Chemistry Reference
In-Depth Information
Table 10.4 Description of salt structures based on the filling of interstitial gaps
Packing of the anions
Occupation by cations
Examples
Cubic closed
all OG
NaCl, KCI, AgCl, MgO, CaO, FeO, PbS
Cubic closed
1/2 OG
MgCI
2
, CaCl
2
, ZnCl
2
, FeCl
2
, CdCl
2
, CaH
2
Cubic closed
All TG
Li
2
O, Na
2
O, K
2
O
Cubic closed
1/2 TG
ZnS (zinc sulfide), CuCl, Agl, HgS
Cubic closed
1/2 OG, 1/8 TG
MgAl
2
O
4
(spinelle), Fe
3
O
4
Hexagonal closed
All OG
NiAs, FeS
Hexagonal closed
2/3 OG
Al
2
O
3
(corunde), Fe
2
O
3
Hexagonal closed
1/2 OG
CdI
2
, PbI
2
, Mg(OH)
2
, Ca(OH)
2
Hexagonal closed
1/2 OG
ZnS (wurtzite), ZnO, CdS
Simple cubic
All HG
CsCl, CsBr, CsI, NH
4
Cl
Simple cubic
1/2 HG
CaF
2,
PbF
2
, BaF
2
Cubic closed by
All OG
FeS
2
(pyrite)
S
2
2
dumb-bell
OG
octahedral gap,
TG
tetrahedral gap,
HG
hexahedral gap
Fig. 10.20 Chemical symbols for the NaCl-structure with decreasing information content
Ionic structures
: The mentioned structures of ionic lattices are known from
instrumental analysis, especially from X-ray structure analysis. In the simplest
case, a Laue diagram can be obtained by bombarding an aligned salt crystal with
a collimated beam of X-rays for some time [
15
]: the pattern of the diffracted beams
on a photo plate reflects the symmetry of the structure. Experts are even able
to produce a three-dimensional structural model of the crystal. Other methods of
X-ray structure analysis can be found in the specific literature.
Symbols for ionic structures
: The structural symbol and the stoichiometric
symbol of a salt can be derived from the known ionic structure by shortening the
structural model. An example is shown for the common structure of sodium
chloride (see Fig.
10.20
). Derived from the 3D sphere packing, the Parth´ symbol
not only outlines the infinite (
) and three-dimensional (3) structure but also
includes the coordination of other ions (6o: six Cl
ions coordinate one Na
+
ion
in octahedral symmetry), and in addition the coordination of the same kind of ions
(Na
+
ions) is given (12c: twelve Na
+
ions coordinate one in the cubic face-centered
geometry).
1
