The mechanism of the A7 to sc phase transition in black phosphorus

Since its recent synthesis, phosphorene [1] has increasingly attracted the attention of chemists, physicists and materials scientists about the layered phases of phosphorus, particularly orthorhombic A17, which is the stable allotrope of the element, and rhombohedral A7, which is obtained by room temperature compression of A17 at ~5 GPa [2]. On further compression at ~11 GPa, A7 is reported to transform to a simple-cubic (sc) structure. Despite the phase diagram of phosphorus being known up to 340 GPa [3], the mechanism of the interlayer bond reconstruction during the transition from the A7 to the sc structure in the low pressure regime has been investigated only by computational studies, with no experimental confirmation so far. In this study we report experimental X-ray diffraction results acquired during the room T compression of black phosphorus in He up to 30 GPa using a DAC and synchrotron radiation (ESRF-ID27). Our data, in excellent agreement with theoretical predictions [4], reveal a two-step mechanism for the A7 to sc transition, demonstrating the existence of an intermediate pseudo simple-cubic structure. From a chemical point of view, the results of this study provide a fundamental insights on the effects determining the formation of interlayer bonds under high density conditions and are of importance for designing and synthesizing phosphorene based materials and heterostructures. Furthermore, the existence of the pseudo simple-cubic structure provides strong experimental evidence to explain the long debated anomalous pressure evolution of Tc in black phosphorus within the 10-30 GPa pressure range. Acknowledgments: Thanks are expressed to EC through the European Research Council (ERC) for funding the project PHOSFUN "Phosphorene functionalization: a new platform for advanced multifunctional materials" (Grant Agreement No. 670173) through an ERC Advanced Grant. [1] Batmunkh et al. Adv. Mater. 2016, 28, 8586-8617. [2] Zhu et al. Phys. Rev. Lett. 2014, 112, 176802. [3] Sugimoto et al. Phys. Rev. B 2012, 86, 024109. [4] Chang et al. Phys. Rev. B 2013, 88, 064517.