Diborane (${\mathrm{B}}_2{\mathrm{H}}_6$), a high energy density material, was believed to be stable in a wide $P$, $T$ interval. A systematic investigation of the B-H system using the ab initio variable-composition evolutionary simulations shows that boron monohydride (BH) is thermodynamically stable and can coexist with solid B, ${\mathrm{H}}_2$, and ${\mathrm{B}}_2{\mathrm{H}}_6$ in a wide pressure range above 50 GPa. ${\mathrm{B}}_2{\mathrm{H}}_6$ becomes unstable and decomposes into the Ibam phase of BH and ${\mathrm{H}}_2$ ($C2/c$) at 153 GPa. The semiconducting layered Ibam structure of BH at 168 GPa transforms into a metallic phase with space group $P6/mmm$ and a 3D topology with strong B-B and B-H covalent bonds. The Ibam-$P6/mmm$ transformation pathway suggests the possibility of obtaining the metastable Pbcm phase on cold decompression of the $P6/mmm$ phase. The electron-phonon coupling calculations indicate that $P6/mmm$-BH is a phonon-mediated superconductor with a critical temperature of superconductivity ($T_c$) of 14.1–21.4 K at 175 GPa.

• Received 12 January 2013

DOI:https://doi.org/10.1103/PhysRevLett.110.165504