Materials with close packing can crystallize in both cubic and hexagonal crystal systems. Close packing refers to the arrangement of atoms or ions in a crystal lattice where the packing is as dense as possible. In both cubic and hexagonal close packing, the atoms or ions are arranged in layers, and the layers are stacked in a regular pattern to maximize the packing efficiency.
In cubic close packing (CCP), also known as face-centered cubic (FCC) structure, the layers are arranged in an ABCABC... sequence, where each layer is shifted with respect to the adjacent layers. The coordination number (the number of nearest neighbors) in FCC is 12. Some examples of materials that crystallize in FCC structure include copper, aluminum, and gold.
In hexagonal close packing (HCP), the layers are arranged in an ABAB... sequence, where each layer is aligned directly on top of the adjacent layer. The coordination number in HCP is 12 as well. Materials such as magnesium, zinc, and titanium can crystallize in HCP structure.
The coordination polyhedra, also known as coordination spheres or coordination environments, refer to the geometric arrangement of neighboring atoms or ions around a central atom or ion in a crystal lattice. The coordination polyhedra play a role in determining the packing arrangement in different Bravais lattices. In close-packed structures, each atom or ion is surrounded by a coordination polyhedron formed by its nearest neighbors.
In FCC and HCP structures, the coordination polyhedron is typically an octahedron. In FCC, each atom is surrounded by six nearest neighbors arranged in an octahedral coordination polyhedron. In HCP, each atom is surrounded by three nearest neighbors in the same layer and three nearest neighbors in the adjacent layers, forming an overall octahedral coordination polyhedron.
The bond type (ionic, metallic, or covalent) can influence the crystal structure and packing to some extent. Ionic bonds tend to result in structures with higher coordination numbers and less dense packing compared to metallic or covalent bonds. Covalent bonds can result in more directional bonding and affect the arrangement of atoms in the crystal lattice. However, it is important to note that the specific crystal structure and packing arrangement also depend on other factors such as the size of the atoms or ions, electronegativity differences, and temperature.