CSCL Crystal Structure - Confused by Clarity?

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Discussion Overview

The discussion revolves around the crystal structure of cesium chloride (CsCl) and its comparison to the structures of cesium (Cs) metal. Participants explore the characteristics of simple cubic and body-centered cubic (bcc) structures, questioning the classification and symmetry of these arrangements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how CsCl can be classified as simple cubic despite appearances suggesting a bcc structure, particularly when comparing it to the structure of cesium metal.
  • Another participant explains that the lattice formed by cesium and chloride ions in CsCl is primitive cubic due to the presence of one Cs and one Cl atom in the elementary cell.
  • A participant questions the classification of CsCl as simple cubic rather than bcc, noting that the structure appears similar to bcc when considering two cesium atoms.
  • It is noted that the symmetry of Cs is higher than that of CsCl because Cs atoms are identical, while Cs and Cl in CsCl are not, which affects the classification.
  • One participant clarifies that for a structure to be considered bcc, all atoms in the basis set must be the same, indicating that CsCl, despite its appearance, does not meet this criterion.
  • A participant inquires about the number of nearest and second nearest neighbors in the CsCl structure, suggesting that the four Cl atoms at the corners of each face are bonded to a Cs atom.

Areas of Agreement / Disagreement

Participants express differing views on the classification of CsCl and its structural symmetry, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

Participants discuss the implications of symmetry and atomic identity in determining crystal structure classification, highlighting potential limitations in understanding based on visual similarity alone.

leviathanX777
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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?
 
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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.
 
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?
 
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.
 
leviathanX777 said:
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
 
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?
 

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