Structural Phase Transitions and Possibility of the Relaxor-like State in Improper Ferroelectric Strontium-Substituted Calcium Sulfoaluminates

Structural phase transitions of strontium-substituted calcium sulfoaluminates, (Ca1-xSrx)(8)[A(1)O(2)](12)(SO4)(2)[(CS)AS-x], are systematically investigated by powder X-ray diffraction (PXRD) and dielectric measurements. In a pure calcium sulfoaluminate (CS)AS-0.0, an unreported phase transition and dielectric anomaly are found in the low-temperature region below the ferroelectric phase-transition temperature. The XRD pattern recorded as a function of Sr substitution and temperature clearly resolves T-c suppression with Sr content x from a structural change perspective. Although no macroscopic change in the crystal structure is observed for (CS)AS-0.4, the broad dielectric peak with frequency dispersion remains for this composition, indicating the fragmentation of polar ordering. Furthermore, (CS)AS-0.4 shows a specific heat anomaly along with the broad dielectric peak, suggesting a relaxor-like short-range ordered state of the system.