Phase transitions and superconductivity of black phosphorus and phosphorus-arsenic alloys at low temperatures and high pressures

X-ray diffraction of black phosphorus and phosphorus-arsenic alloys with composition ${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{As}}_{\mathit{x}}$ (x=0.05,0.1) has been carried out with synchrotron radiation at low temperatures and high pressures. The first transition from the orthorhombic to the rhombohedral state started at around 7.5 GPa at 21 K. The transition pressure at 21 K is about 3 GPa higher than that at room temperature. The orthorhombic and rhombohedral phases coexisted over the pressure range of 7.5--10.5 GPa at 21 K. The second transition to a simple-cubic phase appeared at around 12 GPa. The rhombohedral and the simple-cubic phases coexisted over the pressure range of 12--15.5 GPa at 21 K. This region was much wider than that observed at room temperature. Similar behavior was found for ${\mathrm{P}}_{0.95}$${\mathrm{As}}_{0.05}$ and ${\mathrm{P}}_{0.9}$${\mathrm{As}}_{0.1}$ at low temperatures and high pressures. Superconductivity of black phosphorus and the alloys has been investigated at low temperatures and high pressures. Interestingly, superconductivity is observed when pressure is increased at liquid-helium temperature. The superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) of these materials rapidly increased to about 10 K at around 25 GPa. The single phase of the simple cubic form was observed at around 16 GPa for black P and 22 GPa for ${\mathrm{P}}_{0.9}$${\mathrm{As}}_{0.1}$ at 21 K. A bulk thermal expansion coefficient of the simple-cubic phase was approximately 13\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ (deg${)}^{\mathrm{\ensuremath{-}}1}$. This value is considerably smaller than that of ordinary metals. The small thermal expansion is due to the instability of the simple-cubic structure at low temperatures. The high ${\mathit{T}}_{\mathit{c}}$ of the simple-cubic structure may arise from the lattice instability at very low temperatures.