# Triphosphorus pentanitride

Triphosphorus pentanitride is an inorganic compound with the chemical formula P$_3$N$_5$. Containing only phosphorus and nitrogen, this material is classified as a binary nitride. While it has been investigated for various applications this has not led to any significant industrial uses. It is a white solid, although samples often appear colored owing to impurities.

## Synthesis

Triphosphorus pentanitride can be produced by reactions between various phosphorus(V) and nitrogen anions (such as ammonia and sodium azide):
    3 PCl$_5$ + 5 NH$_3$ → P$_3$N$_5$ + 15 HCl
    3 PCl$_5$ + 15 NaN$_3$ → P$_3$N$_5$ + 15 NaCl + 20 N$_2$
The reaction of the elements is claimed to produce a related material. Similar methods are used to prepared boron nitride (BN) and silicon nitride (Si$_3$N$_4$); however the products are generally impure and amorphous.
Crystalline samples have been produced by the reaction of ammonium chloride and hexachlorocyclotriphosphazene or phosphorus pentachloride.
    (NPCl$_2$)$_3$ + 2 [NH$_4$]Cl → P$_3$N$_5$ + 8 HCl
    3 PCl$_5$ + 5 [NH$_4$]Cl → P$_3$N$_5$ + 20 HCl
P$_3$N$_5$ has also been prepared at room temperature, by a reaction between phosphorus trichloride and sodium amide.
    3 PCl$_3$ + 5 NaNH$_2$ → P$_3$N$_5$ + 5 NaCl + 4 HCl + 3 H$_2$

## Reactions

P$_3$N$_5$ is thermally less stable than either BN or Si$_3$N$_4$, with decomposition to the elements occurring at temperatures above 850 °C:
    P$_3$N$_5$ → 3 PN + N$_2$
    4 PN → P$_4$ + 2 N$_2$
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 [NH$_4$]$_2$HPO$_4$ and [NH$_4$]H$_2$PO$_4$.
Triphosphorus pentanitride reacts with lithium nitride and calcium nitride to form the corresponding salts of PN7−4 and PN4−3. Heterogenous ammonolyses of triphosphorus pentanitride gives imides such as HPN$_2$ and HP$_4$N$_7$. It has been suggested that these compounds may have applications as solid electrolytes and pigments.

## Structure and properties

Several polymorphs are known for triphosphorus pentanitride. The alpha‑form of triphosphorus pentanitride (α‑P$_3$N$_5$) is encountered at atmospheric pressure and exists at pressures up to 11 GPa, at which point it converts to the gamma‑variety (γ‑P$_3$N$_5$) of the compound. Upon heating γ‑P$_3$N$_5$ to temperatures above 2000 K at pressures between 67 and 70 GPa, it transforms into δ-P$_3$N$_5$. The release of pressure on the δ-P$_3$N$_5$ polymorph does not revert it back into γ‑P$_3$N$_5$ or α‑P$_3$N$_5$. Instead, at pressures below 7 GPa, δ-P$_3$N$_5$ converts into a fourth form of triphosphorus pentanitride, α′‑P$_3$N$_5$.
| Polymorph | Density (g/cm$^3$) |
| α‑P$_3$N$_5$ | 2.77 |
| α′‑P$_3$N$_5$ | 3.11 |
| γ‑P$_3$N$_5$ | 3.65 |
| δ‑P$_3$N$_5$ | 5.27 (at 72 GPa) |
The structure of all polymorphs of triphosphorus pentanitride was determined by single crystal X-ray diffraction. α‑P$_3$N$_5$ and α′‑P$_3$N$_5$ are formed of a network structure of PN$_4$ tetrahedra with 2- and 3-coordinated nitrides, γ‑P$_3$N$_5$ is composed of both PN$_4$ and PN$_5$ polyhedra while δ-P$_3$N$_5$ is composed exclusively of corner- and edge-sharing PN$_6$ octahedra. δ-P$_3$N$_5$ is the most incompressible triphosphorus pentanitride, having a bulk modulus of 313 GPa.

## Potential applications

Triphosphorus pentanitride has no commercial 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 P$_3$N$_5$ prior to being sealed into the bulb. After bulb closure, but while still on the pump, the lamps are lit, causing the P$_3$N$_5$ to thermally decompose into its constituent elements. Much of this is removed by the pump but enough P$_4$ vapor remains to react with any residual oxygen inside the bulb. Once the vapor pressure of P$_4$ 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 P$_3$N$_5$ allows sealing machines to run faster and hotter than was possible using red phosphorus.
Related halogen containing cyclic polymers, trimeric hexabromophosphazene (PNBr$_2$)$_3$ (melting point 192 °C) and tetrameric octabromophosphazene (PNBr$_2$)$_4$ (melting point 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 some benefits over the more commonly used red phosphorus, owing mainly to its higher chemical stability. Unlike red phosphorus, P$_3$N$_5$ 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, P$_3$N$_5$ is much more resistant to hydrolysis than red phosphorus, giving pyrotechnic mixtures based on it greater stability under long-term storage.
Patents have been filed for the use of triphosphorus pentanitride in fire fighting measures.