Pressure-Induced Evolution of Crystal and Electronic Structure of Ammonia Borane.

Ammonia borane (NH3BH3) has long attracted considerable interest for its high hydrogen content and easy dehydrogenation conditions which make it a promising hydrogen storage material. Here, we report on a computational study of the structural stability and phase transition sequence of NH3BH3 and associated lattice dynamics and electronic properties in a wide pressure range up to 300 GPa. The results confirm previously reported structures, including the experimentally observed orthorhombic Pmn21 structure at low temperature and ambient pressure, and predict the phase transition sequence Pmn21 → Pc → P21 → P1̅ for NH3BH3. Our calculations also reveal systematic trends of monotonically decreasing band gap with rising pressure in the three high-pressure NH3BH3 phases, which nevertheless all remain nonconducting up to the highest pressure of 300 GPa examined in this work. The present findings elucidate structural and electronic properties of NH3BH3 over an extensive pressure range, providing knowledge essential to further study of NH3BH3 in an expanded pressure-temperature phase space.