Although strain gradients hold great promise for tailoring emergent functionalities, their flexible global control remains a fundamental challenge. Here, we address this concern in ${\mathrm{PbTiO}}_3$ under biaxial tensile strain by uncovering the spontaneous appearance of strain gradients and 90° ferroelectric domain walls and demonstrating their electric control. Combining symmetry analysis and first-principles calculations, we reveal hidden polar-acoustic instabilities within conventionally assumed ground states. We further demonstrate that the trilinear coupling among the modulated polar mode, acoustic mode with modulated strain, and uniform polar mode spontaneously stabilizes a lower-energy $a_1/a_2$ 90° domain structure with $Pmc{2}_1$ symmetry and intrinsic strain gradients. Remarkably, we discover similar polar-acoustic coupling that governs both the $c/a$ domain patterns and their dramatic piezoelectric enhancement. Leveraging these strong lattice couplings, we develop a unified framework for describing and electrically switching spatially varying strain and polarization fields. Our Letter resolves long-standing cross-scale discrepancies in the strain phase diagrams of ${\mathrm{PbTiO}}_3$, reveals a new phonon-mediated mechanism for giant piezoelectricity, and provides the atomistic foundation for the electrical control of strain gradients and domain walls.

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• Elasticity
• Electric polarization
• Electrostriction
• Ferroelectricity
• Lattice dynamics
• Order parameters
• Stress
• Density functional calculations