Phase transformations in the relaxor Na1/2Bi1/2TiO3 studied by means of density functional theory calculations

Abstract

AbstractThe relaxor material Na1/2Bi1/2TiO3 (NBT) is an important basis for the development of lead‐free piezoceramics, but still many features of this material are not well understood. Here, we study the kinetics of phase transformations by octahedral tilts and A‐cation displacements in NBT by means of density functional theory calculations, employing ab initio molecular dynamics and nudged elastic band calculations. Our results show that the energetic differences between the low temperature rhombohedral, intermediate orthorhombic and other metastable phases are close to the room temperature thermal energy. Therefore, it is likely that above room temperature, several octahedral tilt patterns are present simultaneously on the local scale, just because of thermal vibration of the oxygen ions. Octahedral tilt transformations and A‐cation displacements show similarly high energy barriers, however, since the vibrational frequency of oxygen is higher, tilt transformations occur more frequently. Further, tilt transformations in which the oxygen octahedra get deformed the least are more probable to occur. We also find that the chemical A‐cation order affects energy barriers, influences the coupling between rotational and displacive modes and determines the stability of certain octahedral tilt orders. We conclude that the so‐called polar nanoregions in this material result from local octahedral tilt transformations and subsequent A‐cation displacements, which are driven by thermal vibration and are mediated by the underlying chemical order.