Improper origin of polar displacements at CaTiO 3 and CaMnO 3 twin walls

Recent interest in novel functionalities arising at domain walls of ferroic materials naturally calls for a microscopic understanding. To this end, first-principles calculations have been performed in order to provide solid evidence of polar distortions in the twin walls of nonpolar CaTiO${}_{3}$ and magnetic CaMnO${}_{3}$. We show that such polar displacements arise from rotation and/or tilting octahedral distortions---cooperatively acting at the twin wall in both considered systems---rather than from a proper secondary ferroelectric instability, as often believed. Our results are in excellent agreement with experimental observations of domain walls in CaTiO${}_{3}$. In addition, we show that magnetic properties at the twin wall in CaMnO${}_{3}$ are also modified, thus suggesting an unexplored route to achieve and detect multiferroic ordering in a single-phase material.