Black phosphorus
Black phosphorus is stable allotrope of elemental phosphorus. It is of two types known as α–Black and β–Black phosphorus.
Metallic or α–Black:
This is obtained by dissolving red phosphorus in lead at 673K in a scaled tube for sufficiently long time. On cooling crystals
of α–Black phosphorus are obtained. Lead is dissolved out by treatment with dilute nitric acid.
It is highly stable allotrope of phosphorus and does not oxidize in air unless heated very strongly. It does not conduct electricity.
Crystal structure of α–black and β–black phosphorus
β–Black phosphorus:
It is obtained by heating white phosphorus at 470K under very high pressure (4000 atm) in an inert atmosphere or by keeping white
phosphorus for 8 days in presence of Hg at 200atm pressure and 650K. It is a crystalline solid and has layered structure. Each
layer is made up of zig–zag chains in which each phosphorus atoms is covalently bonded to the three neighboring phosphorus
atoms. The P–P–P angles are 99° and the P–P distance between two nearest phosphorus atoms is 2.18A°.
Structure of black phosphorus is similar to that of graphite.
Properties:
- It has a sharp melting point of 760K
- It has higher density (2.69 gcm−3) due to polymeric structure.
- It is a fairly good conductor of heat and electricity.
- It is a stable form of phosphorus and does not burn in air even up to 673K.