Electron microscopic studies of the antiferroelectric phase in Sr0.60Ca0.40TiO3 ceramic

The structural variants and their coexistence across the antiferroelectric phase transition in Sr0.60Ca0.40TiO3 ceramic has been studied through transmission electron microscopy (TEM) at room temperature and ∼100 deg. C. A clear evidence of the presence of superlattice reflections, corresponding to the cell doubling along the c-axis of Pbnm (or b-axis along Pnma), occurring during paraelectric to antiferroelectric transition, has been obtained through selected area electron diffraction, convergent beam electron diffraction and lattice-resolution imaging. Coexistence of the Pbnm and Pbcm phases at room temperature has been observed and attributed to the strain/disorder-induced broadening of the first-order antiferroelectric phase transition. Drastic changes in the domain structure during Pbnm to Pbcm transformation have been observed. This clearly indicates that the antiferrodistortive transition responsible for the occurrence of the antiferroelectric phase is of completely different origin and it is not just an additional follow-up of the already-existing ordering due to a-a-c+ tilt schemes in the Pbnm domain. Thermal cycling studies on microstructural changes indicate some kind of memory mechanism, which retains the memory of the original a-a-c+ tilt schemes in the Pbnm phase. This has been attributed to the symmetry conforming short-range order (SC-SRO) of the point defects. - Graphical abstract: Selected area diffraction (SAD) and convergent beam electron diffraction (CBED) patterns taken along the (a,b) [001] and (c,d) [110] zones of the Pbcm phase from different domains. The occurrence of the FOLZ ring corresponding to 15.5 A and the superlattice spots as indicated by black arrows confirming the existence of the cell-doubled antiferroelectric phase. Coexistence of the Pbnm and Pbcm phases across the transition, i.e. at room temperature, has been observed and attributed to the strain/disorder-induced broadening of the first-order antiferroelectric phase transition