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

TL;DR

This study uses ab initio methods to explore the phase diagram of phosphorus-hydride compounds under high pressure, questioning the stability of these phases and their role in observed superconductivity.

Contribution

It provides the first comprehensive computational analysis of PHn compounds, showing they are not thermodynamically stable and challenging their direct link to experimental superconductivity.

Findings

PHn compounds are not thermodynamically stable at high pressure.

The lowest energy phases of PH1, PH2, and PH3 have T_c's similar to experiments.

Uncertainty remains whether experimental T_c values are due to pure PHn phases.

Abstract

Hydrogen-rich compounds have been extensively studied both theoretically and experimentally in the quest for novel high-temperature superconductors. Reports on sulfur-hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for room-temperature superconductors in hydride materials. Recently, compressed 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. In this work the phase diagram of PH$_n$ ($n=1,2,3,4,5,6$) was extensively explored by means of {\it ab initio} crystal structure predictions using the Minima Hopping Method (MHM). 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$_n$.