Random local strain effects in homovalent-substituted relaxor ferroelectrics: A first-principles study of BaTi0.74Zr0.26O3

We present first-principles supercell calculations on BaTi0.74Zr0.26O3, a prototype material for relaxors with a homovalent substitution. From a statistical analysis of relaxed structures, we give evidence for four types of Ti-atom polar displacements: along the < 111 >, < 110 >, or < 100 > directions of the cubic unit cell or almost canceled. The type of a Ti displacement is entirely determined by the Ti/Zr distribution in the adjacent unit cells. The underlying mechanism involves local strain effects that ensue from the difference in size between the Ti4+ and Zr4+ cations. These results shed light on the structural mechanisms that lead to disordered Ti displacements in BaTi1-xZrxO3 relaxors, and probably in other BaTiO3-based relaxors with homovalent substitution.