STRUCTURAL CHARACTERIZATION OF THE HIGH-TEMPERATURE PHASE-TRANSITIONSIN CA-8[AL12O24](MOO4)(2) ALUMINATE SODALITE USING X-RAY-POWDER DIFFRACTION

The structures are reported of the room-temperature and the three high -temperature phases of Ca-8[Al12O24](MoO4)(2) aluminate sodalite CAM. Structure refinements have been performed with the Rietveld method usi ng synchrotron radiation X-ray powder diffraction data. The cubic phas e has symmetry I (4) over bar 3m with a 9.29377(4) Angstrom. The tetra gonal phases have symmetry P (4) over bar c2, and their unit cells cor responds to a root 2a X root 2a X c supercell of the cubic phase. The second tetragonal phase (T2) exists for 614<T<624 K and has lattice pa rameters a = 13.14536(6) Angstrom and c = 9.29224(8) Angstrom. The fir st tetragonal (T1) phase is stable for 590 < T < 614 K and has lattice parameters a = 13.12263(5) Angstrom and c = 9.32081(5) Angstrom, The orthorhombic phase has symmetry Aba2 with a 26.14683(8) Angstrom, b = 13.07061 Angstrom, and c = 9.31413 (2) Angstrom. The transition (on de creasing temperatures) at T-c approximate to 624 K, from the cubic to the T2 phase, is of second order and is found to be related to change in the orientational order of the cage anions MoO42-. The T2 to T1 tra nsition at T-c approximate to 614 K is of first order, and it correspo nds to a displacement of the cage anions to an off-center position in the cages, while keeping orientational disorder over at least two orie ntations. The transition at T-c approximate to 590 K, from the T1 phas e to the orthorhombic phase, is again a first-order transition, and it is related to a complete ordering of the cage anions. A detailed desc ription is given of the structural distortions of the framework, accom panying the phase transitions. (C) 1997 Academic Press.