting points and electrical conductivities.

Some metal phosphides have been used as rodenticides, primarily calcium phosphide, zinc phosphide, and aluminum phosphide. Zinc phosphide baits have strong, pungent garlic-like odor characteristic for phosphine liberated by hydrolysis. The odor attracts rodents, but has a repulsive effect on other animals. Aluminum phosphide is also wide band gap semiconductor. Calcium phosphide has uses in incendiary bombs. On contact with acids or water, calcium phosphide releases phosphine, which ignites spontaneously. Calcium phosphide is also used in fireworks, torpedoes, self-igniting naval pyrotechnic flares, and various water-activated ammunition.

Gallium phosphide is an important semiconducting phosphide used in the manufacture of low-cost red, orange, and green light-emitting diodes (LEDs) with low to medium brightness since the 1960s. It has a relatively short life at higher current and its lifetime is sensitive to temperature. Pure gallium phosphide LEDs emit green light at a wavelength of 555 nm. Nitrogen-doped gallium phosphide emits yellow-green (565 nm) light, zinc oxide doped gallium phosphide emits red (700 nm). Indium phosphide is a semiconductor and is used in high-power and high-frequency electronics because of its superior electron velocity with respect to the more common semiconductors such as gallium arsenide. Indium phosphide also has one of the longest-lived optical phonons of any compound with the zincblende crystal structure.

Sodium phosphide is a source of the highly reactive phosphide anion, and is often employed commercially in conjunction with zinc phosphide and aluminum phosphide as a catalyst in polymer production. Sodium phosphide is highly dangerous, releasing toxic phosphine upon hydrolysis, a process that is so exothermic that it may result in fires.