---
title: "Ferroelectricity versus piezoelectricity"
subject: "Electrical Engineering Material"
chapter: 3
key: "semester_3/electrical_engineering_material:ferroelectric-vs-piezoelectric"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/ferroelectric-vs-piezoelectric
site: BCT Engineering
---

# Ferroelectricity versus piezoelectricity

Electrical Engineering Material · Chapter 3: Dielectric Materials

- Asked in: 2075 Asoj, 4 marks; 2072 Kartik, 4 marks (asked 2 times)
- Video: [Ferroelectricity||Applied Physics || Dielectrics|| Piezoelectricity](https://www.youtube.com/watch?v=6mYxuCg4gSQ) — SRI ACADEMY OF PHYSICS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/ferroelectric-vs-piezoelectric
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-3

## Question

What is ferroelectricity and piezoelectricity? Write their similarities and differences.

## Short answer

Ferroelectricity is spontaneous reversible polarization with P–E hysteresis, lost above Curie temperature (e.g., BaTiO3). Piezoelectricity generates polarization under stress and deformation under field in non-centrosymmetric crystals (e.g., quartz). Both lack symmetry centre; every ferroelectric is piezoelectric but not vice versa—ferroelectrics have high permittivity and hysteresis, piezoelectrics show linear stress–polarization and no Curie point.

## Answer

**Ferroelectricity** is the property of some crystals to have a **spontaneous polarization** (permanent net dipole moment) even without an applied field, whose direction can be reversed by an external field. They show a P-E hysteresis loop and lose this property above the Curie temperature $T_C$. Examples: barium titanate (BaTiO₃, $T_C \approx 120°\text{C}$), Rochelle salt, PZT.

**Piezoelectricity** is the property of some crystals to develop electric polarization (and a voltage) when mechanically stressed (direct effect), and to change shape when an electric field is applied (converse effect). It needs a crystal with **no centre of symmetry**. Examples: quartz, PZT, Rochelle salt, ZnO.

#### Similarities

1. Both occur only in crystals without a centre of symmetry (non-centrosymmetric).
2. Both are linked to displacement of ions creating dipoles.
3. Every ferroelectric is also piezoelectric (and pyroelectric).
4. Both are used in transducers, sensors and capacitors; PZT is both.

#### Differences

| Point | Ferroelectric | Piezoelectric |
|---|---|---|
| Polarization source | Spontaneous, present without field | Induced by mechanical stress |
| P with no stress, no field | Non-zero | Zero |
| Hysteresis (P–E loop) | Yes | No (linear P vs stress) |
| Curie temperature | Lost above $T_C$ | Non-ferroelectric ones (e.g. quartz) have no Curie point |
| Relative permittivity | Very high (1000–10 000) | Moderate (quartz ~4.5) |
| Relation | Subset of piezoelectrics | Wider class (quartz is not ferroelectric) |
| Uses | Capacitors, FeRAM memory | Crystal oscillators, microphones, ultrasonic transducers, gas lighters |

```text
  P                    P
  ^   ____             ^      /
  |  /   /             |     /  slope = d
 -+-/---/--> E        -+----/----> stress
  |/___/               |   /
 Ferroelectric loop    Piezoelectric line
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What defines a ferroelectric material?
   A. Polarization only under mechanical stress
   B. Spontaneous polarization that can be reversed by an applied field, with a P-E hysteresis loop
   C. Zero polarization at all fields
   D. Magnetization reversible by a field
   Answer: B. Ferroelectrics have permanent polarization without field, reversible by field, and lose it above the Curie temperature. (Key point: Ferroelectricity)

2. What is the direct piezoelectric effect?
   A. Shape change under an electric field
   B. Polarization or voltage developed when the crystal is mechanically stressed
   C. Spontaneous polarization at zero stress
   D. Heat produced by a field
   Answer: B. The converse effect is the shape change under a field. (Key point: Piezoelectric effect)

3. Which statement about the two classes is correct?
   A. Every piezoelectric is ferroelectric
   B. Quartz is ferroelectric
   C. Every ferroelectric is also piezoelectric, but quartz is piezoelectric and not ferroelectric
   D. Piezoelectrics show hysteresis, ferroelectrics do not
   Answer: C. Ferroelectrics are a subset of piezoelectrics. (Key point: Relationship)

4. Both effects require a crystal that:
   A. Has a centre of symmetry
   B. Is amorphous
   C. Is metallic
   D. Has no centre of symmetry
   Answer: D. Non-centrosymmetric structure allows net dipoles to appear. (Key point: Symmetry requirement)

## Related in Electrical Engineering Material

- [Ferroelectricity and piezoelectricity with examples](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Aferroelectric-piezoelectric-examples) — Electrical Engineering Material, chapter 3

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