Structural and dynamical properties of solid ammonia borane under high pressure.

The structural and dynamical properties of solid ammonia borane were investigated by means of extensive density functional theory calculation up to 60 GPa. Molecular dynamics simulations suggest that the Cmc2(1) phase found by recent room-temperature x-ray diffraction experiments can be obtained from the Pmn2(1) structure at high pressure and low temperature. Two new high-pressure phases were found on further compression at room temperature. We also found that all three high-pressure phases have proton-ordered structures, and the separation of the NH(3) and BH(3) rotation observed in the simulations can be explained by their distinct rotational energy barriers. The role of dihydrogen bonds in the high-pressure phases is discussed.