THE STRUCTURES OF ANHYDROUS SILVER SODALITE AG-3[AL3SI3O12] AT 298, 623, AND 723 K FROM RIETVELD REFINEMENTS OF X-RAY-POWDER DIFFRACTION DATA - MECHANISM OF THERMAL-EXPANSION AND OF THE PHASE-TRANSITION AT 678 K

The phase transition behavior of anhydrous silver sodalite (ASS) Ag-3[Al3Si3O12] differs from that of other compounds with a sodalite structure in that the transition detected T-c = 678 K by differential scanning calorimetry does not involve the occurrence of peak splittings and/or superstructure reflections in the powder X-ray diffraction pattern of the law-temperature phase. Variable-temperature powder X-ray diffraction experiments show that the transition is from cubic to cubic and that there is a discontinuity in the thermal expansion of ASS at T-c. In order to investigate the mechanisms of thermal expansion and of the phase transition, Rietveld refinements of powder X-ray diffraction data collected at temperatures of 298, 623, and 723 K were carried out. These structure refinements show that the thermal expansion behavior between 298 K and T-c, which can be described by a quadratic function of the temperature, is determined mainly by the untilting of the sodalite framework, an experimental confirmation that a tilting mechanism is operative in the thermal expansion of sodalite frameworks. In the structures determined at 298 and 623 K, Ag+ ions occupy positions in the center of the large windows of the sodalite cage, which are lined by six [(Al,Si)O-4] tetrahedra (six-ring windows). As a consequence of the untilting, the coordination of the Ag+ ions by framework oxygen atoms changes from a (favorable) threefold planar arrangement with Ag-O bond lengths d(Ag-O) Of 2.347(5) Angstrom at 298 K to an (unfavorable) environment with six O neighbors arranged in a plane at longer distances (d(Ag-O) = 2.50(1) Angstrom (3x) and 2.79(1) Angstrom (3x)) at 623 K. At 723 K, above T-c, the Ag+ ions have been shifted away from the center of the six-ring window, allowing the framework to collapse. Then, Ag+ is again in a threefold oxygen coordination (d(Ag-O) = 2.375(6) Angstrom) with silver at the apex of a hat trigonal [AgO3] pyramid. The occurrence of the phase transition can be rationalized by the demand of the Ag+ ion for small coordination numbers and short, covalent bonds and thus probably is a consequence of the specific bonding characteristics of the Ag+ ion. (C) 1995 Academic Press, Inc.