Triphosphorus pentanitride is an inorganic compound with the chemical formula P3N5. Containing only phosphorus and nitrogen, this material is classified as a binary nitride. No applications have been developed for this material. It is a white solid, although samples often appear colored owing to impurities.
P3N5 is thermally less stable than either BN or Si3N4, with decomposition to the elements occurring at temperatures above 850 °C: It is resistant to weak acids and bases, and insoluble in water at room temperature, however it hydrolyzes upon heating to form the ammonium phosphate salts 2HPO4 and NH4H2PO4. Triphosphorus pentanitride reacts with lithium nitride and calcium nitride to form the corresponding salts of PN47− and PN34−. Heterogenous ammonolyses of triphosphorus pentanitride gives imides such as HPN2 and HP4N7. It has been suggested that these compounds may have applications as solid electrolytes and pigments.
Several different polymorphs are known for triphosphorus pentanitride. The alpha‑form of triphosphorus pentanitride is encountered at atmospheric pressure and exists at pressures up to 6 GPa, at which point it converts to the gamma‑variety of the compound. Computational chemistry indicates that a third, delta‑variety, will form at around 43 GPa with a Kyanite-like structure.
Triphosphorus pentanitride has no large scale applications, although it found use as a gettering material for incandescent lamps, replacing various mixtures containing red phosphorus in the late 1960s. The lighting filaments are dipped into a suspension of P3N5 prior to being sealed into the bulb. After bulb closure, but while still on the pump, the lamps are lit, causing the P3N5 to thermally decompose into its constituent elements. Much of this is removed by the pump but enough P4 vapor remains to react with any residual oxygen inside the bulb. Once the vapor pressure of P4 is low enough, either filler gas is admitted to the bulb prior to sealing off or, if a vacuum atmosphere is desired, the bulb is sealed off at that point. The high decomposition temperature of P3N5 allows sealing machines to run faster and hotter than was possible using red phosphorus. Related halogen containing polymers, trimeric bromophosphonitrile, 3, m.p. 192 °C and tetrameric bromophosphonitrile, 4, m.p. 202 °C find similar lamp gettering applications for tungsten halogen lamps, where they perform the dual processies of gettering and precise halogen dosing. Triphosphorus pentanitride has also been investigated as a semiconductor for applications in microelectronics, particularly as a gate insulator in metal-insulator-semiconductor devices. As a fuel in pyrotechnic obscurant mixtures, it offers various benefits over the more commonly used red phosphorus, owing mainly to its higher chemical stability. Unlike red phosphorus, P3N5 can be safely mixed with strong oxidizers, even potassium chlorate. While these mixtures can burn up to 200 times faster than state-of-the-art red phosphorus mixtures, they are far less sensitive to shock and friction. Additionally, P3N5 is much more resistant to hydrolysis than red phosphorus, giving pyrotechnic mixtures based on it greater stability under long-term storage. Several patents have been filed for the use of triphosphorus pentanitride in fire fighting measures.
