Wurtzite 6: holes that bridge unit cells

The green tetrahedron at the top right of the structure when you open the page is the one that was shown in the unit cell on the previous page. The one to the left of it is around one of the zinc ions that was at the edge of the unit cell of the previous page. The structure to the left illustrates that the corner zinc ions in the unit cell reside at the centre of tetrahedral holes that are shared with four other unit cells. This is analogous to edge-centred ions in cubic unit cells being shared between four unit cells. The difference is that the four unit cells are at 60 and 120 degrees to each other rather than just 90 degrees as is the case with cubic cells. Since there are four edge-centred zinc ions in each wurtzite unit cell, each unit cell contains a third or a sixth of a hole from each of the four edge-centred zinc ions, summing to give one hole altogether on the edge-centres of the unit cell. This adds up with the one occupied tetrahedral hole that is fully inside the cell to give a total of two occupied tetrahedral holes in the cell, as we predicted on the previous page.

Note that the zinc ions occupy alternate segments within the hexagonal framework such that they form tetrahedra, half of which are occupied by sulfide ions. This will be easier to appreciate on the coming pages.

The following pages take a more advanced approach.

Go to page 7 to consider the equivalence of the zinc and the sulfide ions in the wurtzite structure.

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