Phase transition and antiferroelectric-like switching inside ferroelastic domain walls

Ferroelastic domain walls (DWs), traditionally considered functionally restricted due to antagonistic structural order parameters, are revealed to host electrically switchable polar states through synergistic flexoelectric and biquadratic coupling mechanisms. Using large-scale molecular dynamics (MD) simulations on orthorhombic $Pnma {\mathrm{CaTiO}}_{3}$ (CTO) with (110)-type DWs, this study demonstrates that a thermally-driven structural phase transition within DWs is related to the reduced out-of-phase ${a}^{\ensuremath{-}} {\mathrm{TiO}}_{6}$ octahedral tilt (Glazer tilt notation ${a}^{\ensuremath{-}}{a}^{\ensuremath{-}}{c}^{+}$) model, activating an achiral-to-chiral polar transition. The DW structural phase transition occurs more than 300 K below the bulk transitions, and the $z$-axis $({[001]}_{pc})$ polar component becomes switchable at temperatures above 1100 K. Electric-field-driven DW switching, mediated by octahedral tilt suppression, DW flexoelectricity and biquadratic coupling, manifests volatile, antiferroelectric-like double hysteresis loops. At room temperature, a critical field of $\ensuremath{\sim}3.6\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.28em}{0ex}}\mathrm{kV}/\mathrm{cm}$ triggers polarization reversal by suppressing octahedral tilts both in the DWs and bulk, while thermal broadening of DWs at elevated temperatures (e.g., 1000 K) reduces strain gradients and lowers the coercive field by $\ensuremath{\sim}42%$, enhancing switchability. These results reveal the dual role of DWs as both structural interfaces and active functional elements, offering pathways to manipulate nanoscale polar textures and engineer novel storage/memory/piezoelectric devices leveraging tunable domain-wall functionalities.