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

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.Comment: 23 pages, 7 figure

Solid-state NMR in materials science : principles and applications

xv, 264 p. : ill. ; 25 cm

Do Cooperative Proton-Hydride Interactions Explain the Gas-Solid Structural Difference of BH3NH3?

The solid/gas structural differences in the weak donor-acceptor complex BH3NH3 is theoretically studied using density functional and the topological analysis of the electron density. The analysis shows that the cooperative dihydrogen interactions are not the main organizing factors in the molecular aggregations affecting donor-acceptor bond lengths. These aggregations are primarily controlled by electrostatic dipole-dipole interactions. This fact can be exploited in the development of simple electrostatic models that will allow the prediction of the basic structure of supramolecular architecture. I

Reversible Dehydration Behavior Reveals Coordinatively Unsaturated Metal Sites in Microporous Aluminum Phosphonates

Incorporation of the same ligand into three different aluminum phenylenediphosphonates (Al­(H₂O)­(O₃PC₆H₄PO₃H) (<b>1</b>), Al₄(H₂O)₂­(O₃PC₆H₄PO₃)₃ (<b>2</b>), and Al₄(H₂O)₄­(O₃PC₆H₄PO₃)_2.84­(OH)_0.64 (<b>3</b>)) was accomplished by varying the synthetic conditions. The compounds have different sorption properties; however, all exhibit reversible dehydration behavior. The structures of the hydrated and dehydrated phases were determined from powder X-ray diffraction data. Compounds <b>2</b> and <b>3</b> were found to be microporous, while compound <b>1</b> was found to be nonporous. The stability of the dehydrated phase and the resulting porosity was found to be influenced by the change in the structure upon loss of water