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

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.