LOW-TEMPERATURE DYNAMICS OF MOLECULAR H2O IN BASSANITE, GYPSUM AND CORDIERITE INVESTIGATED BY HIGH-RESOLUTION INCOHERENT INELASTIC NEUTRON-SCATTERING

The low temperature dynamics of molecular H2O has been studied in gypsum (CaSO4.2H2O), bassanite (CaSO4.0.5 H2O) and cordierite (Mg2Al4Si5O18.H2O) using neutron spectroscopy. In gypsum, where the H2O is structurally bound and the protons can be located in diffraction studies, inter- and intramolecular motions of the H2O and SO4 groups have been observed. In bassanite, two broad bands in a spectrum recorded at 150 K imply that at this temperature the H2O is dynamically disordered and a static description of the room temperature structure is inappropriate. At low temperatures the H2O molecules order, and at 3 K the spectrum consists of a number of sharp bands. In synthetic, alkali-free hydrous cordierite the H2O molecules are dynamically disordered. This has been shown by earlier NMR and room temperature quasielastic neutron scattering studies, and has been confirmed here by low temperature (50 K) quasielastic neutron scattering. A temperature decrease down to 3 K induces only gradual changes in the inelastic spectrum, and the three bands which we have observed are significantly broadened. It is therefore concluded, that the dynamic disorder persists even down to 3 K. This is in agreement with results from NMR spectroscopic experiments for beryl. High resolution time-of-flight spectra obtained at 3 K exclude tunneling.