Neutron diffraction structure refinement of the zeolite gismondine at 15 K

A sjngle crvstal neutron diffraction study was carried out at 15 K on g'ismondine (Ca3.9.,AI7.77Sia.22032. 17.6H20; a= 10.011(3),. b=l 0.614(6).; c=9.853(3) À,13 = 93.11 (2)°; P2,/c). The final agreement factor for the anlsotropic refinement is R(P)=0.081 (Rw(P)=0.099) using 1941 diffraction intensities collected at the Brookhaven National Laboratory High Flux Beam Reactor (HFBR). Refinement of the, neutron .scatterinq le)l.gths far tetrahedral atoms and mean tetrahedral T-O distances are consistent with an essentially ordered distribution of Si and AI and the. possibilitv of a minor substitution of excess Si in thé AI(1) site. The extraframework catioh site has full Ca occupancy and a coordination involving two framework oxygens· and 4.3:water oxygen atoms in two geometrical configurati.ons. One configuration involves thres fùlly occupied water rnolecules (OW1, OW2., OW3) and one (OW4), with a statistical occurrence of 70%; the second confiquration shows two sites (OW5, OW6) replacing OW4 with an occurrence of 30%. Proton positions were located for ali the water molecules; disordered hydrogen sites were found for OW3 and OW4. The hydrogen bonding configuration is related to the hvdroqen-hvdroqen and hydrogen-caldum electrostatic repulsion.