From orientation disordered to ordered--an ab initio simulation on ammonia borane phase transition within van der Waals corrections.

In this work, we report a detailed theoretical investigation of the phase transition of ammonia borane (NH(3) BH(3); AB), from a tetragonal I4mm (C(4v)(9)) phase with disordered orientation of hydrogen to an orthorhombic phase with Pmn2(1) (C(2v)(7)) symmetry, as a function of temperature based on Density Functional Theory calculations with semiempirical dispersion potential correction. We define a series of substructures with the NH(3) BH(3) moiety always in C(3v) symmetry and the partially occupied high temperature state can be described as a continuous transformation between these substructures. To understand the role of the van der Waals corrections to the physical properties, we use the empirical Grimme's dispersion potential correction (PBE-D2). Both Perdew-Burke-Emzerhof (PBE) and PBE-D2 functional yield almost the same energy sequence along the transition path. However, PBE-D2 functional shows obvious advantage in describing the lattice parameters of AB. The rigid rotor harmonic oscillator approximation is used to compute the free energy and the entropies contribution along the transition pathway. With knowledge of free energy surfaces along rotations of the --[NH(3)] and --[BH(3)] groups, complete transformation paths are mapped out. The phase transition is found to follow the sequence of partially occupied tetragonal system (I4mm) of a mixture of states with monoclinic (Cc), (CM) and orthorhombic (Pmn2(1)) symmetries to fully occupied quasitetragonal system (the intermediate phase, Pmn2(1)) to fully occupied orthorhombic system (Pmn2(1)).