Average and local structural comparison of BaTi1-xCexO3 by pair distribution function

BaTiO3 is considered as a typical ferroelectric material. It presents, with a temperature decrease, the classical phase transition from a paraelectric cubic structure to ferroelectric phases (progressively: tetragonal, orthorhombic and rhombohedral). The solid solutions with other BaMIVO3 perovskites (M=Sn, Zr, Hf, Ce) can strongly modify these transitions and the related polar behaviors. Some materials show, dependently from the dopant type and amounts, a variation from conventional ferroelectric, via diffuse ferroelectric transition to a clear relaxor state and further to dipolar glass behaviour [1]. Considering that BaTiO3-based solid solutions are environment-friendly dielectrics with similar performances as many toxic Pb-based electroceramics [2], the study of these compounds becomes a topic of great interest and a current matter. Different researches have investigated these systems, but the relation with the average crystallographic structure, the local order and electric properties is still mostly unknown. Just few previous studies, as for the BaTi1-xZrxO3 [3], have addressed the issue demonstrating the existence of a structural local disorder linked to the evolution of the polar behaviour. In this work BaTi1-xCexO3 ceramic solid solutions with different cerium amounts (x = 0.05, 0.10, 0.20) are presented. This system represents a limit and interesting case due to a large difference in the Ti and Ce atomic radius ( rCe4+ = 0.87 Å and rTi4+= 0.605 Å), and because this kind of substitution does not involve the creation of charge compensating lattice defects. 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 has been collected ID22 high resolution beamline, ESRF (European Synchrotron Radiation Facility; Grenoble, France) in a range between 100 and 400 K every 100 K to explore the phase transitions associated to each sample. Pair Distribution Function (PDF) refinements have been performed with the aim of understanding the induced differences between average and local structure as a function of composition and temperature. The Rietveld and average PDF results show a good agreement with the structural hypothesis suggested by the dielectric permittivity measurements. On the other hand, PDFs analysis also highlights a clear local disorder for all the samples. Often local and average structure have required different space groups to be fitted appropriately. PDFs comparison and structural refinements demonstrate in fact a clear local evolution of Ti-O and Ce-O bond distances with temperature and by the increase of the cerium amount. [1] V.V. Shvartsman and D.C. Lupascu J. Am. Ceram. Soc. 2012, 95(1), 1-26. [2] C. Ciomaga, M. Viviani, M.T. Buscaglia, V. Buscaglia, L. Mitoseriu, A. Stancu, P. Nanni J. Eur. Ceram. Soc. 2007, 27, 4061-4064. [3] V. Buscaglia, S. Tripathi, V. Petkov, M. Dapiaggi, M. Deluca, A. Gajovic, Y.J. Ren J. Phys.: Condens. Matter. 2014, 26, 065901.