THE STRUCTURE OF CALCINED ALPO(4)-41 - A NEW FRAMEWORK TOPOLOGY CONTAINING ONE-DIMENSIONAL 10-RING PORES

Synchrotron powder data for calcined never-rehydrated AlPO4-41 were initially indexed on a C-centered orthorhombic cell. Possible systematic absences suggested a c-glide plane perpendicular to a. The structure was determined by model building in the idealized space group Cmcm (ignoring Al,P alternation). The new framework topology (designated ''AFO'') consists of layers of 4.6(2). 4.10-rings that are connected by UDUD chains, similar to the UDUD chains in AlPO4-5 (AFI), -8 (AET), -11 (AEL), -25 (ATV), -D (APD), and -54/VPI-5/H1 (VFI) and -H2. The widest pore in AlPO4-41 is a distorted elliptical one-dimensional 10-ring channel that is slightly larger than the 10-ring channel in AlPO4-11, consistent with observed sorption properties. A full-pattern Rietveld refinement assuming alternation of Al and P in space group Cmc2(1) gave R(wt. profile) = 0.273 and R(F) = 0.087. However, the fit between observed and calculated profiles was not satisfactory, nor were distances and angles involving atoms on the mirror plane. Removal of the mirror planes suggested various monoclinic space groups, which were evaluated by a DLS refinement of the alternating Al,P trial model in a properly transformed cell. The lowest DLS R factor (= 0.00045) was obtained in P2(1). A Rietveld refinement in space group P112(1) gave R(wt. profile) = 0.164 and R(F) = 0.074 with refined cell parameters a = 9.7180(1), b = 13.7915(1), c = 8.3591(1) Angstrom, and gamma = 110.59(1)degrees. The agreement between experimental and calculated profiles and the mean values for Al-O and P-O distances and Al-O-P angles are acceptable. The differences between the idealized orthorhombic structure and the monoclinic structure are described. Solid-state Al-27 and P-31 MAS n.m.r. spectra show a single tetrahedral site for Al and for P and do not distinguish between monoclinic or orthorhombic symmetry.