Question

What kind of defects, if any, would you expect in the following crystals: (a) \(\mathrm{KF}\) (b) \(\mathrm{MoO}_{3-x}\) (c) \(\mathrm{AgBr}\).

Short answer

In the given crystals: (a) KF most likely exhibits Schottky defects, which maintain overall charge neutrality. (b) MoO$_{3-x}$ has oxygen vacancies, and to maintain overall charge neutrality, some Mo ions will adopt a lower oxidation state. (c) AgBr is more prone to exhibit Frenkel defects, with an Ag+ ion occupying an interstitial position, maintaining overall charge neutrality.

Step-by-step solution

(a) KF Crystal defects

KF is an ionic compound formed by potassium (K) and fluorine (F), where K+ ions and F- ions are arranged in a regular crystal lattice. Since the K+ and F- ions have the same charges but opposite signs, KF crystals are most likely to exhibit Schottky defects. Schottky defects maintain the overall charge neutrality of the crystal.

(b) MoO3-x Crystal defects

MoO3-x is a nonstoichiometric compound where "x" indicates that there are fewer oxygen atoms than required for a stoichiometric MoO3 compound. Due to this imbalance in the number of Mo and O atoms, MoO3-x crystals can have oxygen vacancies. These deficiency-type defects do not maintain charge neutrality, and to maintain overall charge neutrality, some Mo ions will adopt a lower oxidation state.

(c) AgBr Crystal defects

AgBr is an ionic compound formed by silver (Ag) and bromine (Br) ions, with Ag+ ions and Br- ions arranged in a regular crystal lattice. In AgBr, the difference in sizes between the Ag+ and Br- ions is significant. As a result, AgBr crystals are more prone to exhibit Frenkel defects. In this case, an Ag+ ion may be displaced from its normal lattice site and instead occupy an interstitial position in the crystal, maintaining the overall charge neutrality.