AN XRD AND ELECTRON-DIFFRACTION STUDY OF CRISTOBALITE-RELATED PHASES IN THE NAALO2-NAALSIO4 SYSTEM

A detailed study of the sodium aluminate-carnegieite system (Na2-xAl2- xSixO4, 0 less than or equal to x less than or equal to 1) at temperat ures between 800 and 1300 degrees C has revealed five previously unrep orted phases. All of the new phases can be described as modulated vari ants of an underlying beta-cristobalite parent structure. At x approxi mate to 0.05 the gamma-NaAlO2-type structure (P4(1)2(1)2, a = 1/root 2 a(p), c = c(p)) (p = parent) is stabilized to room-temperature; at x a pproximate to 0.2-0.45 an orthorhombic (Pbca, a = root 2a(p), b = 2b(p ), c = 1/root 2c(p)) KGaO2-type structure is obtained, except at x app roximate to 0.35 where a new tetragonal (P4(1)2(1)2, a = root 2a(p), c = c(p)) phase is observed; at x approximate to 0.5-0.6 a new cubic (P 2(1)3, a = 2a(p)) phase is obtained; at x approximate to 0.7-0.9 a new orthorhombic (Pc2(1)b, a = root 2a(p), b = 2b(p), c = root 2c(p)) pha se is obtained. XRD and electron diffraction data, refined unit cell d imensions, and the phase relationships at 1300 degrees C are presented . The proposed space group symmetries are based on observed extinction conditions and constraints provided by a modulated structure descript ion of the new phases. The materials are prepared by gel synthesis fol lowed by solid state reaction in air. (C) 1997 Academic Press.