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MINERALS AND SYNTHETIC COMPOUNDS, AmXn, IN WHICH X
ATOMS ARE |
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CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES |
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Wurtzite |
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click here to see a key to this data bar
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tetra |
50% |
hcp |
3-d |
4
vertices |
A-
4c, X- 4c/hcp |
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structure |
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In the wurtzite
structure A atoms occupy fifty percent of the interstitial voids of |
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HCP packed X atoms. Wurtzite is the high
temperature polymorph of the FCC |
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zincblende (sphalerite) structure. |
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Ex. ZnS ( zinc sulfide, wurtzite ) - high
temperature polymorph ( Zn - A, S -
X ) |
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Crystal lattice |
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The wurzite lattice
structure is modeled as a HCP packing of sulfur anions (X). The |
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occupied AX4 groups are
colored differently and pointed in the opposite direction |
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than tetrahedrally coordinated groups of sulfur
atoms whose interstitial voids are |
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unoccupied (blue). |
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Figure 64 -
Wurtzite lattice structure |
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click image to enlarge |
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The lattice model
clearly shows the eclipsed HCP arrangement of the AX4 groups |
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in the successive vertical layers as well as the
fifty percent occupancy rate.
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Polyhedral framework |
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linking surfaces |
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Figure 65 -
Wurtzite polyhedral |
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framework model |
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( 59 T, 66 pinges) |
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4
vertices |
click image to enlarge |
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The vertex linkage of
the AX4 tetrahedral coordination groups is clearly evident |
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in this model. Compare the HCP wurtzite
model with that of the FCC zincblende |
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polyhedral framework model shown previously.
The following images portray |
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overhead images of them looking down on the
(111) plane. |
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| Figure 66 - Comparison of |
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the wurtzite and |
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zincblende structures |
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wurtzite (HCP) |
zincblende (FCC) |
click image to enlarge |
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Compare the eclipsed
arrangement of the AX4 coordination groups in wurtzite and |
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the staggered arrangement in zincblende.
Recall that the only difference between |
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the HCP and FCC sphere packings is the
positioning of the third, differentiating, layer |
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of triangulated close packed spheres relative to
the first layer. Only a slight shift in |
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their relative positions brings about the change
in the structure. Both minerals have |
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the same chemical composition. The
increased excitation of the atoms caused by |
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the higher temperatures present when wurtzite
crystallizes imparts enough energy to |
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the system to cause the shift. |
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Crystal habit |
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short prismatic to
tabular |
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Isostructural minerals/compounds |
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AgI,
AlN, BeO, CdS, CdSe, Cu3AsS4, CuBr, CuCl, CuH, CuI,
GaN, InN,
MgTe, |
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MnS, MnSe, MnTe, NbN, TaN, ZnO, ZnSe, ZnTe |
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Page
45 - Structure matters - Wurtzite |
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