Close Packed Structures

The concept of close packing is fundamental to understanding the structure and properties of crystalline solids, particularly metals and many ionic compounds. It describes the most efficient way to arrange identical spheres (representing atoms or ions) in a given volume, minimizing empty space. This efficiency is quantified by 'packing efficiency' and the local environment of each sphere is described by its 'coordination number'.

1. One-Dimensional (1D) Close Packing

In one dimension, spheres are arranged in a linear fashion, touching each other along a straight line. Each sphere has two immediate neighbors, one on either side. Thus, the coordination number in 1D close packing is 2. This is the simplest arrangement and serves as a building block for higher dimensions.

2. Two-Dimensional (2D) Close Packing

Building upon 1D rows, there are two principal ways to arrange spheres in a plane:

a. Square Close Packing (SCP)

In this arrangement, rows of 1D packed spheres are stacked directly on top of each other such that the spheres in adjacent rows align vertically. If we label the first row as 'A', then the second row is also 'A', and so on.

The stacking pattern is AAAA... In this configuration, each sphere in the interior of the layer touches four other spheres (one above, one below, one left, one right). The centers of these four spheres form a square.

Therefore, the 2D coordination number is 4. The empty spaces (voids) formed are square-shaped and relatively large. This arrangement is less efficient than hexagonal close packing.

b. Hexagonal Close Packing (HCP)

This is a more efficient 2D packing. The spheres in the second row are placed in the depressions (valleys) formed by the spheres of the first row. If the first row is 'A', the second row is 'B' (as its spheres are offset).

The third row's spheres are then placed in the depressions of the 'B' row, which align with the 'A' row spheres. The stacking pattern is ABAB... In this arrangement, each sphere in the interior of the layer touches six other spheres, forming a hexagonal pattern around it.

The 2D coordination number is 6. The voids formed are triangular and smaller than the square voids in SCP, indicating higher packing efficiency.

3. Three-Dimensional (3D) Close Packing

Three-dimensional close-packed structures are formed by stacking 2D hexagonal close-packed layers. When a second layer (B) is placed on a first layer (A) such that the spheres of layer B sit in the depressions of layer A, two types of voids are created in the space between the layers:

a. Tetrahedral Voids (T-voids)

These voids are formed when a sphere in the upper layer rests on three spheres in the lower layer, creating a small triangular depression. The void is surrounded by four spheres (three from the bottom layer and one from the top layer), forming a tetrahedron. If there are 'N' spheres in the close-packed structure, there will be $2N$ tetrahedral voids.

b. Octahedral Voids (O-voids)

These voids are larger and are formed when a sphere in the upper layer rests on three spheres in the lower layer, and simultaneously, a void in the upper layer is directly above a void in the lower layer. Essentially, an octahedral void is surrounded by six spheres (three from the bottom layer and three from the top layer), forming an octahedron. If there are 'N' spheres in the close-packed structure, there will be $N$ octahedral voids.

When placing a third layer on top of the second layer (B), there are two distinct ways to achieve close packing, leading to two different 3D structures:

i. Hexagonal Close Packing (HCP)

In HCP, the third layer (C) is placed such that its spheres are directly aligned with the spheres of the first layer (A). The stacking pattern is A-B-A-B-A-B... This results in a hexagonal unit cell. Each sphere in an HCP structure has a coordination number of 12 (6 in its own layer, 3 above, and 3 below). The packing efficiency for HCP is 74%. Examples of metals adopting HCP structure include Magnesium (Mg), Zinc (Zn), Titanium (Ti), and Cadmium (Cd).

ii. Cubic Close Packing (CCP) / Face-Centered Cubic (FCC)

In CCP, the third layer (C) is placed in a new set of depressions, such that its spheres are not aligned with either the 'A' or 'B' layers. The stacking pattern is A-B-C-A-B-C... This arrangement results in a face-centered cubic (FCC) unit cell.

The CCP structure is identical to the FCC lattice. Like HCP, each sphere in a CCP/FCC structure also has a coordination number of 12 (4 in its own plane, 4 above, and 4 below, when viewed from a specific perspective within the FCC unit cell).

The packing efficiency for CCP/FCC is also 74%. Examples of metals adopting CCP/FCC structure include Copper (Cu), Silver (Ag), Gold (Au), Aluminum (Al), and Nickel (Ni).

Comparison of HCP and CCP

Both HCP and CCP are equally efficient in packing (74%) and have the same coordination number (12). The primary difference lies in their stacking sequence and the resulting symmetry of their unit cells. HCP has A-B-A-B stacking and a hexagonal unit cell, while CCP has A-B-C-A-B-C stacking and a face-centered cubic unit cell. While the local environment (first coordination shell) of an atom is identical in both, the long-range order differs.

Packing Efficiency Calculations (for reference, though specific derivations are often beyond NEET scope, the values are important):

Packing efficiency is defined as the percentage of the total volume of the unit cell that is occupied by the spheres. It is calculated as:

Simple Cubic (SC): — Packing efficiency = 52.4%
Body-Centered Cubic (BCC): — Packing efficiency = 68% (Note: BCC is not a close-packed structure, as its coordination number is 8, not 12. It's often discussed for comparison.)
Hexagonal Close Packing (HCP): — Packing efficiency = 74%
Cubic Close Packing (CCP) / Face-Centered Cubic (FCC): — Packing efficiency = 74%

Real-World Applications and Examples

Understanding close-packed structures is vital in materials science. The arrangement of atoms influences properties like density, ductility, malleability, and electrical conductivity. For instance, metals with CCP/FCC structures (like copper and gold) are generally more ductile and malleable than those with HCP structures (like zinc and magnesium) due to the greater number of slip planes available for deformation in FCC lattices.

Common Misconceptions

1
Confusing HCP and CCP: — Students often struggle to differentiate between HCP and CCP, especially since both have the same coordination number and packing efficiency. The key is to remember the stacking sequence (A-B-A-B for HCP, A-B-C-A-B-C for CCP) and the resulting unit cell type (hexagonal for HCP, FCC for CCP).
2
Number of Voids: — A common error is to miscalculate the number of tetrahedral and octahedral voids. Remember, for 'N' atoms in a close-packed structure, there are 'N' octahedral voids and '2N' tetrahedral voids.
3
BCC as Close-Packed: — Body-centered cubic (BCC) is sometimes mistakenly grouped with HCP and CCP as a close-packed structure. While BCC is a common metallic structure, it is not truly close-packed because its coordination number is 8, not 12, and its packing efficiency (68%) is lower than the ideal 74% of HCP/CCP.

NEET-Specific Angle

For NEET, the focus on close-packed structures typically revolves around:

Coordination Number: — Knowing the coordination number for 1D, 2D (SCP, HCP), and 3D (HCP, CCP) structures.
Packing Efficiency: — Recalling the packing efficiency values for SC, BCC, HCP, and CCP/FCC.
Stacking Patterns: — Identifying HCP (A-B-A-B) and CCP (A-B-C-A-B-C) from their stacking sequences.
Types and Number of Voids: — Understanding the formation of tetrahedral and octahedral voids and their numerical relationship to the number of constituent atoms (N octahedral, 2N tetrahedral).
Relationship between FCC and CCP: — Recognizing that cubic close packing is equivalent to the face-centered cubic unit cell.
Examples: — Knowing which common metals adopt HCP or CCP structures.