Chemical order and local structure of the lead-free relaxor ferroelectric Na1/2Bi1/2TiO3Na1/2Bi1/2TiO3

The A-site mixed perovskite sodium bismuth titanate (Na1/2Bi1/2)TiO3(Na1/2Bi1/2)TiO3 (NBT) is investigated by means of first-principles calculations based on density functional theory. By studying different geometries with varying occupations of the A-site, the influence of chemical order on the thermodynamic stability and local structure is explored. We find that the hybridization of Bi 6sp with O 2p  -states leads to stereochemically active Bi3+Bi3+ lone pairs and increases the stability of structures with high Bi concentrations in {001}-planes. This goes along with displacive disorder on the oxygen sublattice, which up to now has been neglected in experimental studies. The calculated ordering energies are, however, small as compared to the thermal energy and therefore only short-range chemical order can be expected in experiments. Thus, it is conceivable that chemically ordered local areas can act as nucleation sites for polar nano-regions, which would explain the experimentally observed relaxor behavior of NBT.

First-principles calculations give relative stabilities of different chemically ordered structures. The results suggest a new model for the local structure of Na1/2Bi1/2TiO3 with 001-ordered nano-regions embedded in a chemically disordered matrix. Chemical order/disorder additionally induces displacive disorder within the oxygen sublattice.Figure optionsDownload as PowerPoint slideHighlights
► Lead-free relaxor ferroelectric Na1/2Bi1/2TiO3Na1/2Bi1/2TiO3 is studied by ab initio  -calculations.
► Structural relaxations in the oxygen sublattice are decisive for relative stabilities.
► Chemical environment of oxygen determines relaxation ability.
► Bi 6s26s2 lone pair formation is the driving force for relaxation.
► New structure model: Chemically 001-ordered nano-regions embedded in disordered matrix.