= Piezoelectric material
A <piezoelectric material> couples mechanical deformation with electric polarization. In a linear small-signal description, one convention writes $D_i=d_{ijk}\sigma_{jk}+\epsilon_{ij}E_j$: applied stress contributes to electric displacement. The converse effect produces strain when an <electric field> is applied.
A quartz element can generate an electrical signal under mechanical loading, while an applied alternating voltage can drive a mechanical resonance. This reciprocity makes piezoelectric materials useful for sensors, actuators and <frequency> control. The coefficients depend on direction and crystal symmetry; an isotropic scalar coefficient cannot describe every orientation of a single crystal.
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