Pair Distribution Function structural investigation: BaTi1-xCexO3 as a locally disordered perovskite

Nowadays barium titanate has become the most widely used functional ceramic material in electronics, finding wide applications in devices as microphones, ultrasonic and underwater transducers, multilayer ceramic capacitors and spark generators [1]. In order to tailor the properties for the specific applications, BaTiO3 is easily and usually doped to improve the material performance, for example modifying the long-range order and consequently the ferroelectric and dielectric properties. The solid solutions with other BaMIVO3 perovskites (M =Sn, Zr, Hf, Ce) 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]. Different researches have investigated these systems, but the relation with the average crystallographic structure and the local order is still mostly unknown. Just few previous works, 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 order to improve our knowledge on the evolution of polar order in these systems more structural investigations are necessary. In this work the case of BaTi1-xCexO3 is presented and the average and local structure variations as a function of composition and temperature are shown. Ce4+ (r =0.87 Å) is much bigger than Ti4+ (r = 0.605 Å), so this system represents a limit and interesting case also because this kind of substitution does not involve the creation of charge compensating lattice defects. In particular BaTi1-xCexO3 ceramic solid solutions with x = 0.05, 0.10, 0.20 have been investigated between 100 and 400 K. 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). This because there are subtle phase transitions in the studied temperature range, that need very high resolution data to be properly detected. Pair Distribution Function (PDF) refinements have been performed with the aim of understanding the induced differences between average and local structure. The results show a clear local disorder which exhibits displacements in atomic positions or even different crystallographic space group compared with the average structure. These local deviations are caused by the evolution of Ti-O and Ce-O bond distances with temperature and by the increase of the cerium amount. [1] S. Yasmin, S. Choudhury, M.A. Hakim, A.H. Bhuiyan and M.J. Rahman J. Mater. Sci. Technol. 2011, 27(8), 759-763. [2] V.V. Shvartsman and D.C. Lupascu J. Am. Ceram. Soc. 2012, 95 (1), 1-26. [3] V. Buscaglia, S. Tripathi, V. Petkov, M. Dapiaggi, M. Deluca, A. Gajovic, Y.J. Ren J. Phys.: Condens. Matter 2014, 26, 065901.