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Hexagonal Close-Packed (Hcp);
a reduced- sphere unit cell for this structure, which is termed
Crystalline
material is one in which the atoms are situated in a repeating or periodic array over large atomic distances;
Crystalline
That is, long-range order exists, such that upon solidification, the atoms will position them- selves in a repetitive three-dimensional pattern, in which each atom is bonded to its
nearest-neighbor atoms
Crystal Structure
the manner in which atoms, ions, or molecules are spatially arranged.
Atomic Hard-Sphere
When describing crystalline structures, atoms (or ions) are thought of as being
solid spheres having well-defined diameters. This is termed the
Lattice
is used in the context of crys- tal structures;
Lattice
means three-dimensional array of points coinciding with atom positions (or sphere centers).
Unit Cell
in describing crystal structures, it is often convenient to sub-
divide the structure into small repeat entities called
Unit Cell
cell is the basic structural unit or building block of the crystal structure and defines the crystal structure by virtue
of its geometry and th
face-Centered cubic (FCC)
The crystal structure found for many metals has a unit cell of cubic geometry, with atoms located at each of the corners and the centers of all the cube faces. It is aptly
Coordination Number And The Atomic Packing Factor (Apf).
Two other important characteristics of a crystal structure are the
Atomic Packing
Factor (Apf)
is the sum of the sphere volumes of all atoms within a unit cell (as-
suming the atomic hard-sphere model) divided by the unit cell volume—that is,
Body-Centered Cubic
(Bcc)
Another common metallic crystal structure also has a cubic unit cell with atoms located at all eight corners and a single atom at the cube center.
Polymorphism.
Some metals, as well as nonmetals, may have more than one crystal structure, a phe- nomenon known as
Allotropy.
When found in elemental solids, the condition is
often termed
Lattice Parameters
The unit cell geometry is completely defined in terms of six parameters: the three edge lengths a, b, and c, and the three interaxial angles a, b, and g.
