Neutron diffraction structure refinement of the zeolite gismondine at 15 K

Abstract

A single crystal neutron diffraction study was carried out at 15 K on gismondine (Ca 3.91Al 7.77Si 8.22O 32 17.6H 2O; a=10.011(3), b=10.614(6), c=9.853(3) A ̊ , β = 93.11(2)°; P2 1/c) . The final agreement factor for the anisotropic refinement is R( F 2)=0.081 ( R w( F 2)=0.099) using 1941 diffraction intensities collected at the Brookhaven National Laboratory High Flux Beam Reactor (HFBR). Refinement of the neutron scattering lengths for tetrahedral atoms and mean tetrahedral TO distances are consistent with an essentially ordered distribution of Si and Al and the possibility of a minor substitution of excess Si in the Al(1) site. The extraframework cation site has full Ca occupancy and a coordination involving two framework oxygens and 4.3 water oxygen atoms in two geometrical configurations. One configuration involves three fully occupied water molecules (OW1, OW2, OW3) and one (OW4), with a statistical occurrence of 70%; the second configuration shows two sites (OW5, OW6) replacing OW4 with an occurrence of 30%. Proton positions were located for all the water molecules; disordered hydrogen sites were found for OW3 and OW4. The hydrogen bonding configuration is related to the hydrogen—hydrogen and hydrogen—calcium electrostatic repulsion.