Black phosphorus was compressed at room temperature across the A17, A7 and simple‐cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X‐ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple‐cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two‐step mechanism for the A7 to simple‐cubic phase transition, indicating the existence of an intermediate pseudo simple‐cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non‐layered simple‐cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene‐based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.

High pressure state-of-the-art synchrotron XRD in black-phosphorus has solved apparent contradictions about the stability of the A7 layered structure in pnictogens, highlighting the importance of the s–p orbital mixing in the formation of the p-sc structure.

Blackphosphorus was compressed at room temperature across the A17, A7 and simple-cubic phases up to 30 GPa, using ad iamond anvil cell and He as pressure transmitting medium. Synchrotron X-ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple-cubic phase.T he Rietveld refinement of the data demonstrates the occurrence of atwo-step mechanism for the A7 to simple-cubic phase transition, indicating the existence of an intermediate pseudo simple-cubic structure.From ac hemical point of view this study represents ad eep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non-layered simple-cubic phase of phosphorus,o pening new perspectives for the design, synthesis and stabilization of phosphorene-based systems.A ss uperconductivity is concerned, anew experimental evidence to explain the anomalous pressure behavior of T c in phosphorus below 30 GPaisprovided.The current renaissance [1] of black phosphorus (bP), first obtained under high pressure by Bridgman in 1914, [2] is intimately related to the recent synthesis of phosphorene, [3,4] a2 Dc orrugated monoatomic layer of P, where each atom is single-bonded to 3n earest neighbors.T he advent of phosphorene [5][6][7][8][9][10] has indeed dramatically raised the interest of the scientific community about bP,w hose crystalline structure is actually made by the periodic stacking of phosphorene layers, in as imilar way as graphene is related to graphite,a nd the layered phases of this element currently represent an extremely active research topic [11,12] (see the Supporting Information, SI-1).At the moment crystalline bP,which is typically obtained from amorphous red phosphorus at high temperature and/or high pressure, [13,14] is the starting material for the synthesis of phosphorene,either by liquid or mechanical exfoliation. The layered semiconducting orthorhombic bP (A17, Cmce, Z = 8) [15,16] is the thermodynamically stable allotrope of the element at ambient conditions (Figure 1). [17] By increasing pressure at room temperature bP undergoes ac haracteristic phase transition at~5GPa to al ayered semimetallic rhombohedral phase (A7, R3 m, Z = 2), which is reported to transform to ametallic simple-cubic phase (sc, Pm3 m, Z = 1) at~11 GPa. [18][19][20] Thes cs tructure,r arely observed in nature (a-Poa nd high-pressure Ca-III and As-II), [21] is stable up to 103 GPa, where it transforms to an incommensurate modulated structure (P-IV,I M, Cmmm(00g)s00). [22,23] At 137 GPa P-IV converts to asimple-hexagonal phase (P-V,sh, P6/mmm, Z = 1) observed up to 282 GPa, [24,25] which is also ar arely adopted structure.[21] Ab ody-centered cubic structure (bcc), [26,27] successively identified as as uperlattice structure (P-VI, cI16(I4 3d)), has been reported to form above 262 GPa and to be stable up to 340 GPa. [28] However,despite the phase diagram of phosphorus under high pressure being known up to 340 GPa, [28] the details of the tra...

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