Origin of relaxor behavior in barium titanate solid solutions

Relaxors are a special class of disordered ferroelectric materials with peculiar dielectric and electromechanical properties, which make them suitable in particular for microwave and energy storage applications. Relaxor behavior is present only in chemically substituted systems and its origins are different depending on the modifications induced by substituents into the parent lattice. In perovskites, it is universally accepted that B-site substitution is the key to relaxor behavior, since it influences the long-range spatial correlation of Ti displacements within the ferroelectric phase [1]. The role of ionic radii, valence, defects and especially the nanoscale distribution of substituent species in modifying the local polar state is, however, largely unknown. Chemically ordered regions, random cation substitution or segregation issues might impact differently on the macroscopic behavior. The most intriguing yet not fully explored question is: by which mechanism do homo- or heterovalent substituents contribute to breaking the long-range ferroelectric order? In this work we address this question in barium titanate solid solutions. We show, using a combination of Raman spectroscopy as a function of composition/temperature and ab initio phonon calculations that different characteristic atomic features appear in homovalent and heterovalent solid solutions. These features may impact differently in producing relaxor behavior. Our results show that in homovalently substituted systems, the introduction of a large B-site cation promoting a non-polar lattice fragments the long-range Ti-O correlation chains [1,2,3], whereas in heterovalently substituted systems strong polar disorder is induced, which also disrupts the ferroelectric long-range order. The latter effect, being charge-mediated, is likely much stronger and leads to the appearance of relaxor behavior in barium titanate for lower substituent contents. References: [1] G. Canu et al., Acta Materialia, 152, 258-268 (2018), [2] D. Amoroso, A. Cano, Ph. Ghosez, Physical Review B, 97, 174108 (2018), [3] A. Pramanick et al., Physical Review Letters, 120, 207603 (2018)