Phosphorus(V) nitride, Phosphorus nitride
|Density||α-P3N5 = 2.77 g/cm3|
|Melting point||decomposes at ≥850°C|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Triphosphorus pentanitride is an inorganic compound with the chemical formula P3N5. Containing only phosphorus and nitrogen, this material is classified as a binary nitride. Related compounds are boron nitride and silicon nitride. No applications have been developed for this material, which remains a topic of research. It is a white solid, although samples often appear colored owing to impurities.
- 3 PCl5 + 5 NH3 → P3N5 + 15 HCl
- 3 PCl5 + 15 NaN3 → P3N5 + 15 NaCl + 5 N2
- (NPCl2)3 + 2 NH4Cl → P3N5 + 8 HCl
- 3 PCl5 + 5 NH4Cl → P3N5 + 20 HCl
- 3 PCl3 + 5 NaNH2 → P3N5 + 5 NaCl + 4 HCl + 3 H2
- 2 P3N5 → 6 PN + 2 N2
- 4 PN → P4 + 2 N2
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.
Structure and properties
Several different polymorphs are known for triphosphorus pentanitride. The alpha‑form of triphosphorus pentanitride (α‑P3N5) is encountered at atmospheric pressure and exists at pressures up to 6 GPa, at which point it converts to the gamma‑variety (γ‑P3N5) of the compound. Computational chemistry indicates that a third, delta‑variety (δ‑P3N5), will form at around 43 GPa with a Kyanite-like structure.
Triphosphorus pentanitride currently has no large scale applications although it had 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 whilst 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, (PNBr2)3, m.p. 192oC and tetrameric bromophosphonitrile, (PNBr2)4, m.p. 202oC find similar lamp gettering applications for tungsten halogen lamps, where they perform the dual processies of gettering and precise halogen dosing. 
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