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Structure World

The Cesium Chloride Structure

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General Notes

CsCl has a cubic unit cell. At first glance you might think that it is body-centered, but this would be true only if the atom at the body center was the same kind of atom as those on the corners of the cells. This is clearly not the case.

CsCl can be thought of as two interpenetrating simple cubic arrays where the corner of one cell sits at the body center of the other. As with NaCl, the 1:1 stoichiometry means that the cell will look the same regardless of whether we start with anions or cations on the corner. Note that each ion is 8-coordinate rather than 6-coordinate as in NaCl.

Structural Information

CsCl Vital Statistics
Formula	CsCl
Crystal System	Cubic
Lattice Type	Primitive
Space Group	Pm3m, No. 221
Cell Parameters	a = 4.123 Å, Z=1
Atomic Positions	Cl: 0, 0, 0 Cs: 0.5, 0.5, 0.5 (can interchange if desired)
Density	3.99
Melting Point	646 degrees C
Alternate Names	none
Isostructural Compounds	NH₄X, TlX (X = halide), CuZn (beta-brass), AgZn, LiHg, MgSr

Structure


Fig. 1 A single unit cell of CsCl	Fig. 2 A 3x3x3 lattice of CsCl

not

NaCl


Fig. 3 The (111) plane of CsCl.	Fig. 4 The (100 plane of CsCl.

Something that surprises novices is that the (111) plane does not contain the body centered atom. It is important for the beginning student to recognize that the (111) does not cut through 1/2, 1/2, 1/2! Take a look at the models above and below to convince yourself of this very important fact!
Self-Study: Which set of planes contain the body center and the points (0,0,1), (1/2,0,1/2) and (0,1/2,1/2)?
Self-Study: What would a polyhedral representation of this structure look like?

Chime models

~~Here are two Chime models you can rotate and manipulate~~ (Chime is obsolete. I need to update these to jmol; just haven't found the time yet). Notice the following important features:

The structure is not closest packed, even though it may appear to be hcp in one orientation.
Each Cl has only 6 next nearest neighbor chlorides, unlike NaCl which has 12.
The Cl and Cs occur in simple interpenetrating primitive cubic lattices.


Fig. 5 A single unit cell of CsCl.	Fig. 6 A 2x2x2 chunk of CsCl lattice.

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