Simplest example is 2-d hexagonal ionix crystal. Charges alternate on hexagonal grid. Stepping on grid creates electric dipole.

For square matrix there is a center of symmetry which disallows dipole creation.

magnitude of hexagonal dipole:

so dipole size of . The density of dipoles is . Electric field induced is .

Then the electric charge density is . For quartz,

For Rochelle Salt and Barium Titamate , it is . These are ferromagnetic with the curie temperature just below room temperature, so small electric fields can produce large domains.

uses:

• cameras
• oscillating quartz in watches
• microphones
• spark toy. strain, = . This means .

source psfile jl@crush.caltech.edu index