First-principles study of competing ferroelectric and antiferroelectric instabilities in BaTiO_3/BaO superlattices

摘要

We report a first-principles study of (BaTiO_3)_m/(BaO)_n superlattices for a wide range of periodicities m. We show that such a system develops a polar zone-center instability for sufficiently large m ratio, which can be understood, at least qualitatively, from a simple electrostatic model and should lead to a ferroelectric ground state. However, the analysis of the phonon-dispersion curves also points out the appearance of stronger anti-ferroelectric instabilities at the zone boundaries around m = 4, before the critical ratio for ferroelectricity is reached and which still dominate beyond it. The dominant character of the antiferroelectric instability is explained from the depolarizing field which hardens the ferroelectric mode. This analysis allows us to predict that, (BaTiO_3)_m/(BaO)_n superlattices should present an antiferroelectric ground state for m larger than 4, which should smoothly evolve to a multidomain structure for increasing m values and only become ferroelectric for large m.