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(PH3)2H2, a crystalline van der Waals (vdW) compound (I4cm) made of PH3and H2molecules, in a Diamond Anvil Cell by direct catalyst-free high pressure (1.2 GPa) and hightemperature (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 notknown to form H2-containing vdW compounds of their molecular hydrides. The observationof (PH3)2H2, identified by synchrotron X-ray diffraction and vibrational spectroscopy (FTIR,Raman), therefore represents the discovery of a previously missing tile, specifically corre-sponding 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 con-ditions, with the observation of the P analogue of the Haber-Bosch reaction for N, funda-mental bond theory, and predicted high pressure superconductivity in P-H systems.