Solid State 04: Class Notes

Flashcards covering various types of crystal defects including stoichiometric defects (Vacancy, Interstitial, Schottky, Frenkel), impurity defects (Substitutional, Interstitial, Aliovalent), and non-stoichiometric defects (Metal Excess, Metal Deficiency).

What are the three main categories of point defects?

Stoichiometric, Non-stoichiometric, and Impurity defect.

How does a vacancy defect affect the density of a substance?

The mass of the substance decreases while the volume remains unchanged, resulting in a decrease in density.

In what two ways can a self-interstitial defect occur in an elemental solid?

1) An extra particle occupies an empty interstitial space in the crystal structure. 2) A particle gets shifted from its original lattice point and occupies an interstitial space.

What defines a Schottky defect?

In an ionic solid, equal numbers of cations and anions are missing from their regular positions, creating paired cation-anion vacancies.

List three conditions for the formation of a Schottky defect.

1) High degree of ionic character. 2) High coordination number of anion. 3) Small difference between the size of cation and anion ($r_{cation}/r_{anion}$ is not far below unity).

What are the consequences of a Schottky defect on mass and density?

The mass decreases as the number of ions decreases, while the volume remains unchanged, causing the density of the substance to decrease.

Define a Frenkel defect.

It arises when an ion of an ionic compound is missing from its regular lattice site and occupies an interstitial position.

Why does the density of a solid remain unchanged in a Frenkel defect?

Because no ions are missing from the crystal lattice as a whole; they are simply displaced to interstitial sites, meaning mass and volume remain constant.

What are the conditions for a Frenkel defect to occur?

It occurs in ionic compounds with a large difference between sizes of cation and anion and a low coordination number.

Name some ionic crystals that exhibit the Frenkel defect.

$ZnS$, $AgCl$, $AgBr$, $AgI$, and $CaF_2$.

What is an impurity defect?

It arises when foreign atoms, which are atoms different from the host atoms, are present in the crystal lattice.

Give an example of a substitutional impurity defect.

Brass, where Copper ($Cu$) is the host atom and Zinc ($Zn$) is the foreign atom found at the lattice sites.

What characterizes an interstitial impurity defect?

The impurity atoms occupy interstitial spaces of the lattice structure, such as Carbon atoms occupying spaces between Iron ($Fe$) atoms in steel.

How does the addition of $SrCl_2$ to $NaCl$ create a vacancy through aliovalent impurity?

One $Sr^{2+}$ ion (O.S. $+2$) replaces two $Na^+$ ions (O.S. $+1$) to maintain electrical neutrality; the $Sr^{2+}$ occupies one site while the other becomes a vacancy.

When does a non-stoichiometric defect arise?

When the ratio of the number of atoms of one kind to another, or the ratio of cations to anions, becomes different from that indicated by the chemical formula.

What are the requirements for a metal deficiency defect?

It is possible only in compounds of metals that show variable oxidation states.

How is electrical neutrality maintained in a metal deficiency defect?

The extra negative charge from missing positive metal ions is balanced by the presence of a cation of the same metal with a higher oxidation state than the missing cation.

Why does $NaCl$ turn yellow when heated in sodium vapour?

$Cl^-$ ions diffuse to the crystal surface to combine with $Na$ atoms, which release electrons that occupy the vacant anion sites to form F-centres.

Give an example of a metal excess defect where a neutral atom occupies an interstitial position.

$ZnO$.