Powder neutron diffraction study of the antiferroelectric phase transition in Sr_0.75Ca_0.25TiO₃

Abstract

The structure of Sr_0.75Ca_0.25TiO₃ 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 etal [Phys. Rev. Lett. 84, 3726 (2000)] for the antiferroelectric phase. The most likely space group for the paraelectric phase of Sr_0.75Ca_0.25TiO₃ 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.