Occurrence of a new superlattice phase across the antiferroelectric phase transition inSr1−xCaxTiO3 (x = 0.30 and 0.40)

Abstract

Low-temperature x-ray diffraction, low-temperature transmission electronmicroscopy (TEM) and dielectric permittivity studies have been carried out onSr1−xCaxTiO3 (x = 0.30 and 0.40) ceramics across its antiferroelectric phase transition. TEM has beenperformed for its structural and microstructural characterizations. Togetherwith the occurrence of the usual structural modulation along [001] of theP 21212 (andPbnm), which givesrise to the Pbcm phase, clear evidences of the occurrence of a new phase, appearing asa result of structural modulation along the [110]-type direction of thePbnm, has also been found. The temperature dependent dielectric permittivity data appears toindicate the paraelectric nature of the new phase. The coexistence of this new phase with thePbcm phase has beenattributed to Δ-point instability, which is equally probable along any of the [100]-type directions of the basicperovskite structure. It appears that irrespective of the existing perovskite distortion, which resultsin P 21212 for x = 0.3 and Pbnm for x = 0.4; the Δ-point instability follows the cubic symmetry of the very basic perovskite.