The Structure of Crystalline Solids Flashcards
illustrating how the spots (i.e., the diffraction pattern) in (a) are produced
schematic diagram
a light-weight
automobile wheel that is
made of magnesium
mag wheel
___ 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 neighbour atoms.
crystalline
of the material, the manner in which atoms, ions, or
molecules are spatially arranged.
crystal structure
constituent particles are arranged in a regular fashion containing short range as well as long range order
crystalline solids
crystalline particles are NOT arranged in any regular fashion; they may be at the most some range order
amorphous solids
All metals
Most Ceramics
Some polymers
crystalline solids
Rubber
Glass
Plastics
Polymers
amorphous solids
___ is used in the context of crystal structures; in this sense ___ means a
three-dimensional array of points coinciding with atom positions (or sphere centres)
lattice
small repeat entities
unit cells
Three relatively simple crystal structures are found
for most of the common metals
•face-centered cubic (FCC) crystal structure
•body centered cubic (BCC) crystal structure
•hexagonal close-packed (HCP) crystal structure
It contains 4 atoms in a unit cell. The atomic packing factor for ___ is
0.74 which means it has 26% void space per unit cell.
FCC
a = 2R√2
where:
a = atomic lenght weight
R = atomic radius
FCC
Two other important characteristics of a crystal structure
Coordination number
Atomic packing factor (APF)
For metals, each atom has the
same number of nearest-neighbour or touching atoms
Coordination number
is the sum of the sphere
volumes of all atoms within a unit cell (assuming the atomic
hard-sphere model) divided by the unit cell volume
Atomic packing factor (APF)
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.
Body-Centered Cubic (BCC) Crystal Structure
The atomic packing factor for BCC is 0.68 which means it has 32%
void space per unit cell. Contain 2 atoms in a unit cell
Body-Centered Cubic (BCC) Crystal Structur
The equivalent of 6 atoms is contained in each unit cell; one-sixth of each
of the 12 top and bottom face corner atoms, one-half of each of the 2
center face atoms, and all 3 midplane interior atoms. The atomic packing
factor for HCP is 0.74 which means it has 26% void space per unit cell.
Hexagonal Close-Packed (HCP) Crystal Structure
Not all metals have unit cells with cubic symmetry;
the final common metallic crystal structure has a unit
cell that is hexagonal
Hexagonal Close-Packed (HCP) Crystal Structure
