Relaxors is the name given to a group of ferroelectric materials which strongly change their shape in electric fields.
Examples include : lead magnesium niobate, lead magnesium niobate-lead titanate, lead lanthanum zirconate titanate, and lead scandium niobate. There are also examples based on barium.
They have been studied for more than 50 years, but the physics underlying their properties is unknown.
Relaxors are characterized by a frequency-dependent peak in the dielectric permittivity and are critical to modern technological applications because they exhibit large dielectric constants and unparalleled piezoelectric coefficients. Despite decades of study a fundamental understanding of the origin of relaxor behavior is lacking.
Proc Natl Acad Sci U S A. 111(5)
Note : Many non-conducting materials (especially crystals) change their shape to some extent if subjected to large electric fields, but the effect is comparatively very small.