High pressure synthesis of phosphine from the elements and the discovery of the missing (PH3)2H2 tile

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 (PH 3 ) 2 H 2 , a crystalline van der Waals (vdW) compound ( I 4 cm ) made of PH 3 and H 2 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 H 2 -containing vdW compounds of their molecular hydrides. The observation of (PH 3 ) 2 H 2 , 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 PH 3 from black phosphorous and hydrogen, and the crystallization of the van der Waals compound (PH 3 ) 2 H 2 which fills a gap in the chemistry of adjacent elements in the periodic table.