Rietveld refinement of the as synthesized and partially calcined forms of the molecular sieve GaPO4-ZON

The similarity between the lattice parameters of the gallophosphate molecular sieve DAB-2 (orthorhombic with a = 14.170 Angstrom, b = 15.959 Angstrom, c = 17.156 Angstrom) and those of several aluminophosphates with the ZON topology indicated that this material might be GaPO4-ZON, so a structural investigation was undertaken. Rietveld refinement of the structures of the as synthesized and partially calcined forms of DAB-2 assuming this topology converged with R-wp = 0.129 (R-F = 0.047) and R-wp = 0.097 (R-F = 0.036), respectively. In the as synthesized form, all cages are occupied by H(2)DABCO(2+) ions, whose charges are balanced by fluoride ions coordinated to framework Ga atoms. One of these F- ions bridges across a 4-ring between two Ga atoms and the other is coordinated to just one of these Ga atoms and hydrogen bonds to the H(2)DABCO(2+) ion in the cage. Thus, of the four Ga sites, two are 4-, one is 5- and one is 6-coordinate. Mild heat treatment at 573 K for six hours results in a partial decomposition of the H(2)DABCO(2+) ion and the loss of the fluoride ion hydrogen bonding to the organic cation. The changes in the unit cell parameters of different materials with the ZON topology can be traced to the influence of the fluoride ions of the framework geometry. The removal of the non-bridging F- ion causes a to increase and b to decrease, while the elimination of the second F- ion causes c to increase and b to decrease.