Vibrational dynamics and quantum tunneling of water molecules in bassanite*

Using inelastic neutron scattering (INS) we studied dynamics of bassanite (CaSO₄*0.5H₂O), a structure of which has channels formed by CaO₈ and CaO₉ polyhedra with water molecules residing in the channels and occupying two different positions. The INS spectra showed that at low temperature (T=5 K) intramolecular O-H stretching modes of water are at high energy, around 445 meV (compared to 410 meV in ice-Ih), and the intermolecular librational band is at low energies, 35-90 meV (65-125 meV in ice-Ih), indicating weak hydrogen bonds acting on water molecules. At lower energies we observed a peak at about 1 meV, which shows the behaviour of tunneling mode: its intensity decreases with temperature increase and shows nonmagnetic momentum transfer dependence. In the talk we will discuss the observed INS results and attempt to explain the tunneling of water molecule, which was not observed before in other INS experiments involved confined water molecules in the presence of hydrogen bonds.