Ionic Dielectrics – Components of Polarization (Assertion–Reason) Assertion (A): In ionic dielectrics, the total polarization can be written as P = Pe + Pi (sum of electronic and ionic polarization). Reason (R): In equilibrium, an ideal ionic crystal has zero permanent electric dipole moment per unit cell (no intrinsic orientation polarization).

Introduction / Context:
Polarization mechanisms in solids include electronic polarization (displacement of electron clouds relative to nuclei), ionic polarization (relative displacement of positive and negative ions), and orientation polarization (alignment of permanent molecular dipoles). Ionic crystals like NaCl do not possess permanent dipoles in their centrosymmetric equilibrium structures, so their polarization under field arises from induced mechanisms only.

Given Data / Assumptions:

• Ideal stoichiometric ionic crystal with centrosymmetric unit cell.
• No pre-existing permanent molecular dipoles in equilibrium.
• Linear response regime under applied electric field.

Concept / Approach:

Because orientation polarization requires permanent dipoles, an ionic crystal contributes primarily Pe (electronic) and Pi (ionic) when an external field is applied. The electronic part arises from deformation of electron clouds; the ionic part from small relative shifts of sublattices of cations and anions. Since there is no intrinsic permanent dipole moment in equilibrium (by symmetry), orientation polarization is absent, justifying the decomposition P = Pe + Pi.

Step-by-Step Solution:

Verification / Alternative check:

Infrared/optical spectroscopy shows optic phonon modes associated with ionic displacement (Pi) and electronic polarizability (Pe); absence of permanent dipole alignment confirms R.

Why Other Options Are Wrong:

• If R were false, one would expect an orientation term; typical ionic crystals lack it due to symmetry.
• Declaring A false contradicts standard dielectric theory for ionic solids.

Common Pitfalls:

Confusing ferroelectric ionic crystals (where symmetry is broken and spontaneous polarization exists) with ordinary ionic dielectrics; mixing up induced and permanent dipoles.

Final Answer:

Both A and R are true and R is correct explanation of A