Abstract
We report a first-principles study of (BaTiO${_3}$)$_m$/(BaO)$_n$ superlattices for a wide range of periodicities $m/n$. We show that such a system develops a polar zone-center instability for sufficiently large \textit{m}/\textit{n} 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 antiferroelectric 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 anti-ferroelectric 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 $m$.
Highlights
We study the case ofBaTiO3͒m / ͑BaOn superlattices epitaxially grown on a SrTiO3 substrate as a prototypical example of FE/I superlattices
When grown on a SrTiO3 substrate, assuming a theoretical cubic in-plane lattice constant of 3.84 Å, BaTiO3 is under compressive strain and becomes tetragonal, as well as BaOthe epitaxial strain being applied on the Ba-Ba distancein a smaller extent.[34]
There is no continuity of the Ti-O chains from one perovskite block to the one. This will prevent the possibility of huge polarization currents along the stacking direction as those associated to the ferroelectric mode in BaTiO3 / BaO system and could explain why Ruddlesden-Popper materials do not develop any tendency to become ferroelectric with polarization aligned along the stacking direction.[27]
Summary
Numerous works have been devoted to the study of interfacial effects in ferroelectricFEnanostructures.[1,2] Both theoretical and experimental works have focused on various kinds of perovskite ferroelectric multilayers and superlattices including ferroelectric ultrathin films between metallic electrodes[3–7] and superlattices combining either ferroelectric materials,[8–10] ferroelectric and incipient-ferroelectric materials,[11–21] or even incipientferroelectric and paraelectric materials.[22,23] These studies highlighted the fact that three main factors govern the physics of multilayers: epitaxial strain, electrical boundary conditions, and interfacial effects. Numerous works have been devoted to the study of interfacial effects in ferroelectricFEnanostructures.[1,2] Both theoretical and experimental works have focused on various kinds of perovskite ferroelectric multilayers and superlattices including ferroelectric ultrathin films between metallic electrodes[3–7] and superlattices combining either ferroelectric materials,[8–10] ferroelectric and incipient-ferroelectric materials,[11–21] or even incipientferroelectric and paraelectric materials.[22,23]. These studies highlighted the fact that three main factors govern the physics of multilayers: epitaxial strain, electrical boundary conditions, and interfacial effects. We show that the predicted ground state differs from that of the simple electrostatic model and explain the reason for the discrepancy
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