High pressure and high temperature chemistry of the lowest Z pnictogens: Phosphorus and Nitrogen

The recent synthesis of Phosphorene [1,2] and the theoretical predictions of a variety of related 2D allotropes [3] have raised a growing interest from the scientific community about the layered structures of Phosphorus. Indeed, black Phosphorus (bP, A17), formed by the ordered stacking of Phosphorene layers in a similar way as graphene is related to graphite, is actually the starting material for the synthesis of Phosphorene by exfoliation techniques. However, while the synthesis and stabilization of Phosphorene are still challenging tasks, its chemistry and functionalization represent frontier research topics. Within this perspective, pressure has been shown to be a very effective tool. Besides bP, another layered structure of P, rhombohedral A7, made by the stacking of blue Phosphorene layers, is indeed experimentally accessible by pressure. Remarkably, pressure has recently allowed to gain fundamental insight about the mechanism ruling the formation of chemical bonds between P layers, unveiling the existence of an intermediate p-sc structure between the layered rhombohedral A7 and the non-layered simple-cubic phases of P, significantly raising the pressure limit where the layers of P can be observed up to at least 30 GPa [4]. In this study we report the high pressure and high temperature chemistry of Phosphorus in the presence of N2 by means of state-of-the-art synchrotron X-ray diffraction (at ESRF-ID27), FTIR and Raman spectroscopy, using membrane Diamond Anvil Cell for the generation of static high pressure and laser heating for in situ generation of high temperature. Besides the N- functionalization of Phosphorene layers, this study is relevant to the substantially unexplored chemistry of the lowest Z pnictogens and to the synthesis of new PN compounds. Crystalline black Phosphorus was laser heated under different pressure and temperature conditions, corresponding to different crystalline structures of Phosphorus (A17, A7 and sc), above and below the melting line of N2. The results indicate a pressure dependent chemical reactivity, leading to different reaction products, according to the applied pressure. The low pressure data demonstrated the first direct synthesis of gamma-P3N5 from the elements, avoiding any precursor or byproduct [5]. Raman spectra and XRD patterns were acquired at high and ambient pressure, tracing the equation of state of the material, and providing new experimental input about the existence of the predicted delta- P3N5 phase [6-7]. References [1] Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility, ACS Nano 2014, 8, 4033-4041. [2] Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black phosphorus field-effect transistors, Nat. Nanotechnol. 2014, 9, 372-377. [3] Woo Hyun Han, W.H. Sunghyun Kim, S.; Lee, I-H.; Chang, L.J. Prediction of Green Phosphorus with Tunable Direct Band Gap and High Mobility, J. Phys. Chem. Lett. 2017, 8, 4627-4632. [4] Scelta, D.; Baldassarre, A.; Serrano-Ruiz, M.; Dziubek, K.; Cairns, A. B.; Peruzzini, M.; Bini, R.; Ceppatelli, M. Interlayer Bond Formation in Black Phosphorus at High Pres- sure, Angew. Chem. Int. Ed. 2017, 56, 14135-14140. [5] Landskron, K.; Huppertz, H.; Senker, J. and Schnick, W. High-Pressure Synthesis of gamma- P3N5 at 11 GPa and 1500 °C in a Multianvil Assembly: A Binary Phosphorus (V) Nitride with a Three-Dimensional Network Structure from PN4 Tetrahedra and Tetragonal Pyramids Angew. Chem. Int. Ed. 2001, 40, 2643-2645. [6] Dong, J.; Kinkhabwala, A. A.; McMillan, P. F. , High-pressure polymorphism in phosphorus nitrides Physica Status Solidi B, 2004, 210, 2319-2325. [7] Kroll, P.; Schnick, W. A Density Functional Study of Phosphorus Nitride P3N5 : Refined Geometries, Properties, and Relative Stability of alpha-P3N5 and gamma-P3N5 and a Further Possible High-Pressure Phase delta-P3N5 with Kyanite-Type Structure Chem. Eur. J. 2002, 8, 3530- 3537.