Question

What is black alpha phosphorus?

Answer 1

Black alpha phosphorus is an allotrope of phosphorus consisting of multiple layers with two-dimensional structures, weakly bonded to one another by van der Waals forces. When separated, the resulting monolayer material is known as phosphorene.

Complete answer:
Alpha black phosphorus is formed when red phosphorus is heated in a sealed tube at $803K$. It can be sublimed in air and has opaque monoclinic or rhombohedral crystals. It does not oxidize in air.
We can prepare black phosphorus from white phosphorus by heating it to $470K$ at inert temperature. ... While beta black phosphorus forms when white phosphorus is heated at $473K$, alpha black phosphorus forms when we heat red phosphorus at $803K$.
Alpha Black phosphorus is more stable than beta one. Alpha consists of opaque monoclinic OR rhombohedral crystals, while beta having corrugated sheets that form flaky layered crystals.
Black phosphorus is most stable then all other allotropes of Phosphorus. It melts at $860K$. And structurally resembles graphite like. It is least reactive among all.
Black phosphorus is thermodynamically the most stable form at room temperature and pressure; the atoms are linked together in puckered sheets, like graphite.

Note:
Black alpha phosphorus is the least reactive form of phosphorus and has no significant commercial uses. The latest addition to the family of $2D$ layered materials, has attracted much interest owing to potential optoelectronics, nanoelectronics, and biomedicine applications. Little is known about its toxicity, such as whether it could be as toxic as white phosphorus