The Structures of Anhydrous Silver Sodalite Ag3[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
| Autor: | Michael Wiebcke, Jürgen Felsche, P. B. Kempa, Stefanie Assmann, Peter Behrens |
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| Rok vydání: | 1995 |
| Předmět: |
Phase transition
Chemistry Coordination number Crystal structure Condensed Matter Physics Thermal expansion Electronic Optical and Magnetic Materials Inorganic Chemistry Bond length Crystallography chemistry.chemical_compound Differential scanning calorimetry Materials Chemistry Ceramics and Composites Sodalite Physical and Theoretical Chemistry Powder diffraction |
| Zdroj: | Journal of Solid State Chemistry. 115:55-65 |
| ISSN: | 0022-4596 |
| Popis: | The phase transition behavior of anhydrous silver sodalite (ASS) Ag3[Al 3 Si 3 O 12 ] 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 low-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) A at 298 K to an (unfavorable) environment with six O neighbors arranged in a plane at longer distances ( d Ag-O = 2.50(1) A (3×) and 2.79(1) A (3×)) 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) A) with silver at the apex of a flat trigonal [AgO 3 ] 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. |
| Databáze: | OpenAIRE |
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