Metallization and superconductivity of NBH12 compounds stabilized by dihydrogen bonds

Abstract

As the most promising hydrogen storage material, ammonia borane (NH₃BH₃, AB) has been the subject of study in a variety of scientific fields. Here, we predict for the first time that the hydrogen storage capacity of AB can be increased from 19.6 wt% up to 32.8 wt% via insertion of H₂ molecules into its crystal structure. We present particle swarm optimization algorithms combined with density functional theory calculations which predicted a new hydrogen-rich phase of AB in which AB and H₂ have a ratio of 1 : 3. The new phase, AB–(H₂)₃, is stable over a wide pressure range from 20–240 GPa. Interestingly, the AB–(H₂)₃ crystal structure maintains the same P2₁ space-group symmetry as pure AB and the inserted hydrogen molecules are located between the layers of ammonia borane molecules, thereby introducing a network of dihydrogen bonds between ammonia borane and hydrogen molecules which greatly stabilizes the structure and reduces the metallization pressure of AB from 600 GPa to 240 GPa. The calculated electronic properties show that the electronic band gap of P2₁-AB(H₂)₃ gradually closes with increasing pressure, finally reaching a semi-metallic state at about 240 GPa. Surprisingly, the electron–phonon coupling calculations show that P2₁-AB(H₂)₃ exhibits superconductivity after metallization (at 240 GPa and 12 K) which is primarily attributed to acoustic phonon mode softening. This study provides insights into hydrogen storage materials with increased storage capacity under high pressure conditions, and it also provides new directions for the synthesis of covalent hydrogen-rich compounds.