CRISTOBALITE-RELATED PHASES IN THE NAALO2-NAALSIO4 SYSTEM - I - 2 TETRAGONAL AND 2 ORTHORHOMBIC STRUCTURES

The crystal structures of five new cristobalite-related sodium alumino silicates with four different structure types from the system Na2-xAl2 -xSixO4, 0 less than or equal to x less than or equal to 1 [Na(1.95)A( 1.95)Si(0.05)O(4), P4(1)2(1)2, a = 5.2997 (6), c = 7.0758 (9) Angstrom ; Na1.75Al1.75Si0.25O4, Pbca, a = 10.4221 (11), b = 14.264 (3), c = 5. 2110 (5) Angstrom; Na1.65Al1.65Si0.35O4, P4(1)2(1)2, a = 10.3872(7), c = 7.1589 (8) Angstrom; Na1.55Al1.55Si0.45O4, Pbca, a = 10.385 (1), b = 14.198 (3), c = 5.1925 (6) Angstrom; Na1.15Al1.15Si0.85O4, Pb2(1)a, a = 10.214 (2), b = 14.226 (7), c = 10.308 (1) Angstrom], have been re fined by the Rietveld method from X-ray powder diffraction data. Plaus ible starting models were derived for the x = 0.05, 0.25 and 0.45 stru ctures by analogy. Starting models for the x = 0.35 and 0.85 structure s, with previously unreported structure types, were derived from a mod ulation wave approach based on distortion of the ideal C9 structure ty pe and assuming regular SiO4 and AlO4 tetrahedra.