Understanding and Revisiting Properties of ${\mathrm{EuTiO}}_3$ Bulk Material and Films from First Principles

Ab initio computations are performed to investigate properties of bulk material and epitaxial films made of ${\mathrm{EuTiO}}_3$ (ETO). A whole family of nanoscale twinned phases, that present complex oxygen octahedra tilting (OOT) and unusual antiferroelectricity, is found to be degenerate in energy with simpler phases (all possessing typical OOT) in bulk ETO. Such degeneracy provides a successful explanation of recently observed anomalous phenomena. The calculations also lead to revisiting the (rich) phase diagram of ETO films.

Figure 1

Predicted total energy of the $G$-type AFM state [panel (a)] and energy difference between the $G$-type AFM and FM states [panel (b)] versus the misfit strain for low-in-energy phases in epitaxial (001) ETO films. The inset of panel (a) displays a schematic of the $a^{-}{a}^{-}{c}^m$-type nanoscale twinned phases possessing the $mmmppp$ tilting pattern. In this inset, the curled and straight arrows represent oxygen octahedra tilting about [001] or [$00\overline{1}$] and antiferroelectric Eu displacements, respectively. The different tilting angles about [001] are also provided here.

Figure 2

Evolution of the polarization [panel (a)] and AAFD vector [panel (b)] with strain for equilibrium phases. The inset of Fig. 2b shows the IAFD vector of the $Pnma$ and $Pmc{2}_1$ state. Open symbols display the $x$ component (which is identical to the $y$ component) while solid symbols show the $z$ component. For strain ranging between 0.25% and 1.9%, the rhombus, inverted triangles, and triangles correspond to $Imma$, $Pnma$, and $C2/c$ states, respectively, in panel (b).