Triphosphorus pentanitride

Triphosphorus pentanitride can be produced by reactions between various phosphorus(V) and nitrogen anions (such as ammonia and sodium azide):[1]

3 PCl₅ + 5 NH₃ → P₃N₅ + 15 HCl

3 PCl₅ + 15 NaN₃ → P₃N₅ + 15 NaCl + 5 N₂

The reaction of the elements is claimed to produce a related material.[2]Similar methods are used to prepared boron nitride (BN) and silicon nitride (Si₃N₄); however the products are generally impure and amorphous.[1][3]

Crystalline samples have been produced by the reaction of ammonium chloride and hexachlorocyclotriphosphazene[4] or phosphorus pentachloride.[1]

(NPCl₂)₃ + 2 NH₄Cl → P₃N₅ + 8 HCl

3 PCl₅ + 5 NH₄Cl → P₃N₅ + 20 HCl

P₃N₅ has also been prepared at room temperature, by a reaction between phosphorus trichloride and sodium amide.[5]

3 PCl₃ + 5 NaNH₂ → P₃N₅ + 5 NaCl + 4 HCl + 3 H₂

P₃N₅ is thermally less stable than either BN or Si₃N₄, with decomposition to the elements occurring at temperatures above 850 °C:[1]

2 P₃N₅ → 6 PN + 2 N₂

4 PN → P₄ + 2 N₂

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 (NH₄)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.[6]

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.[7]

The structure of α‑P₃N₅ has been determined by single crystal X-ray diffraction, which showed a network structure of edge‑sharing PN₄ tetrahedra.[8]

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, (PNBr₂)₃, m.p. 192 °C and tetrameric bromophosphonitrile, (PNBr₂)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.[9]

Triphosphorus pentanitride has also been investigated as a semiconductor for applications in microelectronics, particularly as a gate insulator in metal-insulator-semiconductor devices.[10][11]

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.[12]

Several patents have been filed for the use of triphosphorus pentanitride in fire fighting measures.[13][14]

• Polyphosphazene