3. Explain why ferroelectric crystals already have, with zero applied electric field, a net dipole moment. What is the Curie temperature?

Ferroelectric materials possess a dielectric polarization without an applied electric field. Examples of ferroelectric materials include potassium dihydrogen phosphate (KH2PO4), barium titanate (BaTiO3) and Lithium niobate (LiNbO3).

Ferroelectric materials undergo a change based on temperature. Above a temperature called the Curie temperature, the crystal is cubic, but below the Cure temperature they are tetragonal. Undergoing the change to tetragonal, with an electric polarization.

Ferroelectric materials are also piezoelectric, meaning that they produce an electric charge under a mechanical load.

4. Quote at least one important ferroelectric material exploited in electrooptic modulators.

LiNbO3 is a highly attractive ferroelectric crystal used in electrooptic modulators, due to the strong electrooptic effect, or change in refractive index under an electric field.