Size effect of the guest cation on the AlO4 framework in aluminate sodalite-type oxides M 8[Al12O24](SO4)2 (M = Sr2+ and Ca2+) in the I 43m phase

Abstract

Sr8[Al12O24](SO4)2 (SAS) and Ca8[Al12O24](SO4)2 (CAS) are members of the aluminate sodalite-type oxides with the general chemical formula M 8[Al12O24](XO4)2 (M 2+ is the guest cation and XO4 2− is the guest anion). To discuss the role of the guest cations (M 2+ = Sr2+ and Ca2+) on the rotation of AlO4 in the oxygen tetrahedral framework in the I 43m phase, the crystal structure parameters and the probability density function of the guest ions in SAS and CAS have been investigated via synchrotron radiation X-ray powder diffraction by considering Gram–Charlier expansions. The interatomic distances between the M 2+ and O2− ions evaluated from the maximum positions in the probability density distribution are almost equal to the sum of the ideal ionic radii of the M 2+ and O2− ions. This result suggests that the geometry of the AlO4 tetrahedral framework and the fluctuation of the guest ions are mainly caused by steric effects between the M 2+ and O2− ions.