Crystal structure of MAPO-20 sodalite: theoretical analysis of three-color ordering of Mg, Al and P in a sodalite unit
Han, Shaoxu; Smith, Joseph V.; Pluth, Joseph J.; Richardson, James W.
European Journal of Mineralogy Volume 2 Number 6 (1990), p. 787 - 798
published: Dec 19, 1990
manuscript accepted: Jul 12, 1990
manuscript received: May 27, 1990
ArtNo. ESP147050206006, Price: 29.00 €
Abstract The crystal structure of synthetic MAPO-20 with the sodalite topology, including a possible position for encapsulated tetramethylammonium species (TMA), was determined at room temperature by X-ray single-crystal diffraction, and at room temperature, 200 K and 20 K by neutron powder diffraction. Unit cell composition: Mg2Al4P6O24 • 2(CH3)4N, Mr = 682.7, cubic, Pm3̅n (223), a = 9.0437/8) Å (X-ray). Because refinement in P4̅3n of the room-temperature data gave no improvement over that in Pm3̅n, the framework must be fully expanded at room temperature. The short C-O distances (3.05-3.20 Å) indicate weak hydrogen bonding between framework oxygen and the hydrogens in the encapsulated molecule. Broadening of peaks in the neutron data at 20 K suggests a transition to non-cubic symmetry. MAPO-20 is a derivative of ALPO-20, which has strict alternation of Al and P over the tetrahedral vertices. One third of the Al atoms are substituted by Mg. Theoretically, there are many ways to accomplish this substitution. A three-color mathematical enumeration of the 4 Mg, 8 Al and 12 P atoms over the 24 vertices of the truncated octahedron yielded 27 distinct models and 18 mirror images. No diffraction evidence for long-range order was obtained from the present study. The nuclear magnetic resonance data of Barrie & Klinowski (1989) can be explained by local order of several types in which no 4-ring contains 2 Mg.