Intermediate phases in the Ca-rich part of the system (Ca1−x Sr x )8[Al12O24](WO4)2

Abstract

Abstract The Ca-rich part of the cation substitution series of aluminate sodalites (Ca1−x Sr x )8[Al12O24](WO4)2 has been studied as a function of temperature and composition. The intermediate β-phase of pure calcium tungstate aluminate sodalite was found to have a (3 + 2)-dimensional commensurately modulated structure with an isometric unit cell. The probable superspace group is Abm2(α 0 γ) 0s0, 000 with α = ¼, γ = ¼, a = b = [unk] · 9.301(1) Å, c = 9.301(1) Å at 636 K. Its average structure was found to be similar to that of the room-temperature γ-phase. In crystals with 0 < x ≤ 0.18 the existence of a new intermediate phase could be confirmed. At variance with the otherwise very similar room-temperature phase with x = 0, this γ′-phase has an incommensurate modulation vector. Its proposed superspace group is Abm2(½ 0 γ) 0ss with γ = 0.05(1). Within the precision of our TEM-experiments γ does not change with temperature or composition. The average structure of the γ′-phase could be refined and was found closely related with the average structure of γ-CAW.