But pure copper and pure silver are far better conductors than they would be if their positive ions scattered electrons. The phenomenon of piezoelectricity arises directly from crystal structure. Certain crystals become electrically polarized when they are squeezed or stretched. Conversely, they become mechanically deformed under the influence of an electric field. These piezoelectric crystals are used in some radio earphones to change electrical energy to mechanical energy and in phonograph pickups to change mechanical energy to electrical energy.
Piezoelectricity depends on the absence of a certain kind of symmetry, called inversion symmetry.
The inverse of a figure is something like its mirror image turned upside-down. If the figure and its inversion look the same, as with a cube, then the figure has inversion symmetry. If they do not look the same, as with a tetrahedron, then the figure does not have inversion symmetry. Crystals that lack inversion symmetry usually can be squeezed so that the positive and negative charges of the atoms do not shift equally, generating an electric potential. No crystal is completely free from defects.