The
structure of crystalline solids is determined by packing of their constituents
.In order to

understand the packing of the constituent particles in a crystal;
it is assumed that these particles are

hard spheres of identical size (eg those
of metal). The packing of these spheres takes place in such a

way that they
occupy the maximum available space and hence the crystal has maximum density.
This

type of packing is called close packing.

**TWO DIMENSIONAL PACKING (2D):**

When the
rows are combined touching each other, the crystal plane is obtained. The rows
can be combined in two different ways

**(A) Square close packing:**

**(1)**

**The particles when placed in the adjacent rows show a horizontal as well as vertical (head to head) alignment and form squares. This type of packing is called**

**square close packing.**

**(2) Packing efficiency calculation:**

**(1)**One sphere will be in constant contact with 4 other spheres hence coordination number is 4

**(2)**Side of Square is

**(a)**and

**a=2r**where

**r**is the radius of atom (sphere).

**(3)**Area of square = a2 = 4r

^{2}

**(4)**Area of atoms in the square is

**(5)**

**Packing fraction (PE)**, fraction of area occupied by spheres .

**(B)**

**Hexagonal close packing:**

**(1)**

**The particles in every next row are placed in the depressions between the particles of the first row. The particles in the third row will be vertically aligned with those in the first row. This type of packing gives a hexagonal pattern and is called**

**hexagonal close packing**

**(2) Packing efficiency calculation:**

**(1)**One sphere will be in constant contact with 6 other spheres hence coordination number is 6

**(2)**Side of hexagon is

**a**and a=2r where r is the radius of atom (sphere) .

**(3)**Area of hexagon unit cell is 6*area of six equilateral triangles

**(4)**Area of atoms in the hexagon unit

**(5)**

**Packing fraction (PE)**, fraction of area occupied by spheres

**THREE DIMENSIONAL PACKING (3D):**

**(1) 3D SIMPLE CUBIC CELL AND BODY CENTRED CELL (SCC AND BBC):**

**(A) Simple (primitive) Cubic Cell (SCC) structure:**

**(1)**2D square close packing sheets are involved to generate

**simple cubic cell**as well as body centred cell. In which each corner atom is touching potion with its adjacent corner atom.

**(2)**Take two 2D square close packing sheet and Placing a second square packing layer (sheet) directly over a first square packing layer forms a

**"simple cubic"**structure.

**(3)**The simple “cube” appearance of the resulting unit cell is the basis for the name of this three dimensional structure.

**(4)**This packing arrangement is often

**symbolized as "AA..."**, the letters refer to the repeating order of the layers, starting with the bottom layer.

**(5)**The coordination number of each lattice point is six. This becomes apparent when inspecting part of an adjacent unit cell.

**(6)**The unit cell contain eight corner spheres, however, the total number of spheres within the unit cell is 1 (only 1/8th of each sphere is actually inside the unit cell). The remaining 7/8ths of each corner sphere resides in 7 adjacent unit cells.

**(7) PACKING EFFICIENCY AND COORDINATION NUMBER:**

**(2) Body Centered Cubic (bcc) Structure:**

**(1)**A more efficiently packed cubic structure is the "body-centered cubic" (bcc).

**(2)**The first layer of a square array is expanded slightly in all directions. Then, the second layer is shifted so its spheres nestle in the spaces of the first layer.

**(3)**

**This repeating order of the layers is often symbolized as "ABA..."**. Like Figure 3b, the considerable space shown between the spheres in Figure 5b is misleading: spheres are closely packed in bcc solids and touch along the body diagonal.

**(4)**The packing efficiency of the bcc structure is about 68%. The coordination number for an atom in the bcc structure is eight.

**(5)**PACKING EFFICIENCY AND COORDINATION NUMBER:

**THREE DIMENSIONAL CLOSED PACKING (3D):**

**(2) CUBIC CLOSE PACKING AND HEXAGONAL PACKING (CCP AND HCP)**

**Step-(1)**In order to develop

**three dimensional close packing**take a 2D hexagonal close packing sheet as first layer

**(A- layer**).

**Step-(2)**Another 2D hexagonal close sheet

**(B-layer)**is taken and it is just over the depression (Pit) of the first layer (A) .When the second layer is placed in such a way that its spheres find place in the

**‘b’ voids**of the first layer, the

**‘c’ voids**will be left unoccupied. Since under this arrangement no sphere can be placed in them, (c voids), i.e. only half (50%) the triangular voids in the first layer are occupied by spheres of the second layer (i.e. either b or c)

**Step-(3)**There are two alternative ways in which spheres in the third layer can be arranged over the second layer

**(1)**When a third layer is placed over the second layer in such a way that the spheres cover the

**tetrahedral or ‘a’ voids;**a three dimensional closest packing is obtained where the spheres in every third or alternate layers are vertically aligned (i.e. the third layer is directly above the first, the fourth above the second layer and so on) calling the first layer A and second layer as layer B,

**the arrangement is called A**BAB …………. pattern or

**hexagonal close packing (HCP)**as it has hexagonal symmetry.

**(2)**

**When a third layer is placed over the second layer in such a way that spheres cover the**

**octahedral or ‘c’ voids,**a layer different from layers A and B is produced. Let it be layer ‘C’. Continuing further a packing is obtained where the spheres in every fourth layer will be vertically aligned to the spheres present in the first layer.

**This pattern of stacking spheres is called ABCABC ……….. pattern or cubic close packing (CCP).**It is similar to face centred cubic (fcc) packing as it has cubic symmetry

In both
HCP and CCP methods of stacking, a sphere is in contact with 6 other spheres in
its own

layer. It directly touches 3 spheres in the layer above and three
spheres in the layer below. Thus

sphere has 12 close neighbours.
The number of nearest neighbours in a packing is called coordination

number. In close packing arrangement (HCP & CCP) each sphere has
a coordination number of 12.

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