Atomistic origin and strain control of the finite-temperature dielectric response in BaTiO3

The dielectric response of BaTiO3 (BTO) varies strongly with temperature, yet its local atomic origin remains unclear. Using electric-field-coupled molecular dynamics with a machine-learning force field, we show that the permittivity tracks the field-induced angular redistribution of local Ti-O off-centering rather than its magnitude or mean polar angle. Temperature and biaxial strain modify the local structure differently, but both responses follow a common relation with the same orientational descriptor, providing a real-space counterpart to soft-mode behavior.