QB_PH_SEC7_029: Dielectric, Magnetic Properties & Superconductivity

A ferroelectric crystal (such as $\text{BaTiO}_3$) exhibits which characteristic physical property below its Curie temperature $T_c$?

Correct Answer

### Step-by-Step Derivation & Verification: 1. **Ferroelectric State Definition**: Ferroelectricity is the electrical analogue of ferromagnetism. 2. **Key Physical Signatures**: - Possesses a non-zero macroscopic electric polarization $\vec{P}_s$ even with applied electric field $\vec{E} = 0$ (spontaneous polarization). - Displays an electric hysteresis loop ($P$ vs $E$). - The direction of $\vec{P}_s$ can be reversed/reoriented by applying an electric field greater than the coercive field $E_c$. - Above $T_c$, it undergoes a structural phase transition into a paraelectric state where susceptibility obeys the Curie-Weiss law: $\varepsilon_r \propto \frac{1}{T - T_c}$. 3. **Conclusion**: Option (A) is the authentic definition.