Superconductivity in metastable phases of phosphorus-hydride compounds under high pressure

Abstract

Reports on sulfur hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for another room-temperature superconductor among hydrogen-rich compounds. Recently, compressed phosphorus hydride (phosphine) was reported to metallize at pressures above 45 GPa, reaching a superconducting transition temperature (${T}_{c}$) of 100 K at 200 GPa. However, neither the exact composition nor the crystal structure of the superconducting phase have been conclusively determined. This work reports an extensive study of the phase diagram of PH${}_{n}$ ($n$=1--6) by means of \textit{ab initio} crystal structure predictions using the minima hopping method. The results do not support the existence of thermodynamically stable PH${}_{n}$ compounds, which exhibit a tendency for elemental decomposition at high pressure even when vibrational contributions to the free energies are taken into account. Although the lowest energy phases of PH${}_{1,2,3}$ display ${T}_{c}$'s comparable to experiments, it remains uncertain if the measured values of ${T}_{c}$ can be fully attributed to a phase-pure compound of PH${}_{n}$.