BaCexTi1-xO3 perovskite: how the local disorder influences the material properties

Recently, among BaTiO3 based ferroelectric-relaxors, the BaCexTi1-xO3 (BCT) solid solution, in which Ti4+ is substituted by Ce4+, has emerged as a promising lead-free material suitable in applications [1,2] as tunable capacitor [3], multilayer ceramic capacitors with high capacitance [4] or as actuator [5]. As a function of cerium amount, an almost continuous variation of the ferroelectric behaviour is observed, from conventional ferroelectric, via diffuse ferroelectric transition to a clear relaxor state. This is linked to changes in the phase transition temperatures and to average structural modifications, as described in details in Canu et al. [6]. Observations there reported, however, underline how the macroscopic properties of the material are not defined by the long-range structure, but rather by the short-range atomic interactions. Therefore, in the present work, Pair Distribution Function (PDF) was used investigating BaCexTi1-xO3 (x=0.02-0.30) local structure. The insertion of the much bigger cerium atoms ( rCe4+ = 0.87 Å and rTi4+= 0.605 Å) was proved to create a high level of local structural disorder, increasing with x. In order to properly fit this disorder, an original structural model was designed. This allows: i) different Ti-O and Ce-O distances ii) the typical distortions of the two end-members: Ti off-centre displacement occurring in BaTiO3 and octahedral tilt in BaCeO3. Obtained results show that the local disorder is mainly related to the different behaviour of the oxygen cage in Ce-bearing and Ti-bearing polyhedral. Indeed the respective octahedra tend to keep the volume of their respective end-members polyhedra (14.9 Å3 for CeO6 and 10.7 Å3 for TiO6), causing tilting and distortion of the oxygen cages. In turn, this complex local arrangement entails deep distortions, increasing as a function of doping, which influence the directions of titanium displacements diminishing their local correlation and consequently their long range cooperative effects. [1] D. Makovec, Z. Samardzija, D. Kolar J. Solid State Chem. 1996, 123(1), 30-38. [2] A. Ianculescu, D.C. Berger, C.A. Vasilescu, M. Olariu, B.S. Vasile, L.P. Curecheriu, A. Gajovi?, R. Tru?c?, R. Nanoscale Ferroelectrics and Multiferroics, edited by M. Algueró, J. M. Gregg & L. Mitoseriu, 2016. [3] L.P Curecheriu, C.E. Ciomaga, V. Musteata, G. Canu, V. Buscaglia, L. Mitoseriu Ceram. Int. 2016, 42(9), 11085- 11092. [4] Y. Park, Y. Kim J. Mater. Res. 1995, 10(11), 2770-2776. [5] C. Ang, Z. Yu, Z. Jing, Z., R. Guo, A. Bhalla, L.E. Cross Appl. Phys. Lett. L.E. 2002, 80(18), 3424-3426. [6] G. Canu, G. Confalonieri, M. Deluca, L. Curecheriu, M.T Buscaglia, M. Asandulesa, N. Horchidan, M. Dapiaggi, L. Mitoseriu, V. Buscaglia Acta Mater. 2018, 152, 258-26