Interlayer bond formation in black phosphorus at high pressure

20-10-2017

The observation of a two-step mechanism for the A7 to simple-cubic high-pressure phase transition in phosphorus has unveiled an intermediate pseudo-simple-cubic crystalline structure, raising the pressure limit for 2D vs 3D phosphorus and explaining the long-debated anomalous pressure evolution of its Tc.

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Phosphorene is a 2D corrugated monoatomic layer of phosphorus atoms that has recently gained interest from chemists, physicists and materials scientists for its fascinating properties that are related to the layered phases of phosphorus [1]. Black phosphorus is the starting material in the synthesis of phosphorene. The relationship between black phosphorus and phosphorene can be likened to that of graphite and graphene. Black phosphorus is one of the many solid structures of phosphorus that exist at ambient pressure that are commonly referred to by their colours (i.e. white, red, violet, black). They exhibit extremely diverse chemical and physical properties. Black phosphorus was first synthesised at high pressure by Bridgman in 1914 and its crystalline structure (A17) consists of stacked phosphorene layers. The synthesis, stabilisation and functionalisation of single-layer phosphorene remain a challenge. Knowledge of the effects that stabilise the layered vs non-layered structures of phosphorus and of the mechanism of interlayer bond formation that take place during the corresponding phase transitions are important for the design and synthesis of phosphorene-based materials and heterostructures.

At room temperature and under high-pressure conditions, the layered structure of black phosphorus (A17, orthorhombic) is stable up to about 5 GPa, where it transforms to rhombohedral A7, another phase with a layered structure stable up to about 11 GPa. Here, according to the literature, a transition to a non-layered simple-cubic (sc) phase occurs. This transition is particularly important because the associated bond breaking and reconstruction mechanism sets the stability limit for the layered phases of phosphorus.

Room-temperature compression of black phosphorus was performed at beamline ID27 using X-ray diffraction to follow the phase changes between A17, A7 and sc phases. A membrane diamond-anvil cell (DAC) with He as the hydrostatic pressure transmitting medium was used and pressures up to 27.6 GPa were reached. Two previously-unreported peaks were observed in the diffraction pattern of the sc structure. These peaks persisted up to the highest investigated pressure and have led to a revision of the phosphorus phase diagram up to 30 GPa. Assuming a rhombohedral cell description (R3m) for both A7 and sc structures, the two extra peaks are not expected in the diffraction pattern of an sc structure (angle α = 60.0 degree and atomic position u = 0.250), but they naturally emerge when α ≈ 60.0 degree and atomic position u ≤ 0.250. This condition corresponds to a lattice distortion without atomic displacement. Rietveld refinement of the XRD data provided the experimental values of the lattice parameter a, of the angle α, of the atomic position u and of the nearest neighbour distance nn at each pressure point across the investigated pressure range. The experimental data, in agreement with theoretical predictions, demonstrate a two-step mechanism for the A7 to sc phase transition. 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 structure (Figure 1). The p-sc structure originates from pressure-dependent competition between a strong s-p orbital mixing that prevails at low pressure, which leads to the formation of lone pairs of electron at phosphorus sites and layers in A17 and A7, and a dominating electrostatic contribution at higher density, which is responsible for the octahedral coordination in sc.

Phase diagram of phosphorus between 0.0 and 30.0 GPa

Figure 1. Phase diagram of phosphorus between 0.0 and 30.0 GPa, showing the pressure and temperature ranges where A17 and A7 are reported to be stable. The dashed line at 10.5 GPa marks the A7 to sc phase transition according to current literature. The room temperature data indicate that the sc phase is not achieved up to 27.6 GPa and that an intermediate pseudo simple-cubic (p-sc) structure exists between the A7 and the sc structures. The melting line of He is also displayed (dotted line). Structures generated by the Rietveld refinement of the data in the corresponding phases are also shown, highlighting the transition between the A17 and A7 layered structures and the transformation of layered A7 to non-layered sc.

The existence of the p-sc structure has profound implications. Firstly, from a chemical point view, the presence of three shorter and three longer distances relates p-sc to A7 structurally, which significantly raises the pressure limit where the layered phases of P can exist and opens new perspectives for the synthesis, stabilisation and functionalisation of phosphorene-based systems. Secondly, concerning superconductivity, the identification of the p-sc structure in the 10-30 GPa pressure range provides new experimental evidence to explain the long-debated anomalous pressure evolution of the critical temperature Tc.

 

Principal publication and authors
Interlayer bond formation in black phosphorus at high pressure, D. Scelta (a,b), A. Baldassarre (b,c), M. Serrano-Ruiz (a), K. Dziubek (b), A.B. Cairns (d), M. Peruzzini (a), R. Bini (a,b,c), M. Ceppatelli (a,b), Angewandte Chemie International Edition, (2017); doi: 10.1002/anie.201708368.
(a) CNR-ICCOM Institute of Chemistry of OrganoMetallic Compounds, National Research Council of Italy, Sesto Fiorentino (Italy)
(b) LENS, European Laboratory for Non-Linear Spectroscopy, Sesto Fiorentino (Italy)
(c) Dipartimento di Chimica “Ugo Schiff”, Università degliStudi di Firenze, Sesto Fiorentino (Italy)
(d) ESRF

 

References
[1] M. Batmunkh et al., Adv. Mater. 28, 8586-8617 (2016).
[2] K.T. Chan et el., Phys. Rev. B 88, 064517 (2013).