Question

When a smaller ion (usually a cation) is dislocated from its normal site in a crystal and moves to an interstitial site, it is known as:

• A. Schottky defect
• B. Frenkel defect
• C. Interstitial defect
• D. Vacancy defect

Hint:

A key distinction: Schottky defects involve missing ions from the crystal, while Frenkel defects involve ions moving within the crystal (dislocation) without leaving the crystal structure. Both maintain overall electrical neutrality.

Answer 1

The Correct Option is B

Step 1: Understand different types of crystal defects.
Crystal defects are irregularities in the regular arrangement of constituent particles (atoms, ions, or molecules) in a crystalline solid. Point defects are common types that involve a missing particle, an extra particle, or a misplaced particle. Step 2: Analyze the given description.
The question describes a specific type of point defect where:
A smaller ion (typically a cation, due to its smaller size)
Is dislocated from its normal lattice site
And then moves to an interstitial site (a vacant space between the normal lattice positions) within the crystal.
This dislocation leaves a vacancy at its original normal site.
The overall electrical neutrality of the crystal is maintained because no ions are lost or gained from the crystal. Step 3: Evaluate each option against the description.
(A) Schottky defect: This defect arises when equal numbers of cations and anions are missing from their lattice sites in an ionic crystal, creating vacancies to maintain electrical neutrality. It does not involve an ion moving to an interstitial site.
(B) Frenkel defect: This defect accurately matches the description. It occurs when an ion (usually the smaller cation) leaves its normal lattice site, creating a vacancy there, and then occupies an interstitial site within the crystal. This defect is a combination of a vacancy defect and an interstitial defect.
(C) Interstitial defect: This is a broader term for a defect where an atom or ion occupies an interstitial site. While a dislocated ion moving to an interstitial site \textit{results} in an interstitial ion, the specific phenomenon described (dislocation \textit{from} a normal site \textit{to} an interstitial site, creating a vacancy) is precisely termed a Frenkel defect.
(D) Vacancy defect: This defect simply describes a missing atom or ion from its regular lattice site. While a vacancy is created in a Frenkel defect, the complete description involves the displaced ion occupying an interstitial site. Step 4: Conclude the correct term.
The phenomenon described---a smaller ion leaving its lattice site and moving to an interstitial site---is the definition of a Frenkel defect.