Crystal structure of MAPO-20 sodalite; theoretical analysis of three-color ordering of Mg, Al and P in a sodalite unit

T he crystal structure of synthetic MAPO-20 with the sodalite topology, inc1uding a possible position for eneapsulated tetramethylammonium species (TMA), was determined at room temperature by X-ray single-crystal diffraetion, and at room temperature, 200 K and 20 K by neutron powder diffraction. Unit cell composition: Mg2A14P6024· 2(CHJ)4N, Mr = 682.7, cubic, Pm3n (223), a = 9.0437 (8) A (X-ray). Because refinement in P43n of the room-temperature data gave no improvement over that in Pm3n, the framework must be fully expanded at room temperature. The short C-O distances (3.05-3.20 A) indicate weak hydrogen bonding between framework oxygen and the hydro gens in the encapsulated moleeule. 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 striet alternation of AI and P over the tetrahedral vertices. One third of the AI atoms are substituted by Mg. Theoretically, there are many ways to accomplish this substitution. A three-color mathematical enumeration of the 4 Mg, 8 AI and 12 P atoms over the 24 vertices of the truneated octahedron yielded 27 distinet models and 18 mirror images. No diffraetion evidence for long-range order was obtained from the present study. The nuc1ear magnetic resonanee data of Barrie & Klinowski (1989) ean be explained by loeal order of several types in which no 4-ring eontains 2 Mg. Key-words: sodalite, MAPO-20, crystal strueture, framework ordering.