Introduction

Crystalline solid material is one of the two major types of solid materials. Another is amorphous solid. Generally, crystalline solids are composed of atoms or ions. These atoms, ions, or molecules pack in regular geometric arrays called unit cell. The unit cell is a structural component that, when repeated in all directions, result in a macroscopic crystal.

Simple structures

Seven crystal systems are listed in the following table, which can describe all crystals.

The positions of atoms, ions or molecules are described in lattice points in unit cell. By putting points at the corners of the unit cell, the seven crystal systems can be obtained.

However, there are other arrangements of points. In 1848, Bravais, a French crystallographer, demonstrated that there are 14 point lattices. These 14 possible crystal structures are called Bravais lattices showed in the following table.

Structures of binary compounds

Binary compounds, containing two elements, can be described in seven different specific crystal structures.

1. Sodium chiloride, NaCl (rock salt)

NaCl: FCC Cl with Na in all octahedral sites. There are 4 Na and 4 Cl in each unit cell. Each unit cell has 4 formula units of NaCl structure.

2. Cesium chiloride, CsCl

CsCl: simple cubic Cl with Cs in all cubic sites. There are 1 Cs and 1 Cl in each unit cell. Each unit cell has 1 formula unit of CsCl structure.

3. Zinc blende, ZnS

Zinc blend: FCC S with Zn in half tetrahedral sites. There are 4 Zn and 4 S in each unit cell. Each unit cell has 4 formula units of ZnS structure.

4. Wurtzite, Zns

Wurtzite: HCP S with Zn in half tetrahedral sites. Both Zn and S are in tetrahedral hole of the other lattice. Each type of ion forms a hexagonal closed=packed lattice. There are 4 Zn and 4 S in each unit cell. Each unit cell has 4 formula units of wurtzite structure.

5. Fluorite, CaF2

Fluorite: FCC Ca with F in all tetrahedral sites. There are 4 Ca and 8 F in each unit cell. Each unit cell has 4 formula units of fluorite structure.

6. Nickel arsenide, NiAs

NiAs: HCP As with Ni in all octahedral sites. There are two Ni and 2 As in each unit cell. Each unit cell has 2 formula units of NiAs structure.

7. Rutile, TiO2

Rutile has a primitive tetragonal unit cell. The coordination number for Ti is 6, for O is 3. There are two Ti and four O in each unit cell. Each unit cell has 2 formula units of TiO2 structure.

Reference

Miessler, Gary L., and Donald A. Tarr. Inorganic Chemistry. Upper Saddle River, NJ: Pearson Prentice Hall, 2011. Print.