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

authored by

Peter Behrens, Paul B. Kempa, Stefanie Assmann, Michael Wiebcke, Jürgen Felsche

Abstract

The phase transition behavior of anhydrous silver sodalite (ASS) Ag₃[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 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)O₄] 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) Å at 298 K to an (unfavorable) environment with six O neighbors arranged in a plane at longer distances (d_Ag-O = 2.50(1) Å (3×) and 2.79(1) Å (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) Å) with silver at the apex of a flat 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.

External Organisation(s)

University of Konstanz

Type

Article

Journal

Journal of solid state chemistry

Volume

115

Pages

55-65

No. of pages

11

ISSN

0022-4596

Publication date

15.02.1995

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Electronic, Optical and Magnetic Materials, Ceramics and Composites, Condensed Matter Physics, Physical and Theoretical Chemistry, Inorganic Chemistry, Materials Chemistry

Electronic version(s)

https://doi.org/10.1006/jssc.1995.1101 (Access: Closed)