Structural characterization of the high-temperature phase transitions in Ca{sub 8}[Al{sub 12}O{sub 24}] (MoO{sub 4}){sub 2} aluminate sodalite using X-ray powder diffraction
- Univ. of Bayreuth (Germany)
- Univ. of Kiel (Germany)
The structures are reported of the room-temperature and the three high-temperature phases of Ca{sub 8}[Al{sub 12}O{sub 24}] (MoO{sub 4}){sub 2} aluminate sodalite CAM. Structure refinements have been performed with the Rietveld method using synchrotron radiation X-ray powder diffraction data. The cubic phase has symmetry I{bar 4}3m with a = 9.29377 (4) {Angstrom}. The tetragonal phases have symmetry P{bar 4}c2, and their unit cells corresponds to a {radical}2a x {radical}2a x c supercell of the cubic phase. The second tetragonal phase (T2) exists for 614<624K and has lattice parameters a = 13.14536 (5) {Angstrom} and c = 9.29224 (5) {Angstrom}. The first 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), b = 13.07061 {Angstrom}, and c = 9.31413 (2) {Angstrom}. The transition (on decreasing temperatures) at T{sub c} {approx} 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 MoO{sub 4}{sup 2-}. The T2 to T1 transition at T{sub c} {approx} 614 K is of first order, and it corresponds to a displacement of the cage anions to an off-center position in the cages, while keeping orientational disorder over at least two orientations. The transition at T{sub c} {approx} 590 K, from the T1 phase to the orthorhombic phase, is again a first-order transition, and it is related to a complete ordering of the cage anions. A detailed description is given of the structural distortions of the frame-work, accompanying the phase transitions.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 518428
- Journal Information:
- Journal of Solid State Chemistry, Vol. 129, Issue 1; Other Information: PBD: 15 Feb 1997
- Country of Publication:
- United States
- Language:
- English
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