Question

Discuss the structure of the following ionic crystal types: (a) Rock salt type structure (b) Fluorite-type structure (c) Zinc-blende-type structure

Short answer

Rock salt structure consists of an FCC lattice of anions with a cation in each octahedral hole. Fluorite structure also forms an FCC lattice of anions, but with a cation in each tetrahedral hole. Zinc-blende structure consists of FCC lattices of both cations and anions, with cations occupying half of the tetrahedral holes.

Step-by-step solution

Discuss Rock salt type structure

The rock salt structure is common for salts. This type of crystal structure consists of an FCC (face-centered cubic) lattice of anions (negatively charged ions) with a cation (positively charged ion) in each octahedral hole in the lattice. An example of an ionic solid with this structure is sodium chloride (NaCl). Each ion is surrounded by six ions of the opposite charge, and this configuration is stable due to strong ionic bonds.

Discuss Fluorite-type structure

In the fluorite structure, the anions again form an FCC lattice, but in this case, there is a smaller cation in each tetrahedral hole. This structure is named for the mineral fluorite, calcium fluoride (CaF2), where the Calcium ions (Ca2+) are surrounded by eight fluoride ions (F-) and each F- ion is surrounded by four Ca2+ ions.

Discuss Zinc-blende-type structure

Zinc-blende is another type of FCC lattice, but this time both cations and anions both form these structures, with the cations occupying half of the tetrahedral holes. It is named after the mineral Zinc-blende, in which the zinc and sulphide ions form this structure.

Key concepts

Rock Salt Structure

The rock salt structure is a fundamental concept in understanding ionic crystals, common in many familiar salts. This structure features an FCC lattice where each anion, the negatively charged ion, occupies the corners and face center positions of the cubic unit cell. Meanwhile, each cation, the positively charged ion, is nestled within the octahedral holes created by the anions. A prime example of this structure is seen in sodium chloride (NaCl).

An important characteristic of the rock salt structure is its coordination number. Each ion in this arrangement is coordinated with six ions of the opposite charge. This results in a stable and robust ionic bond, maintaining a strong electrostatic attraction between the oppositely charged ions.

Beyond NaCl, other compounds like lithium bromide (LiBr) and magnesium oxide (MgO) exhibit the rock salt structure, showcasing its prevalent nature in ionic compounds. This structure's efficiency is key in maximizing ionic interactions for stability.

Fluorite Structure

The fluorite structure presents another important type of ionic crystal framework. Named after the mineral fluorite (\[\text{CaF}_2\]), it is characterized by an FCC lattice of anions which houses cations within the tetrahedral holes. In the fluorite structure, calcium ions (\[\text{Ca}^{2+}\]) are surrounded by eight fluoride ions (\[\text{F}^-\]), while each fluoride ion is surrounded by four calcium ions.

This specific arrangement creates a high degree of ionic interaction due to the rectangular array, promoting stability. The high coordination number for the cations, which is eight in this structure, is typical when smaller anions are involved.

The fluorite structure is not only limited to calcium fluoride. Other compounds such as uranium dioxide (\[\text{UO}_2\]) and thorium dioxide (\[\text{ThO}_2\]) also crystallize in a similar pattern, indicating its significance and adaptability in various chemical environments.

Zinc-blende Structure

The zinc-blende structure further diversifies ionic crystal systems. This structure involves an FCC arrangement where both cations and anions contribute to the framework. Unlike the traditional single charge-type dominance seen in other structures, zinc-blende features the cations occupying exactly half of the tetrahedral holes in the lattice. An iconic representative of this structure is zinc sulfide (\[\text{ZnS}\]), after which the structure itself is named.

The zinc-blende structure is particularly noted for its tetrahedral coordination. Each ion, whether a cation or anion, is surrounded tetrahedrally by four oppositely charged ions. This results in a coordination number of four. Such tetrahedral coordination is crucial for the compound's stability and influences its electronic properties.

Interestingly, this structure is prevalent in many semiconductors, bringing attention to its importance in materials science. Other related compounds, such as copper (I) iodide (\[\text{CuI}\]) and cadmium sulfide (\[\text{CdS}\]), exhibit similar structural characteristics related to zinc-blende.