  Recent interests towards novel functionalities arising at domain walls of
ferroic materials naturally call for a microscopic understanding. To this end,
first-principles calculations have been performed in order to provide solid
evidence of ferroelectricity in the twin walls of nonpolar CaTiO3 and magnetic
CaMnO3. We show that ferroelectricity arises from rotation/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 CaTiO3. In addition, we show that magnetic
properties at the twin wall in CaMnO3 are also modified, thus suggesting an
unexplored route to achieve and detect multiferroic ordering in a single-phase
material.
