Hexagonal Close-Packed

Definition

Hexagonal close-packed (HCP) is a type of crystal lattice structure in which atoms or ions are arranged in a repeating three-dimensional pattern. This arrangement is characterized by a close-packed structure with a hexagonal unit cell, providing high packing efficiency and stability for certain solid materials.

5 Must Know Facts For Your Next Test

1. In the hexagonal close-packed (HCP) structure, the atoms or ions are arranged in a repeating pattern of two close-packed layers, with the third layer fitting into the depressions of the first layer.
2. The HCP structure has a coordination number of 12, meaning each atom or ion has 12 nearest neighbors, which is the maximum possible for a close-packed arrangement.
3. The HCP structure is commonly found in many metallic elements, such as magnesium, titanium, and zinc, as well as some ceramic materials.
4. The HCP structure is highly efficient in terms of packing density, with approximately 74% of the available volume occupied by the atoms or ions.
5. The HCP structure is characterized by a unit cell with a hexagonal base and a height that is approximately 1.633 times the length of the base, which is the ideal ratio for close-packing.

Review Questions

• Describe the key features of the hexagonal close-packed (HCP) crystal structure.
  • The hexagonal close-packed (HCP) crystal structure is characterized by a repeating pattern of two close-packed layers of atoms or ions, with the third layer fitting into the depressions of the first layer. This arrangement results in a highly efficient packing density, with approximately 74% of the available volume occupied by the atoms or ions. The HCP structure has a coordination number of 12, meaning each atom or ion has 12 nearest neighbors, which is the maximum possible for a close-packed arrangement. The unit cell of the HCP structure has a hexagonal base and a height that is approximately 1.633 times the length of the base, which is the ideal ratio for close-packing.
• Explain how the hexagonal close-packed (HCP) structure differs from other crystal lattice structures, such as the face-centered cubic (FCC) structure.
  • The key difference between the hexagonal close-packed (HCP) and face-centered cubic (FCC) crystal lattice structures is the arrangement of the atoms or ions. In the HCP structure, the atoms are arranged in a repeating pattern of two close-packed layers, with the third layer fitting into the depressions of the first layer. In contrast, the FCC structure has a cubic unit cell with atoms positioned at the corners and the centers of the faces. While both structures are close-packed, the HCP structure has a slightly higher packing efficiency of approximately 74%, compared to the 74% packing efficiency of the FCC structure. Additionally, the HCP structure has a coordination number of 12, while the FCC structure has a coordination number of 12 as well.
• Analyze the significance of the hexagonal close-packed (HCP) structure in the context of crystalline solids and their properties.
  • The hexagonal close-packed (HCP) crystal structure is significant in the context of crystalline solids because it represents a highly efficient packing arrangement that is commonly observed in many metallic elements and some ceramic materials. The close-packing of atoms or ions in the HCP structure contributes to the overall stability and mechanical properties of these materials, such as hardness, strength, and resistance to deformation. Additionally, the HCP structure's high packing density and coordination number of 12 can influence the materials' thermal, electrical, and magnetic properties, making them suitable for various applications in engineering, technology, and materials science. Understanding the HCP structure and its relationship to the properties of crystalline solids is crucial for designing and developing new materials with desirable characteristics.

Related terms

Crystal Lattice: A crystal lattice is a regular, repeating pattern of atoms, ions, or molecules in a crystalline solid material.

Close-Packing: Close-packing refers to the arrangement of atoms or ions in a crystal structure where they are packed as closely together as possible, maximizing the number of nearest neighbors.

Unit Cell: A unit cell is the smallest repeating unit of a crystal structure that, when repeated in three dimensions, can describe the entire crystal lattice.