INTERMEDIATE PHASES IN THE CA-RICH PART OF THE SYSTEM (CA1-XSRX)8[AL12O24](WO4)(2)

The Ca-rich part of the cation substitution series of aluminate sodali tes (Ca1-xSrx)(8)[Al12O24](WO4)(2) has been studied as a function of t emperature and composition. The intermediate beta-phase of pure calciu m tungstate aluminate sodalite was found to have a (3 + 2)-dimensional commensurately modulated structure with an isometric unit cell. The p robable superspace group is Abm2(alpha 0 gamma) 0s0, 000 with alpha = 1/4, gamma = 1/4, a = b = root 2.9.301(1) Angstrom, c = 9.301(1) Angst rom, at 636 K. Its average structure was found to be similar to that o f the room-temperature gamma-phase. In crystals with 0 < x less than o r equal to 0.18 the existence of a new intermediate phase could be con firmed. At variance with the otherwise very similar room-temperature p hase with x = 0, this gamma'-phase has an incommensurate modulation ve ctor. Its proposed superspace group is Abm2(1/2 0 gamma) 0ss with gamm a = 0.05(1). Within the precision of our TEM-experiments gamma does no t change with temperature or composition. The average structure of the gamma'-phase could be refined and was found closely related with the average structure of gamma-CAW.