Q. 38

Which of the following is not true about the voids formed in 3 dimensional hexagonal close packed structure?
A. A tetrahedral void is formed when a sphere of the second layer is present above triangular void in the first layer.

B. All the triangular voids are not covered by the spheres of the second layer.

C. Tetrahedral voids are formed when the triangular voids in the second layer lie above the triangular voids in the first layer and the triangular shapes of these voids do not overlap.

D. Octahedral voids are formed when the triangular voids in the second layer exactly overlap with similar voids in the first layer.

Answer :

All real structures are three dimensional. Hexagonal close packed structure is formed from three dimensional close packing from two dimensional hexagonal close packed layers placed on top of each other. Here, the second layer is aligned on the first layer such that the spheres of the second layer fits into the depressions i.e. the spaces between the spheres of the first layer. Not all the triangular depressions of the first layer is covered by the spheres of the second layer. Option (i) and (ii) are correct statements. If we observe this structure further, we see the formation of tetrahedral and octahedral voids. When a sphere of the second layer is located above the triangular void of the first layer, the shape of the void formed when the centers of these four spheres are joined is a tetrahedron and is known as tetrahedral void. In other places in the structure, the triangular voids of the second layer lie above the triangular voids of the first and the triangular shapes do not overlap, forming a void surrounded by six spheres and is known as octahedral void and joining the centers of these spheres forms an octahedron. Now in the case of hcp structures, the spheres are alternately aligned, as in the spheres of the third layer are aligned with the spheres of the first layer. From these statements, option (iii) and (iv) are incorrect.

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