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

# Ferroelectricity and piezoelectricity with examples

Electrical Engineering Material · Chapter 3: Dielectric Materials

- Asked in: 2068 Shrawan, 8 marks
- Video: [Ferroelectrics and Piezoelectrics](https://www.youtube.com/watch?v=fI4ihFp_6Pk) — Taylor Sparks
- 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-piezoelectric-examples
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-3

## Question

What is Ferroelectricity and Piezoelectricity? Explain with the help of suitable example.

## Short answer

Ferroelectricity: spontaneous polarization reversible by an applied field, exhibiting a hysteresis loop (e.g., BaTiO₃ below 120 °C). Piezoelectricity: generation of polarization under mechanical stress and the converse (e.g., quartz in crystal oscillators). All ferroelectrics are piezoelectric, but not vice versa.

## Answer

#### Ferroelectricity
**Ferroelectricity** is the property of some crystals to show **spontaneous polarization** (even without an applied field) that can be **reversed** by an external electric field. It is the electrical analogue of ferromagnetism.

Main features:
- **P–E hysteresis loop**, with remanent polarization $P_r$ and coercive field $E_c$.
- **Domains**: regions with the same direction of polarization.
- **Curie temperature $T_c$**: above it the material becomes paraelectric, and $\varepsilon_r = \dfrac{C}{T - T_c}$ (Curie–Weiss law).
- Very high permittivity near $T_c$ ($\varepsilon_r$ of thousands).

```text
          P
          ^    ____
          |   /   /
     -Ec  |  /   /   Pr
    ------+-/---/------> E
         / /  | +Ec
     ___/ /   |
```

**Example: Barium titanate (BaTiO₃)**. Above 120 °C it is cubic, with Ti⁴⁺ at the centre of the O²⁻ octahedron (no dipole). Below 120 °C the cell becomes tetragonal and Ti⁴⁺ shifts off centre, giving each cell a permanent dipole. Neighbouring cells align, forming domains. Other examples: PZT, Rochelle salt, KH₂PO₄.

**Uses**: high-value ceramic capacitors (MLCC), FeRAM non-volatile memory, thermistors (PTC).

#### Piezoelectricity
**Piezoelectricity** is the generation of electric polarization (voltage) when a crystal is **mechanically stressed** (direct effect), and the change of its dimensions when an **electric field** is applied (converse effect). It occurs only in crystals **without a centre of symmetry**.

- Stress shifts the positive and negative ion centres unequally, creating a net dipole moment: $P = d\,T$ (d = piezoelectric coefficient, T = stress).
- Converse: strain $S = d\,E$.

```text
   force
     |
   +---+         + + + +
   |   |  --->   crystal   ---> voltage V
   +---+         - - - -
     |
   force
```

**Example: Quartz (SiO₂)**. Squeezing a quartz plate gives a voltage across its faces; applying AC voltage makes it vibrate at a very stable natural frequency. Other examples: PZT, Rochelle salt, ZnO, PVDF.

**Uses**: quartz crystal oscillators in watches and microprocessors, gas lighters, microphones, ultrasonic transducers, pressure sensors, piezo buzzers.

#### Relation
All ferroelectrics are piezoelectric (they lack a centre of symmetry), but not all piezoelectrics are ferroelectric; quartz is piezoelectric but not ferroelectric.

## Check yourself

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

1. Ferroelectricity is the property of:
   A. Spontaneous polarization that can be reversed by an electric field
   B. Polarization only under mechanical stress
   C. Magnetization reversible by a field
   D. Zero permittivity near Tc
   Answer: A. Ferroelectrics have spontaneous polarization that reverses with field, giving a P-E hysteresis loop. (Key point: Ferroelectricity)

2. Above the Curie temperature a ferroelectric:
   A. Stays ferroelectric
   B. Becomes paraelectric, εr = C/(T - Tc)
   C. Becomes piezoelectric only
   D. Has εr independent of T
   Answer: B. Above Tc spontaneous polarization vanishes and the Curie-Weiss law applies. (Key point: Curie temperature)

3. Why does BaTiO3 have a dipole below 120 °C?
   A. Oxygen leaves the lattice
   B. Ti4+ shifts off centre in the tetragonal cell
   C. Ba shifts off centre
   D. The cell turns cubic
   Answer: B. In the cubic phase Ti sits at the centre with no dipole; below 120 °C it displaces. (Key point: BaTiO3 origin)

4. Piezoelectricity occurs only in crystals:
   A. That are ferromagnetic
   B. That are amorphous
   C. Without a centre of symmetry
   D. With a centre of symmetry
   Answer: C. Stress shifts ion centres unequally only if the crystal lacks inversion symmetry. (Key point: Piezoelectric condition)

## Related in Electrical Engineering Material

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