Polymorf - Knowhwere- Wurtzite

MINERALS AND SYNTHETIC COMPOUNDS, A_mX_n, IN WHICH X ATOMS ARE

CLOSE PACKED AND A ATOMS OCCUPY TETRAHEDRAL INTERSTICES

Wurtzite

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tetra	50%	hcp	3-d	4 vertices	A- 4c, X- 4c/hcp

structure

In the wurtzite structure A atoms occupy fifty percent of the interstitial voids of

HCP packed X atoms. Wurtzite is the high temperature polymorph of the FCC

zincblende (sphalerite) structure.

Ex. ZnS ( zinc sulfide, wurtzite ) - high temperature polymorph ( Zn - A, S - X )

Crystal lattice

The wurzite lattice structure is modeled as a HCP packing of sulfur anions (X). The

occupied AX₄ groups are colored differently and pointed in the opposite direction

than tetrahedrally coordinated groups of sulfur atoms whose interstitial voids are

unoccupied (blue).

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		Figure 64 - Wurtzite lattice structure
click image to enlarge


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The lattice model clearly shows the eclipsed HCP arrangement of the AX₄ groups

in the successive vertical layers as well as the fifty percent occupancy rate.

Polyhedral framework

The vertex linkage of the AX₄ tetrahedral coordination groups is clearly evident

in this model. Compare the HCP wurtzite model with that of the FCC zincblende

polyhedral framework model shown previously. The following images portray

overhead images of them looking down on the (111) plane.

Compare the eclipsed arrangement of the AX₄ coordination groups in wurtzite and

the staggered arrangement in zincblende. Recall that the only difference between

the HCP and FCC sphere packings is the positioning of the third, differentiating, layer

of triangulated close packed spheres relative to the first layer. Only a slight shift in

their relative positions brings about the change in the structure. Both minerals have

the same chemical composition. The increased excitation of the atoms caused by

the higher temperatures present when wurtzite crystallizes imparts enough energy to

the system to cause the shift.

Crystal habit

short prismatic to tabular

Isostructural minerals/compounds

AgI, AlN, BeO, CdS, CdSe, Cu₃AsS₄, CuBr, CuCl, CuH, CuI, GaN, InN, MgTe,

MnS, MnSe, MnTe, NbN, TaN, ZnO, ZnSe, ZnTe

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Page 45 - Structure matters - Wurtzite