MOVEMENT OF CHANNEL H2O IN CORDIERITE OBSERVED WITH QUASI-ELASTIC NEUTRON-SCATTERING

Quasi-elastic neutron scattering experiments have been performed on natural and synthetic, hydrous and anhydrous, powder samples of cordierite to elucidate the dynamics of the H2O molecules incorporated in the cavities of the cordierite structure. At room temperature, all protons are dynamically disordered. A simple jump model yields residence times at room temperature of about 6 ps. These measurements show that a static description of the orientation of the H2O molecules is inappropriate, which is in agreement with earlier NMR measurements. A model is proposed in which the H2O molecules in alkali-free cordierite, where ideally all the proton-proton vectors are aligned parallel-to [001], revolve around their center of gravity while maintaining their orientation. This model is based on the wave-vector dependence of the elastic incoherent structure factor, which leads to a very small radius of gyration (almost-equal-to 0.4 angstrom). The proposed model is compatible with earlier infrared experiments, in which no disorder of the orientation of the proton-proton vector was observed at room temperature. It is also consonant with the large anisotropic atomic displacement parameters observed for the O in H2O in earlier structural studies. In agreement with a recent theoretical study, the model proposed here implies that it is not the O in H2O that is located at the center of the cavity but rather the center of mass of the molecule. Within the resolution of the current experiments, the time scale of the dynamic behavior of the protons in natural or synthetic samples is similar.