Chapter 3: Atomic and Ionic Arrangements

Liquid Crystal

Materials that behave as amorphous materials (liquid-like) in one state but can form crystalline regions in response to an external stimulus

Monocrystalline Materials

Composed of a single crystal and no grain boundaries throughout

Polycrystalline Materials

Composed of many small crystals and many grain boundaries

Lattice

A collection of points arranged in a periodic pattern

Basis

A group of one or more atoms arranged in a particular way with respect to each other & associated with each lattice point

Crystal Structure

= Lattice + Basis

Bravais Lattices

In 3D, there are 14 Bravais Lattices. They are grouped into 7 crystal systems based on the lattice parameters.

Unit Cell

Subdivision of the lattice (i.e. a repeating unit) that still retains the overall characteristics of the entire lattice.

Coordination #

num of an atoms nearest neighbor

Packing Factor (PF), or called Atomic Packing Fraction (APF)

Fraction of space occupied by atoms within the unit cell = (# of atoms/cell)(volume of each atom)/volume of unit cell

Face Centered Cubic (FCC) Structure

Coordination #: 12, # of atoms/unit cell: 4, PF: 74%

Body Centered Cubic (BCC) Structure

Coordination #: 8, # of atoms/unit cell: 2, PF: 68%

Hexagonal Close-packed (HCP) Structure

Coordination #: 12, # of atoms/unit cell: 6, PF: 74%

Close-packed Directions

Directions within the unit cell that the atoms are in continuous contact

Theoretical Density

Density = = (# of atoms/cell)(atomic mass)/((volume of unit cell)(Avogadro number))

Miller Indices

Used to specify lattice directions and planes in unit cells

Crystal Directions

[u v w]

Crystal Planes

(u v w)

Anisotropic

Material properties can vary with direction

Isotropic

Material properties are identical in all direction

3 types of interstitial sites

Trigonal (cubic), tetragonal (tetrahedral), octagonal (octahedral)