CSCL Crystal Structure - Confused by Clarity?

[leviathanX777]

Hello, I'm really confused by something.

Cscl is simple cubic. But how it this possible? With the cl in the middle it looks like a bcc structure. Also howcome when you have 2 caesium its bcc and not simple cubic? its the same structure yet it changes to bcc. Is it because they are both atoms of the same type?

 

[DrDu]

Think of replacing every elementary cell by a single point. The structure formed by these points is called the lattice. In the case of cesium metal, the elementary cell contains just a single atom whence the lattice resembles the origial crystal structure. In the case of CsCl, an elementary cell contains one Cs and one Cl atom. If you replace the two atoms by a single point, the lattice formed from these single points will be primitive cubic.

 

[leviathanX777]

Oh yeah, I know that. But why don't they call it bcc when it looks like a bcc structure and why does it changed from simple cubic to bcc when its 2 caesium?

 

[DrDu]

The symmetry of a Cs crystal is higher than that of CsCl because the Cs atoms are identical while the Cs and Cl in the latter case aren't.

 

[Modey3]

Oh yeah, I know that. But why don't they call it bcc when it looks like a bcc structure and why does it changed from simple cubic to bcc when its 2 caesium?

Because in ordered to be considered BCC every atom (basis set) needs to be the same. Sure CsCl looks like it's BCC, but it doesn't have the same symmetry as pure BCC Cs. BCC is an indicator of symmetry not shape.

modey3

 

[leviathanX777]

Ah ok, thanks for clearing that up. The number of nearest neighbours is 8, is the number of second nearest neighbours 6? As the four cl atoms at the corner of each face are bonded to a cs atom?