High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations

TL;DR

This study investigates the high-pressure structural phases and transition mechanisms of ammonia borane using X-ray diffraction, theoretical calculations, and Landau theory, revealing new ordered phases and transition pathways relevant for hydrogen storage.

Contribution

It provides the first detailed structural analysis of ammonia borane under high pressure, identifying new phases and elucidating transition mechanisms with theoretical support.

Findings

Disordered I4mm transforms into ordered Cmc21 phase above 1.1 GPa.

A generic phase diagram for NH3BH3 is proposed, mapping multiple phases.

Both temperature and pressure induce second-order phase transitions.

Abstract

Structural evolution of a prospective hydrogen storage material, ammonia borane NH3BH3, has been studied at high pressures up to 12 GPa and at low temperatures by synchrotron powder diffraction. At 293 K and above 1.1 GPa a disordered I4mm structure reversibly transforms into a new ordered phase. Its Cmc21 structure was solved from the diffraction data, the positions of N and B atoms and the orientation of NH3 and BH3 groups were finally assigned with the help of density functional theory calculations. Group-theoretical analysis identifies a single two-component order parameter, combining ordering and atomic displacement mechanisms, which link an orientationally disordered parent phase I4mm with ordered distorted Cmc21, Pmn21 and P21 structures. We propose a generic phase diagram for NH3BH3, mapping three experimentally found and one predicted (P21) phases as a function of temperature and pressure, along with the evolution of the corresponding structural distortions. Ammonia borane belongs to the class of improper ferroelastics and we show that both temperature- and pressure-induced phase transitions can be driven to be of the second order. The role of N-H...H-B dihydrogen bonds and other intermolecular interactions in the stability of NH3BH3 polymorphs is examined.