CCP and HCP crystal structure

 Before know about cubic close packing and hexagonal close packing we need to know about close packing. So, close packing is packing of the sphere in such a way that the atoms occupy maximum available space and there is minimum empty space.

   3 dimensionally close packing structures can be divided into two types-

1)     CCP (Cubic close packing) structure or FCC (Face Centered Cubic):

 

                           

It is a type of close packing, in which placing the third layer of spheres on the octahedral voids of the second layer. In this type of arrangement, the third layer is not the repetition of the first layer. So this type of Arrangement is called the “ABCABC” type of arrangement. In this type, each sphere is surrounded by 12 spheres, 6 from the same layer, 3 from the top layer, and 3 from the bottom layer. So the coordination number is 12. The total volume occupied by the sphere is about 74%. Example- Aluminium, Copper, Gold, etc.

      The empty space lies between the spheres after the packing of them, such empty space is called voids or holes or the interstitial sites.

There are three types of voids-

        1) Trigonal voids: The empty space or voids, which are created from three-sphere when they join together, such voids are called trigonal voids. Here the coordination number is 3. 

         

        2) Tetrahedral voids: the voids, which are formed from four spheres when they join together, such voids are called tetragonal voids. When three spheres from one layer and one sphere from the next layer combine together or pack, then these types of voids are generally observed, which are tetrahedral in shape. So, this kind of voids is called tetrahedral voids. Here the coordination number is 4.

 

        3) Octahedral voids: the voids, which are formed from six spheres when they join together, such voids are called octahedral voids. When three spheres from one layer and three spheres from the next layer combine together or pack, then these types of voids are generally observed, which are octahedral in shape. So, this kind of voids is called tetrahedral voids. Though its name "Octahedral" represents the 8 spheres, it is consisting of 6 spheres. So, the coordination number of the octahedral void is 6. The octahedral word only indicates the shape of the voids. The condition behind forming an octahedral void is that the 'apex' of both layers triangles should be opposite in direction. 

           

     

      

         If there are N number of atoms in packing, then the 2N number of Tetrahedral voids and N number of octahedral voids are formed

        Octahedral voids and Tetrahedral voids in FCC:  In FCC, the effective number of atmos is 4. Therefore, the number of tetrahedral voids is twice its atom number that is 8, and the number of octahedral voids is 4.

    In FCC, the octahedral voids are located mainly at the body center (only 1) and the edge center (3)

.       In FCC, the tetrahedral voids are located near the corner atom.

2)    HCP (Hexagonal close packing): It is a type of close packing, in which placing the third layer of the sphere on the tetrahedral voids of the second layer. In this type of arrangement, the third layer spheres are exactly as same as the first layer spheres. So the type of arrangement is “ABAB” type and the structure is called Hexagonal Close Packing. In this type, each sphere is in connecting with 12 other spheres. Six from the same layer, three from the bottom layer, and three from the top layer. So the coordination number is 12. The total volume occupied by the sphere is 74%. Example- Beryllium, Magnesium, cadmium, etc.