# Cesium chloride crystal structure

Cesium chloride has a cubic structure with eight $$Cs$$ atoms at each corner of each cube and one $$Cl$$ in the middle of the cube. It is stated in my solid state book that this crystal cannot be described by a BCC lattice due to the fact that the atoms are not equivalent whereas they should be by definition of a lattice.

But I have some trouble understanding this. I thought that a lattice is just a mathematical handiness of describing the real physical crystal. A lattice consists of points (whereas the atoms are not points) and is independent on the crystal. Why can’t we just put points on the atoms and describe the crystal completely this way? Why do the atoms need to be equivalent such that we can use a certain lattice ?

• You need a lattice (each lattice point looks the same) and a basis, the thing you put on the lattice point. Since a caesium atom is not a chlorine atom, they are not the same. – Jon Custer Jan 24 '19 at 1:31
• @JonCuster That comment looks suspiciously like an answer... ;) – PM 2Ring Jan 24 '19 at 6:17
• FWIW, there are diagrams of the alkali metal halide crystals on Wikipedia. With the lighter metals they are FCC, but with the larger Cs ion they're BCC (except for the fluoride). – PM 2Ring Jan 24 '19 at 9:24