Polymorphism of calcium sulphoaluminate (Ca₄Al₆O16·SO₃) and its solid solutions

Crystallochemical studies of Ca₄Al₆SO₁₆, or 4CaO·3Al₂O₃·SO₃, reveal a hitherto unreported phase transformation at 470 ± 5°C. This transformation is rapidly reversible and non-quenchable for the ideal composition. The low-temperature polymorph is body-centred pseudo-cubic with a ≃ 18·4 Å. High-temperature X-ray powder diffraction shows that above the transformation temperature the structure remains bodycentred cubic, but with a ≃ 9·2 Å. Sodium and iron solid solutions, [(Na₂)_x(Ca_4-x)][Al_6-2y)(Fe_2y)]O₁₂SO₄ (0 ≤ x ≤ O·7 and 0 ≤ y ≤ O·7), have been prepared. Throughout most of the composition range of x and y, the high-temperature phase, a ≃ 9·2 Å, is readily preserved to ambient temperature.