In this three dimensional arrangement of the unit cell, spheres of the third layer are placed on triangular-shaped tetrahedral voids of the second layer.The coordination number is 12 in either of these two structures.Ĭharacteristics of Hexagonal Close Packing:
In either of them, each sphere is in contact with twelve spheres. Types of close packing are highly efficient and 74% space in the crystal isįilled. This structure is called a hexagonal close-packed (hcp) structure.Īrrangement of atoms is found in many metals like magnesium and zinc. This pattern is often written as ABAB ……. Thus, the pattern of spheres is repeated in alternate layers. In this case, the spheres of the third layer are exactly aligned with those of the first layer. Tetrahedral voids of the second layer may be covered by the spheres of the third layer. Tetrahedral Voids (Hexagonal Closed Packing): Have been marked as ‘O’ in the figure above. The apex of the triangle pointing upwards and the other downwards. The triangular voids in the second layer are above the triangular voids in theįirst layer, and the triangular shapes of these do not overlap. These voids are called tetrahedral voids because a tetrahedron isįormed when the centres of these four spheres are joined. At other places, Layer is above the void of the first layer (or vice versa) a tetrahedral void Rise to two different types of holes or voids. Observed from the figure that not all the triangular voids of the first layerĪre covered by the spheres of the second layer. Placing the third layer over the second layer can be done in two ways. T = Tetrahedral void and O = Octahedral void In one row, the apexes of the triangles are pointing upwards and in the next layer downwards. The triangular voids are of two different types. It can be seen in the figure that in these rows there are some voids (empty spaces). Spheres are at the corners of a regular hexagon hence this packing is called Each sphere is in contact with six of its neighboursĪnd the two-dimensional coordination number is 6.
#VOLUME OCCUPIED BY ATOMS HCP FREE#
Hence this arrangement is of ABAB type.Īrrangement, there is less free space and this packing is more efficient than The similarly placed fourth row will be aligned with those of the second Placed adjacent to the second in a staggered manner, its spheres are aligned Second row is different and may be called ‘B’ type. Manner such that its spheres fit in the depressions of the first row.Īrrangement of spheres in the first row is called ‘A’ type, the one in the Arrangement, the second row may be placed above the first one in a staggered
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