Squeezing lone pairs: The A 17 to A 7 pressure-induced phase transition in black phosphorus

We show how the ground state of phosphorus, black phosphorus, and the α-As type modification, next-lying on increasing pressure, are tightly connected over a Peierls intermediate. We unravel an intrinsic reactivity given by lone pairs, which are conserved during phase transition and are not suppressing the Peierls state. Along this line we provide a consistent solution to a classical chemical puzzle.