MINERALS AND SYNTHETIC COMPOUNDS, AmXn, WITH CLOSE

PACKED X ATOMS AND A ATOMS OCCUPYING CUBIC INTERSTICES

 
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CsCl

                  click here to see a key to this data bar                                  

cubic

100%

cp

3-d

all faces

A- 8c, X- 8c/cp

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  structure

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. .
     In the CsCl structure the X atoms are packed CP (simple cubic or cubic primitive) .
and the A atoms occupy all of the cubic interstices.  As a result the A atoms are .
coordinated with eight X atoms and vice versa.  The CsCl structure is exhibited by
ionic compounds where the cations and anions have equal but opposite charge and
nearly equal ionic radii.
  .
Ex. CsCl  ( cesium chloride )           ( Cs - A , Cl - X ) .
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Crystal lattice
     The crystal structure could be modeled as a CP lattice with the understanding
that each cube represents an AX8 coordination group of atoms and the vertices
represent the locations of the CP packed X atoms.  However since both the A and
X atoms have the same coordination it is more informative to model the structure as
two interpenetrating CP lattices.  That is, the vertices of one lattice represent the X
atoms and the vertices of the other lattice represent the A atoms.  The following
image shows how the successive layers alternate between A atoms and X atoms.
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CsCl.gif

 
       Figure 111 -  CsCl lattice structure

click image to enlarge

                       
   
 
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The X atoms are located at the intersections of the solid green square grid and the
A atoms are positioned at the intersections of the solid purple grid.
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Polyhedral framework
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linking surfaces

CsCl.gif

 

 

         Figure 112 -   CsCl polyhedral

all faces (CP)

                   framework model
                     ( 66 S,  96 pinges )

 

                          click image to enlarge
.      
     The polyhedral framework could be modeled as a simple CP structure or as two
interpenetrating CP frameworks as pictured here.  The X atoms are located at the
vertices of the green 2o cube and the A atoms at the vertices of the purple 2o cube.
The interpenetrating model demonstrates the topological equivalency of the atoms.
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Isostructural minerals/compounds
AgCd, AgMg, AgZn, AuMg, AuZn, BeCo, BeCu, BePd, CaTl, CoAl, CsBr, CsI, CuPd,
CuZn, LiAg, LiHg, TiTl, MgHg, MgSr, MgTl, NiAl, NiTi, SrTl, TlBi, TlBr, TlCl, TlI, TlSb
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