The CsCl and NaCl structure
The CsCl and NaCl structure
The CsCl and NaCl structure are typical ionic compound structures, consisting of positively charged cations and negatively charged anions.
In these structures a certain number of anions surround each cation and each anion is surrounded by a certain number of anions. The stronger electrostatic attraction there is between these oppositely charged ions, the more energetically stable the structure becomes.
Here we will have a closer look into the CsCl structure and the NaCl structure. You can also make your own 3D paper model, which will help you to explore the differences in the nature. of these structures. Can you answer the questions below?
The caesium chloride structure (CsCl)
The caesium chloride structure adopts a primitive cubic lattice with a two-atom basis. Since there is a centre atom in the unit cell, it is sometimes mistaken as a body-centred cubic structure. However, this centre atom is different from those at the corners of the unit cell. The caesium chloride structure is adopted by ions which are very similar in size. Prominent ionic compounds with this structure are for example CsCl, CsBr and CsI, but also many binary metals adopt this structure.
Make your own 3D paper model of a CsCl crystal with our cutting instructions, and explore its nature.
[Figure] [Instruction] [Template]
The Sodium Chloride structure (NaCl)
Sodium Chloride, also known as Rock salt structure, adopts a face centred cubic (fcc) lattice with a two-atom basis. This can be also represented as two separate fcc arrays for the negatively charged anions (e.g. Cl-) and the positively charged cations (e.g. Na+), which are separated by ½ of the body diagonal of the cubic unit cell.
This structure is adapted for example by potassium chloride (KCl), sodium chloride (NaCl), and lithium fluoride (LiF).
[Figure] [Instruction] [Template]
Questions
Have a close look on your 3D paper model.
How many nearest neighbours does each ion have? Take into account that each cubic unit cell is surrounded by several similar unit cells.
What is the coordination between the oppositely charged ions? Along which lines do they touch?
How many atoms are in each unit cell?
What is the minimal size ratio between the ions to prevent ions of the same kind touching each other?
Na+ and Cs+ have a radius of rNa+ = 0.97Å and rCs+ = 1.69Å respectively, while Cl- has a radius of rCl− = 1.81Å. Calculate the edge length of the cube, which is also known as lattice parameter. Calculate the packing efficiency for NaCl and CsCl and compare the results for both structures.
Why do NaCl and CsCl adopt different crystal structures?
We have mentioned earlier that in ionic crystal there are strong electrostatic forces between the opposite ions, which makes these structures very stable. Therefore, in these structures the ions try to maximize the number of oppositely charged ions around them by filling the holes between the larger ions. The number of oppositely charged nearest neighbours and the coordination between them strongly depends on the structure:
In the CsCl structure each anion is touched by 8 cations and each cation is surrounded by 8 anions
In the NaCl structure each anion is touched by 6 cations and each cation is surrounded by 6 anions
So, when in the CsCl structure each ion can assemble more ions of the opposite charge, why do not Na and Cl ions adopt this structure?
The reason for this lies in the very different size of the Na and Cl ions. Generally, a structure becomes more stable if ions maximise the number of oppositely charged ions around them. But this is limited to some extent. Because if ions of the same kind (like the larger ions) touch each other, repulsion is introduced which makes the crystal lattice less stable. This is the case when the size difference between the ions is too small to effectively separate the larger ions.
In the CsCl structure the size ratio of the ions needs to be larger than 0.732 to prevent touching, which is why this structure is usually adopted by ions of similar size.
The NaCl is typically adapted, when the size ratio of the ions is between 0.414 and 0.732.
For size ratios of only 0.225 to 0.414 a wurtzite or zincblende structure (with only 4 other neighbouring ions) is adapted. We will discuss these two structures in one of the next sections.