The structure determination and rietveld refinement of the aluminophosphate AIPO4-18

The framework structure of AlPO4-18 has been solved by combining X-ray powder diffraction data with direct methods, model building, distance least squares refinement, and powder pattern generation of hypothetical models. Rietveld refinement of the calcined material in the monoclinic space group C2/c (a = 13.711, b = 12.731, c = 18.570 A, and β = 90.01 °) converged with RF = 0.032 and Rwp = 0.108 (Rexp = 0.047). The framework structure consists of layers of tilted double 6-rings that give rise to a three-dimensional 8-ring channel system and a new, pear-shaped cage. The topological symmetry is Cmcm and the structure is related to the CHA structure type. The dehydrated material (C2/c; a = 13.512, b = 12.624, c = 18.439 A, and β = 95.55 °) was also refined using high-resolution synchrotron powder diffraction data. Both the TEA+ ion and a 5-coordinate Al were located, and refinement converged with rf = 0.070 and Rwp = 0.220 (Rexp = 0.204). The code AEI has been assigned to this new structure type.