Occurrence of a new superlattice phase across the antiferroelectric phase transition in Sr1-xCaxTiO3 (x=0.30 and 0.40)

Low-temperature x-ray diffraction, low-temperature transmission electron microscopy (TEM) and dielectric permittivity studies have been carried out on Sr1-xCaxTiO3 (x = 0.30 and 0.40) ceramics across its antiferroelectric phase transition. TEM has been performed for its structural and microstructural characterizations. Together with the occurrence of the usual structural modulation along [001] of the P2(1)2(1)2 (and Pbnm), which gives rise to the Pbcm phase, clear evidences of the occurrence of a new phase, appearing as a result of structural modulation along the [110]-type direction of the Pbnm, has also been found. The temperature dependent dielectric permittivity data appears to indicate the paraelectric nature of the new phase. The coexistence of this new phase with the Pbcm phase has been attributed to Delta-point instability, which is equally probable along any of the [100]-type directions of the basic perovskite structure. It appears that irrespective of the existing perovskite distortion, which results in P2(1)2(1)2 for x = 0.3 and Pbnm for x = 0.4; the Delta-point instability follows the cubic symmetry of the very basic perovskite.