Triphosphorus pentanitride

Triphosphorus pentanitride

Names
IUPAC name Triphosphorus pentanitride
Other names Phosphorus(V) nitride, Phosphorus nitride
Identifiers
CAS Number	12136-91-3
3D model (JSmol)	Interactive image
ECHA InfoCard	100.032.018
EC Number	235-233-9
PubChem CID	15443112
CompTox Dashboard (EPA)	DTXSID6093968
SMILES [N].[N].[N].[N].[N].[P].[P].[P]
Properties
Chemical formula	P3N5
Molar mass	162.955 g/mol
Appearance	White solid
Density	2.77 g/cm3 (α-P3N5)
Melting point	850 °C (1,560 °F; 1,120 K) decomposes
Solubility in water	insoluble
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. 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):^[1]

3 PCl₅ + 5 NH₃ → P₃N₅ + 15 HCl

3 PCl₅ + 15 NaN₃ → P₃N₅ + 15 NaCl + 20 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 [NH₄]Cl → P₃N₅ + 8 HCl

3 PCl₅ + 5 [NH₄]Cl → 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

Reactions

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

P₃N₅ → 3 PN + 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 [NH₄]H₂PO₄.

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 HPN₂ and HP₄N₇. It has been suggested that these compounds may have applications as solid electrolytes and pigments.^[6]

Structure and properties

Several 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 11 GPa, at which point it converts to the gamma‑variety (γ‑P₃N₅) of the compound.^[7][8] Upon heating γ‑P₃N₅ to temperatures above 2000 K at pressures between 67 and 70 GPa, it transforms into δ-P₃N₅.^[9] The release of pressure on the δ-P₃N₅ polymorph does not revert it back into γ‑P₃N₅ or α‑P₃N₅. Instead, at pressures below 7 GPa, δ-P₃N₅ converts into a fourth form of triphosphorus pentanitride, α′‑P₃N₅.^[9]

The structure of all polymorphs of triphosphorus pentanitride was determined by single crystal X-ray diffraction. α‑P₃N₅ and α′‑P₃N₅ are formed of a network structure of PN₄ tetrahedra with 2- and 3-coordinated nitrides,^[7][9] γ‑P₃N₅ is composed of both PN₄ and PN₅ polyhedra^[8] while δ-P₃N₅ is composed exclusively of corner- and edge-sharing PN₆ octahedra.^[9] δ-P₃N₅ is the most incompressible triphosphorus pentanitride, having a bulk modulus of 313 GPa.^[9]

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 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 cyclic polymers, trimeric hexabromophosphazene (PNBr₂)₃ (melting point 192 °C) and tetrameric octabromophosphazene (PNBr₂)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.^[10]

Triphosphorus pentanitride has also been investigated as a semiconductor for applications in microelectronics, particularly as a gate insulator in metal-insulator-semiconductor devices.^[11][12]

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, 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.^[13]

Patents have been filed for the use of triphosphorus pentanitride in fire fighting measures.^[14][15]

See also

• Polyphosphazene
• Phosphorus mononitride

References