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

The thermal expansion behavior of the sodalite cage [Al 12 O 24 ] in cubic ye'elimite, Ca 8 [Al 12 O 24 ](SO 4 ) 2 with space group I3m 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 [Al 12 O 24 ] 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 8.510 -6 K -1 .Two distinct thermal expansion processes were observed above and below approximately 923 K for the [Al 12 O 24 ] cage.Between 773 and 873 K, the values of quadratic elongation and bond angle variance, which describe the distortion of the [AlO 4 ] 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 [AlO 4 ] 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 [Al 12 O 24 ] cage was primarily due to the distortion of the [AlO 4 ] tetrahedron below approximately 923 K and the mutual rotation of the apexsharing [AlO 4 ] 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.2016 The Ceramic Society of Japan.