(s) about point defects in ionic…

(s) about point defects in ionic crystals.

One or more options correct

Choose the correct statement(s) about point defects in ionic crystals.

A. $\mathrm{NaCl}$ crystals are typically dominated by Frenkel defects because $\mathrm{Na^+}$ is much smaller than $\mathrm{Cl^-}$
B. Heating $\mathrm{NaCl}$ in sodium vapour gives yellow colour from F-centres and makes the crystal diamagnetic
C. Doping $\mathrm{NaCl}$ with a small amount of $\mathrm{SrCl_2}$ creates anion vacancies to maintain electrical neutrality
D. A crystal with Frenkel defects has nearly the same density as the perfect crystal, whereas Schottky defects decrease the density

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Answer: D

(A) False:

\mathrm{Na^+}

and

\mathrm{Cl^-}

have similar enough size and a high coordination number (6), so

\mathrm{NaCl}

shows Schottky defects, not Frenkel; Frenkel defects occur in crystals with a large size difference and low coordination number, such as

\mathrm{AgBr}

and

\mathrm{ZnS}

. (B) False: sodium vapour gives a metal excess (anion vacancies occupied by electrons, F-centres). The unpaired trapped electrons make the crystal paramagnetic, not diamagnetic. (C) False: each

\mathrm{Sr^{2+}}

replaces two

\mathrm{Na^+}

ions (charge conservation), leaving one cationic vacancy, so the vacancies are cation vacancies. (D) True: in a Frenkel defect the ion only moves to an interstitial site, so the mass and volume are unchanged, whereas Schottky defects remove ions from the crystal and lower the density. Hence (D) only.

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