Crystal structure

Please subscribe this blog by clicking "Joint this Site" Button.
Fundamental Concept:

 A crystalline material is one in which the atoms are situated in a repeating or periodic array over large atomic distances;  that is, long-range order exists, such that upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded to its nearest-neighbor atoms.

For those that do not crystallize, this long-range atomic order is absent; are called noncrystalline or amorphous materials.

Some of the properties of crystalline solids depend on the crystal structure of the material, the manner in which atoms, ions, or molecules are spatially arranged.

 When describing crystalline structures, atoms (or ions) are thought of as being solid spheres having well-defined diameters. This is termed the atomic hard sphere model in which spheres representing nearest-neighbor atoms touch one another.

 Sometimes the term lattice is used in the context of crystal structures; in this sense “lattice” means a three-dimensional array of points coinciding with atom positions (or sphere centers).

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 α,β  and γ ; are sometimes termed the lattice parameters of a crystal structure.

Table From

Materials Science and Engineering William D. Callister, Jr

Metal	Symbol	Crystal Structure	Atomic  Radius (nm)
Aluminum	Al	FCC	0.1431
Cadmium	Cd	HCP	0.1490
Chromium	Cr	BCC	0.1249
Cobalt	Co	HCP	0.1253
Copper	Cu	FCC	0.1278
Gold	Au	FCC	0.1422
Iron(α)	Fe	BCC	0.1241
Lead	Pb	FCC	0.1750
Molybdenum	Mo	BCC	0.1363
Nickel	Ni	FCC	0.1246
Platinum	Pt	FCC	0.1387
Silver	Ag	FCC	0.1445
Tantalum	Ta	BCC	0.1430
Titanium (α)	Ti	HCP	0.1445
Zinc	Zn	HCP	0.1332
Tungsten	W	BCC	0.1371
		FCC= face-centered cubic; HCP =hexagonal close-packed; BCC =body-centered cubic	1 nm=

                                                                                                                               1 nm =10^-9 mt
There are seven crystal system:

Lattice Parameter Relationships and Figures Showing Unit Cell Geometries for the Seven Crystal Systems:

Crystal System	Axial Relation	Interaxial Angel	Space lattic (3+4+1+2+1+2+1=14)	Unit cell Geometry
Cubic	a=b=c	α=β=γ=900	1.Simple cubic 2.BCC 3.FCC
Orthorhombic	a≠b≠c	α=β=γ=900	1.Simple Orthorhombic 2.Body- Centered Orthorhombic 3.Base-Centered Orthorhombic 4.Face-Centered Orthorhombic
Rhombohedral (trigonal)	a=b=c	α=β=γ≠900	1.Simple Rhombohedral
Tetragonal	a=b≠c	α=β=γ=900	1.Simple Tetragonal 2.Body-centered Tetragonal
Hexagonal	a=b≠c	α=β=900,γ=1200	1.Simple Hexagonal
Monoclinic	a≠b≠c	α=γ=900,β≠90	1.Simple monoclinic 2.Base-Centered Orthorhombic
Triclinic	a≠b≠c	α≠β≠γ≠900	1. Simple Triclinic

Some metals, as well as nonmetals, may have more than one crystal structure, a phenomenon known as polymorphism. When found in elemental solids, the condition is often termed allotropy. The prevailing crystal structure depends on both the temperature and the external pressure. One familiar example is found in carbon: graphite is the stable polymorph at ambient conditions, whereas diamond is formed at extremely high pressures. Also, pure iron has a BCC crystal structure at room temperature, which changes to FCC iron at 912⁰ C

 .