Structural phase transitions of calcium strontium sulfoaluminate series, (Ca1–x Sr x )8­[AlO2]12(SO4)2 ((CS)­AS-x) with x = 0.80–1.00, are systematically investigated by powder X-ray diffraction, dielectric measurements, and pyroelectric measurements, to clarify a phase diagram of (CS)­AS-x (x = 0.80–1.00). A pure strontium sulfoaluminate, (CS)­AS-1.00, is found to undergo three phase transitions, which take place successively on cooling from a prototypical cubic phase with the symmetry of Im3̅m. Though the room-temperature phase of (CS)­AS-1.00 was previously reported to be of polar Pcc2, the pyroelectric measurements clarified a nonpolar character of the crystal symmetry. The dielectric measurements suggest a possibility of an antiferroelectric ground state of (CS)­AS-x in the Sr-rich compositions. As x decreases, the ground state changes to a short-range-ordered state, implying a unique phase transition from the antiferroelectric state to the antiferroelectric-relaxor state. The present study provides an intriguing playground for designing new ferro/antiferroelectric materials.

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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.