The properties of a solid material, from its strength to its ability to conduct electricity, are governed by the arrangement of its constituent atoms. Solids exist in two main forms: amorphous and crystalline. A crystalline solid features a highly ordered, repeating pattern of atoms, ions, or molecules that extends across large distances within the material. This internal order gives crystals their characteristic properties.
Amorphous solids, such as glass, lack this extensive, long-range order, exhibiting a more random arrangement akin to a frozen liquid. This structural distinction results in different behaviors, with crystalline materials often possessing a sharper melting point and greater rigidity than their amorphous counterparts. By controlling this microscopic atomic structure, scientists and engineers can determine the macroscopic properties of a material.
The Building Blocks of Order
Understanding crystal structure begins with two fundamental concepts: the crystal lattice and the unit cell. The crystal lattice is an imaginary, three-dimensional framework of points that represents the periodic arrangement of atomic locations in space. This conceptual grid provides the repeating geometry of the structure.
The unit cell is the smallest volume element of the crystal lattice that retains the full symmetry and characteristics of the entire crystal structure. When translated and stacked repeatedly in three dimensions, this block generates the infinite, ordered pattern of the complete crystal. For instance, in a simple cubic system, the unit cell is a cube containing the equivalent of one atom.
In many structures, the unit cell contains a motif, or a group of atoms, associated with each lattice point. The final crystal structure is the combination of the lattice and the motif placed upon each of its points. Analyzing the geometry and contents of this unit cell allows for the prediction of the bulk material’s behavior.
Primary Geometric Arrangements
The close-packing of atoms in metals results in a few common and highly symmetric unit cell types: Body-Centered Cubic (BCC), Face-Centered Cubic (FCC), and Hexagonal Close-Packed (HCP). Distinctions among these structures are based on the position of atoms within the unit cell, which influences the material’s density and mechanical response.
The Body-Centered Cubic (BCC) structure has atoms at all eight corners of the cube and one atom in the center. This arrangement results in a coordination number of eight (eight nearest neighbors). The BCC structure is less densely packed than the other types, possessing an atomic packing factor of 68% of the total volume.
The Face-Centered Cubic (FCC) structure places atoms at the corners and in the center of each of the six faces. The FCC arrangement achieves a higher density, with an atomic packing factor of 74%, which is the maximum theoretical density for spheres of a single size. Each atom in this structure has a coordination number of twelve.
The Hexagonal Close-Packed (HCP) structure also achieves 74% packing efficiency and a coordination number of twelve, but uses a different stacking sequence. While FCC layers repeat in an ABCABC pattern, HCP follows an ABAB sequence. This difference in stacking symmetry leads to variations in mechanical properties.
Structure Determines Function
The geometric order of a crystal structure dictates the physical and mechanical properties exhibited by the material. For example, the high packing efficiency in FCC and HCP structures translates to greater density compared to the more open BCC arrangement. The atomic arrangement also determines how the material responds to applied forces.
The ability of a metal to deform plastically, or its ductility, is related to slip systems, which are planes of atoms that can easily slide past one another. Face-Centered Cubic metals are highly ductile because they possess twelve slip systems, and these planes are closely packed. This combination allows for easier dislocation movement and greater deformation before fracture.
Body-Centered Cubic metals, while having many potential slip systems, are generally less ductile because their planes are not as densely packed, requiring higher energy to initiate slip. This difference in packing also influences electrical properties, as the free movement of electrons is easier in the highly symmetric, close-packed lattices. Furthermore, the strong, directional covalent bonds in the diamond cubic structure, a variation of the FCC lattice, create a three-dimensional network that resists atomic movement, resulting in diamond’s hardness.
Natural and Engineered Examples
The utility of many common materials is a direct result of their atomic arrangement. Metals like gold, copper, and aluminum all crystallize in the Face-Centered Cubic (FCC) structure. This structure is responsible for their high ductility and malleability, allowing them to be easily drawn into wires or hammered into thin sheets.
Metallic Examples
Metals such as iron (at room temperature), chromium, and tungsten adopt the Body-Centered Cubic (BCC) structure. This less-packed arrangement contributes to their higher strength and resistance to plastic deformation, making them suitable for structural applications and high-temperature filaments. The Hexagonal Close-Packed (HCP) structure is found in metals like magnesium and zinc, which are often less ductile than FCC metals due to their fewer available slip systems.
Non-Metallic Examples
The crystal structure of non-metallic solids also governs their functionality. Common table salt, sodium chloride, forms a structure based on two interpenetrating FCC lattices of alternating positive and negative ions, which explains its regular cubic cleavage and solubility. Semiconductors like silicon and germanium utilize the diamond cubic structure, where the atoms are covalently bonded in a tetrahedral arrangement. This structure is the foundation for their electronic properties used in modern computing.