Thermal expansion behavior of sodalite cage in cubic Ca₈[Al₁₂O₂₄](SO₄)₂

Abstract

The thermal expansion behavior of the sodalite cage [Al₁₂O₂₄] in cubic ye’elimite, Ca₈[Al₁₂O₂₄](SO₄)₂ with space group I43m and Z = 1, was investigated using high-temperature X-ray powder diffraction. A split-atom model previously reported for 1073 K accurately represented the disordered crystal structures at 773–973 K. This model is characterized by the splitting of the O atoms coordinating to the S atom around the triad axes within the [Al₁₂O₂₄] cage structure. As the temperature increased from 773 to 1073 K, both the cage and the unit cell expanded steadily and continuously by 0.255 %, with a coefficient of linear thermal expansion of 8.5 × 10⁻⁶ K⁻¹. Two distinct thermal expansion processes were observed above and below approximately 923 K for the [Al₁₂O₂₄] cage. Between 773 and 873 K, the values of quadratic elongation and bond angle variance, which describe the distortion of the [AlO₄] tetrahedron, increased continuously by 0.16 and 11.74 %, respectively, leading to a cage expansion of 0.073 %. Below approximately 923 K, the Al–O–Al angle of the apex-sharing [AlO₄] tetrahedra remained nearly constant. However, as the temperature increased to 1073 K, this angle increased by 0.77 %, contributing to a further 0.182 % expansion of the cage. Thus, the total expansion of 0.255 % (= 0.073 + 0.182 %) for the [Al₁₂O₂₄] cage was primarily due to the distortion of the [AlO₄] tetrahedron below approximately 923 K and the mutual rotation of the apex-sharing [AlO₄] tetrahedra above this temperature. This study identified, for the first time, the switching between these two distinct processes in the thermal expansion behavior of sodalite cages.