Cristobalite-Related Phases in the NaAlO₂–NaAlSiO₄ System. I. Two Tetragonal and Two Orthorhombic Structures

The crystal structures of five new cristobalite-related sodium aluminosilicates with four different structure types from the system Na_2−xAl_2−xSi_xO₄, 0 ≤ x ≤ 1 [Na_1.95Al_1.95Si_0.05O₄, P4₁2₁2, a = 5.2997 (6), c = 7.0758 (9) Å; Na_1.75Al_1.75Si_0.25O₄, Pbca, a = 10.4221 (11), b = 14.264 (3), c = 5.2110 (5) Å; Na_1.65Al_1.65Si_0.35O₄, P4₁2₁2, a = 10.3872 (7), c = 7.1589 (8) Å; Na_1.55Al_1.55Si_0.45O₄, Pbca, a = 10.385 (1), b = 14.198 (3), c = 5.1925 (6) Å; Na_1.15Al_1.15Si_0.85O₄, Pb2₁a, a = 10.214 (2), b = 14.226 (7), c = 10.308 (1) Å], have been refined by the Rietveld method from X-ray powder diffraction data. Plausible starting models were derived for the x = 0.05, 0.25 and 0.45 structures by analogy. Starting models for the x = 0.35 and 0.85 structures, with previously unreported structure types, were derived from a modulation wave approach based on distortion of the ideal C9 structure type and assuming regular SiO₄ and AlO₄ tetrahedra.

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