  PH3 is studied to understand the superconducting transition and responsible
stoichiometry under high pressure by means of Raman, IR, and x-ray diffraction
(XRD) measurements, and theoretical calculations. It is found PH3 is stable up
to about 8 GPa and then starts to dehydrogenate through two dimerization
processes at room temperature as pressure up to 25 GPa. Two resulting
phosphorus hydrides, P2H4 and P4H6, are verified experimentally and can be
recovered to ambient pressure. On further compression above 35 GPa, P4H6
directly decomposes into elemental phosphorus. The superconductivity transition
temperatures of P4H6 at 100 and 200 GPa have been predicted to be 13 and 67 K
in agreement with reported results, suggesting it might responsible for the
superconductivity at higher pressures. Our results clearly show that P2H4 and
P4H6 are only stable P-H compounds between PH3 and elemental phosphorus,
shedding light on the superconducting mechanism.
