Phase Diagram Analysis of High-Pressure/High-Temperature Polymorphs of Ammonia Borane

Abstract

Ammonia borane (NH₃BH₃) is a promising hydrogen-storage material because of its high hydrogen density. It is employed as a hydrogen source when synthesizing superconducting polyhydrides under high pressure. Additionally, NH₃BH₃ is a crystallographically interesting compound that features protonic hydrogen (H^δ+) and hydridic hydrogen (H^δ-), and it forms a dihydrogen bond, which explains its stable existence as a solid. Herein, X-ray diffraction experiments were performed at high pressures (HPs) and high temperatures (HTs) of up to 30 GPa and 300 °C, respectively, to investigate the HP/HT phase diagram of NH₃BH₃. A new HP/HT phase (HPHT2) was identified above 9 GPa and 150 °C. Crystal-structure analysis using the Rietveld method and stability verification using density functional theory calculations revealed that HPHT2 has a P2₁/n (Z = 4) structure, similar to that of a previously reported HP/HT phase (HPHT) that appears at a lower pressure. HPHT2 is denser than the HP phases that appear at room temperature (HP1 and HP2) at the same pressure (up to ∼17 GPa). In the phase diagram, the phase-boundary line between HPHT and HP1 is a downward convex curve. These unconventional phenomena in the density and phase boundary can be attributed to the influence of dihydrogen bonding on the crystal structure and phase diagram.