Abstract
The structure of Sr0.75Ca0.25TiO3 has been studied using powder neutron diffraction and the Rietveld method in the temperature range 24–300 K. It is shown that new superlattice reflections characteristic of an antiferroelectric phase appear around T=185 K. The unit cell parameters change discontinuously around 185 K indicating first order nature of the antiferroelectric phase transition. Rietveld analysis of the diffraction data confirms the Pbcm space group proposed by Ranjan et al. [Phys. Rev. Lett. 84, 3726 (2000)] for the antiferroelectric phase. The most likely space group for the paraelectric phase of Sr0.75Ca0.25TiO3 is proposed to be Ibmm. The antiparallel arrangement of dipoles in the antiferroelectric phase is shown to be of very novel type mimicking long period modulated phases predicted by axial next nearest neighbor interaction type models.
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