Domain walls in $R3c$-structured NaNbO$_3$

Overview & Synopsis

In this work, we undertake a systematic first-principles study of structural domain walls in the low-temperature rhombohedral $R3c$ phase of perovskite oxide NaNbO$_3$. Employing a group-theoretic classification, we introduce eight unique classes of domain walls in the $R3c$ structure, four of which have a domain wall orientation determined uniquely by the conditions of elastic compatibility and charge neutrality, and perform structural relaxations for each representative domain wall class. We find that one class of 109° domain walls has a notably low energy of 9 mJ m$^{-2}$, comparable to the low-energy ferroelectric walls in BaTiO$_3$. Finally, we find that this class of 109° walls is compatible with the crystal twinning observed in experiments on the $R3c$ phase, and propose an atomistic mechanism for the $R3c \leftrightarrow Pbcm$ phase transition.