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MINERALS AND SYNTHETIC COMPOUNDS, AmXn, WITH CLOSE |
PACKED X ATOMS AND A ATOMS OCCUPYING CUBIC INTERSTICES |
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CsCl |
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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) |
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and the A atoms occupy all of the cubic
interstices. As a result the A atoms are |
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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. |
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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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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 |
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Figure 112 - CsCl polyhedral |
all
faces (CP) |
framework model |
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( 66 S, 96 pinges ) |
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click image to enlarge |
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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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Page
72 - Structure matters - CsCl |
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