Dense superconducting phases of copper-bismuth at high pressure

Although copper and bismuth do not form any compounds at ambient conditions, two intermetallics, CuBi and Cu$_{11}$Bi$_7$, were recently synthesized at high pressures. Here we report on the discovery of additional copper-bismuth phases at elevated pressures with high-densities from ab initio calculations. In particular, a Cu$_2$Bi compound is found to be thermodynamically stable at pressures above 59 GPa, crystallizing in the cubic Laves structure. In strong contrast to Cu$_{11}$Bi$_7$ and CuBi, cubic Cu$_2$Bi does not exhibit any voids or channels. Since the bismuth lone pairs in cubic Cu$_2$Bi are stereochemically inactive, the constituent elements can be closely packed and a high density of 10.52 g/cm$^{3}$ at 0 GPa is achieved. The moderate electron-phonon coupling of $\lambda=0.68$ leads to a superconducting temperature of 2 K, which exceeds the values observed both in Cu$_{11}$Bi$_7$and CuBi, as well as in elemental Cu and Bi .