Molecular dynamics in ammonium dihydrogen phosphate using incoherent neutron scattering

Molecular dynamics of the ammonium dihydrogen phosphate, ADP, was studied by neutron scattering spectroscopy using an energy window of 100 μeV. This compound exhibits an order-disorder phase transition at T_c = 148 K from antiferroelectic phase to paraelectric phase. The entire ADP lattice dynamics is dominated by a network of different N-H⋯O and O-H⋯O hydrogen bonds that are broken above critical temperature T_c. In the ordered phase the molecular electric dipoles appear as a result of hydrogen ordering lateral configuration around PO₄ groups. The dynamics of ammonium group NH₄ plays also an important role in the phase transition. The quasielastic lines give information about ammonium reorientational diffusion above T_c while the inelastic part of the spectrum provides useful information about ordered proton modes coupled with acoustic ADP lattice modes. At the same time the low energy transfer domain between 2 meV and 8 meV is very sensitive to phase transition. In the disordered phase the number of states increases compared with ordered phase, in a way similar with the solid-glass phase transition.