Triphosphorus pentanitride

Triphosphorus pentanitride

Names
IUPAC name Triphosphorus pentanitride
Other names Phosphorus(V) nitride, Phosphorus nitride
Identifiers
CAS Number	12136-91-3
Properties
Chemical formula	P3N5
Molar mass	162.955
Appearance	White solid
Density	α-P3N5 = 2.77 g/cm3
Melting point	decomposes at ≥ 850 °C (1,560 °F; 1,120 K)
Solubility in water	insoluble
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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.

Synthesis

Triphosphorus pentanitride can be produced by reactions between various phosphorus(V) and nitrogen compounds (such as ammonia and sodium azide); including a reaction between the elements.

3 PCl₅ + 5 NH₃ → P₃N₅ + 15 HCl

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

Similar methods are used to prepared boron nitride and silicon nitride; however the products are generally impure and amorphous.

Crystalline samples have been produced by the reaction of ammonium chloride and hexachlorocyclotriphosphazene: or phosphorus pentachloride.

(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.

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

Reactions

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

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 (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.

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.

Polymorph	Density (g/cm3)
α‑P3N5	2.77
γ‑P3N5	3.65
δ‑P3N5	4.02

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

Applications

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. 192^oC and tetrameric bromophosphonitrile, (PNBr₂)₄, m.p. 202^oC find similar lamp gettering applications for tungsten halogen lamps, where they perform the dual processies of gettering and precise halogen dosing.

It has also been investigated as a new material for semiconductor based electronics; particularly as a gate insulator in metal-insulator-semiconductor devices.

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

See also

• Polyphosphazene