The limit of layers in black phosphorous
In the 2D "flatlands", graphene stands tall as the most exciting discovery for future technologies. Despite its many attractive properties however, graphene lacks a natural band gap that would allow electrical flow to be switched on and off (it is a zero gap semiconductor). Luckily the physical realisation of graphene has led to an explosion of interest in other 2D and/or layered (nano)materials for specific applicaitions. For example the elemental allotropes silicene, germanene or stanene, transition metal dichalcogenides such as molybdenum disulfide, metal oxides such as vanadium oxide, and metal-free layered materials such as hexagonal boron nitride, all display a vast array of different properties.
Phosphorene is a 2D material that is made up of a single layer of hexagonally arranged phosphorous atoms. Just as graphene can be synthesised from graphite, phosphorene is synthesised from layered black phosphorous (bP). Unlike graphene phosphorene does have a natural band gap, and also as a consequence of moving to group 15, in-plane bonding is due to sp3 hybridization leading to puckering of the layers. This band gap and ultimate properties of phosphorene can be tuned by altering the number of layers and strain field across the 2D (multi)layered sheets, meaning it has a multitude of possible applications including in electronic devices, sensors, thermoelectrics, batteries, and even as a water splitting photocatalyst.
Black phosphorous was first synthesised at high pressure by Bridgeman in 1914. Indeed the high pressure structural transitions of bP are well documented except for one: the key A7 to simple cubic transition at 11 GPa. This transition from a layered 2D to simple cubic (sc) 3D structure sets the stability limit for the layered phases of phosphorous, and its mechanism is therefore critical to understand the stabilisation of the low dimensional structure.
Using state-of-the-art high pressure X-ray diffraction at ESRF ID27 a two step mechanism has now been revealed in a study led by Matteo Ceppatelli (LENS, Florence). The key observation of two perviously unreported peaks in the diffraction pattern of the sc structure, persisting to the highest pressures we measured, shows a gradual rather than abrupt phase transition. By assuming a rhombohedral cell for all phases (A7, intermediate and sc), these extra peaks naturally emerge when there is a small lattice distortion from the expected cubic structure (α ≈ 60.0 degrees and the atomic position u ≤ 0.250). Full structure refinement of our XRD data proves experimental values of the lattice parameter a, angle α, atomic position u, and of the nearest neighbour distance nn at each pressure. The results clearly show a discontinuity for all the parameters at 10.5 GPa, where the A7 to sc transition is expected to take place. Nevertheless, while at this pressure α almost immediately approaches the limit value of 60.0 degrees, at the same pressure the atomic position u, except for a small discontinuity, is still far from the 0.250 value expected in the sc phase. These observations imply that, in the pressure range from 10.5 up to at least 27.6 GPa, a pseudo simple-cubic (p-sc) structure exists rather than the sc one.
The existence of the p-sc structure has remarkable implications. First, the existence of a p-sc structure significantly rises the pressure limit where the layered phases of P can exist. Second, the identification of the p-sc structure in the 10-30 GPa pressure range provides a new experimental evidence for explaining the long-debated anomalous pressure evolution of the superconducting critical temperature, Tc.