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High pressure reactivity of phosphorus and hydrogen is relevant to fundamental chemistry, energy conversion and storage, and materials science. Here we report the synthesis of (PH3)2H2, a crystalline van der Waals (vdW) compound (I4cm) made of PH3 and H2 molecules, in a Diamond Anvil Cell by direct catalyst-free high pressure (1.2 GPa) and high temperature (T ≲ 1000 K) chemical reaction of black phosphorus and liquid hydrogen, followed by room T compression above 3.5 GPa. Group 15 elements were previously not known to form H2-containing vdW compounds of their molecular hydrides. The observation of (PH3)2H2, identified by synchrotron X-ray diffraction and vibrational spectroscopy (FTIR, Raman), therefore represents the discovery of a previously missing tile, specifically corresponding to P for pnictogens, in the ability of non-metallic elements to form such compounds. Significant chemical implications encompass reactivity of the elements under extreme conditions, with the observation of the P analogue of the Haber-Bosch reaction for N, fundamental bond theory, and predicted high pressure superconductivity in P-H systems., Chemical elements at high pressure may behave more consistently with their periodic properties than they do at ambient conditions. The authors report the synthesis of PH3 from black phosphorous and hydrogen, and the crystallization of the van der Waals compound (PH3)2H2 which fills a gap in the chemistry of adjacent elements in the periodic table.

Black Phosphorus (bP), which is made by the periodic stacking of Phosphorene [1], has a layered orthorhombic A17 structure, stable at room conditions, that transforms into a layered rhombohedral A7 above ~5 GPa, which is reported to further convert into a non-layered simple cubic structure at ~11 GPa. The sequence of the high pressure (HP) structures of P at room T presents two striking anomalies if compared to the other group 15 elements with higher Z: first, the A17 structure is an isolated exception and second, whereas the HP limit for the A7 decreases in group 15 with increasing Z, according to current literature its pressure value in P (11 GPa) is located below that of As (25 GPa) [2]. A recent experiment [3], in agreement with theoretical predictions [4], has revealed a two-step mechanism for the A7 to sc transition and the existence of a previously unreported, intermediate pseudo simple-cubic (p-sc) structure from 10.5 up to at least 30 GPa, which significantly raised the HP limit for the layered structures of P and provided new experimental evidences to account for the long debated anomalous pressure behaviour of the superconducting critical temperature, T c. In this study we performed synchrotron X-ray diffraction (ID27, ESRF) during room T compression of bP up to 30 GPa in Diamond Anvil Cell (DAC) in the presence of He, H2, N2 and Daphne Oil 7474. Our data demonstrated that the p-sc structure is an intrinsic feature of P, independent from the pressure transmitting media. Furthermore, we derived the EOS's of A17, A7 and p-sc phases and demonstrated the first order mechanism of the A7 to p-sc transition. Highlighting the structural relations between A7 and p-sc, here we finally solved the apparent contradictions from previous literature bringing order to the sequence of HP A7 layered structures in group 15 elements [2,5]. Acknowledgements: 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. References: [1] M. Batmunkh et al., Adv. Mater. 28, 8586 (2016) [2] H. Katzke et al., Phys. Rev. B, 2008, 77, 024109 [3] D. Scelta et al., Angew. Chem. Int. Ed. 56, 14135 (2017) [4] K. T. Chan et al., Phys. Rev. B 88, 064517 (2013) [5] D. Scelta et al., submitted (2018)