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