Piezoelectric and ferroelectric materials

Squeeze an ordinary block of metal and nothing electrical happens — its crystal structure has a centre of symmetry, so any internal shift in charge cancels out overall. Squeeze a crystal like quartz, which lacks that symmetry, and the story is different: compressing it shifts the centres of positive and negative charge in each unit cell apart from each other, and those tiny shifts add up across the whole crystal into a measurable voltage across its faces. This is the piezoelectric effect, and it isn't a one-way trick: run it backwards by applying a voltage across the same crystal, and it deforms slightly in response. That converse effect is what drives piezoelectric actuators, ultrasound transducers and the buzzer in a quartz watch, while the forward effect is what makes piezoelectric materials useful as force and vibration sensors, lighters, and microphone pickups.

Ferroelectric materials are built from the same lack-of-symmetry idea, but taken a step further. In a ferroelectric crystal like barium titanate, the positive and negative charge centres are offset even with no external field at all, giving the material a permanent, spontaneous electric polarization — and crucially, that polarization's direction can be flipped by a strong enough applied field and will then stay flipped once the field is removed. That's a direct electrical parallel to ferromagnetism: ferroelectric materials have domains, a polarization-versus-field hysteresis loop, and a Curie temperature above which the spontaneous polarization disappears and the material becomes an ordinary (paraelectric) dielectric.

That retained, switchable polarization is exactly what makes ferroelectrics useful for non-volatile memory: a bit can be stored as one of the two stable polarization directions, and it stays stored with the power off, the same way a permanent magnet stays magnetized without a field holding it there.