Rietveld refinement of the pure aluminophosphate AlPO4-40

A structure analysis of a pure aluminophosphate with the AFR topology has b een performed using the Rietveld method in combination with very-high-resol ution synchrotron powder diffraction data. The results are consistent with the somewhat unexpected Al-27, P-31 and C-13 MAS NMR results reported in th e literature. Although the framework topology is the same as that of SAPO-4 0, the symmetry is lower (Pc2(1)n, a = 21.751, b = 13.730. c = 14.218 Angst rom), so there are eight P, eight Al, 33 O, one N and 12 C atoms in the asy mmetric unit. The reason for the lower symmetry is the presence of an order ed arrangement of hydroxyl groups bridging between framework Al atoms acros s a 4-ring. These hydroxyl groups are located in alternating 'open cube' fr amework building units. Analogous fluoride bridges have been reported for m aterials with a closely related ZON topology. As a result of the hydroxyl b ridge, a quarter of the framework Al atoms are 5- rather than 4-coordinate. As in the case of SAPO-40, the lattice parameters changed during the cours e of the data collection. With 185 non-hydrogen positional parameters, this represents one of the most complex structures yet refined using the Rietve ld (whole profile) refinement technique. (C) 1999 Elsevier Science B.V. All rights reserved.