BaTi1-xCexO3: local disorder modeled by PDF

In the past few decades, scientific community has paid great attention on free-lead ferroelectrics. These materials, in comparison to the toxic lead-based ones, have comparable good performances and can be employed for environmental friendly applications. BaTiO3 perovskite in its pure form does not have ideal properties, but by chemical substitutions, it can be tailored to meet a variety of device and performance requirements [1]. The solid solutions BaTi1-xMIV x O3 perovskites (M = Sn, Zr, Hf, Ce) are a classical example: in fact, these materials show an almost continuous variation of the ferroelectric behaviour with composition, from conventional ferroelectric, via diffuse ferroelectric transition to a clear relaxor state and further to dipolar glass behaviour [2]. In these cases the dipolar behaviour and functional responses seem to depend critically on the order between Ti and its substituents [3]. It is then mandatory to perform deep structural investigations on these materials, in order to clarify the connection between property and structure. In this research work, the solid solution of BaTi1-xCexO3 has been chosen because it represents a limit and interesting case: on one hand Ce4+ (r = 0.87 °A) is much bigger than Ti4+ (r = 0.605 °A) and on the other this kind of substitution does not involve the creation of charge compensating lattice defects. Ceramic solid solutions with x = 0.05, 0.10, 0.20 have been investigated between 100 and 420 K, because, as indicated by dielectric permittivity measurements, the three samples under study correspond to a different polar behaviour: conventional ferroelectric (x = 0.05) but close to the so-called pinched transition, diffuse phase transition (x = 0.10) and non-ergodic relaxor (x = 0.20). Total scattering data have been collected at ID22 high resolution beamline, ESRF (European Synchrotron Radiation Facility; Grenoble, France) and Pair Distribution Function (PDF) refinements, coupled with traditional Rietveld refinements, have been performed with the aim of understanding the induced differences between average and local structure and at the end to describe how the local structure reacts at the substitution between titanium and cerium atoms. As demonstrated by the comparison between local and average analysis, the introduction of cerium in BaTiO3 structure generates a deep local disorder, which is not reflected on the average structure. In order to model it and to understand the local neighbourhood relation between cerium and titanium, lattice energies have been calculated and compared for different structural configurations: from random distribution to a clustered supercell. Different numbers and size of BaCeO3 clusters have been considered. [1] T. Maiti, R. Guo, A.S. Bhalla, J. Am. Ceram. Soc. 91, 6 (2008) [2] V .V. Shvartsman, D.C. Lupascu, J. Am. Ceram. Soc. 95(1), 1-26 (2012) [3] V. Krayzman, I. Levin, J. Appl. Cryst. 41, 386-392 (2008)