Crystal Structure

long-range order exists

• Crystalline material

• Amorphous Material

long-range atomic order is absent

• Crystalline material

• Amorphous Material

Spheres representing nearest-neighbor atoms touch one another.

• Atomic hard-sphere model

• Bohr Model

• Reduced sphere model

• Lattice

means a three-dimensional array of points coinciding with atom positions

• Lattice

• Amorphous

• Unit Cells

• FCC

the basic structural unit or building block of the crystal structure and defines the crystal structure by virtue of its geometry and the atom positions within

• Lattice

• Amorphous

• Unit Cells

• FCC

Cube edge length a or x and the atomic radius R in FCC is related by

• a = 2R√𝟐

• a = $\frac{4R}{\sqrt{3}}$

In order to compute the number of atoms per unit cell for the HCP crystal structure we will use

• 
  

  $N=Ni\ +\frac{Nf}{2}+\frac{Nc}{6}$

• $N=Ni\ +\frac{Nf}{2}+\frac{Nc}{8}$

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)

• Total unit cell volume

• Volume of atoms in a unit cell

when metals and nonmetals have more than one crystal structure

• Polymorphism

• Allotropy

Allotropy is

• when more than one crystal structure is found in elemental solids

• when metals and nonmetals have more than one crystal structure