[CHE217] LECTURE 1-3

Crystallography

CRYSTALLINE SOLID

Solid that contains a regular and repeating atomic or molecular arrangement (atoms vibrate in a fixed pattern)

AMORPHOUS MATERIALS

Materials with a random atomic arrangement (e.g., polymers and glass)

LATTICE

The 3-D space-filling repeating pattern on which atoms are placed to form a crysta

UNIT CELL

Smallest volume that completely describes the crystal pattern, defined by lattice constants (edge lengths) and interaxial angles

CRYSTALS

Materials with an ordered and repeated atomic arrangement. Examples include metals and some ceramics.

LATTICE CONSTANTS

Edge lengths along the major axes

INTERRACIAL ANGLES

Angles between axes

CUBIC

TETRAGONAL

ORTHORHOMBIC

RHOMBOHEDRAL

HEXAGONAL

MONOCLINIC

TRICLINIC

SIMPLE CUBIC (SC)

Atoms are located only at the eight corners of the cubic unit cell. It is the most basic structure and has a Coordination Number of 6

BODY CENTERED CUBIC (BCC)

This structure has atoms at the eight corners of a cube, plus one additional atom sitting squarely in the center of the body.

It contains 2 equivalent atoms per unit cell and has an Atomic Packing Factor (APF) of 0.68.

Example: Iron

FACE CENTERED CUBIC (FCC)

Features atoms at the eight corners, plus one atom located at the center of each of the cube's six faces.

It is a very densely packed structure containing 4 atoms per unit cell with an APF of 0.74

Example: Gold

HEXAGONAL CLOSE PACK (HCP)

Unlike the others, this is shaped like a hexagonal prism. It has atoms at the corners and centers of the top and bottom faces, with three atoms sandwiched in the middle layer.

It is just as densely packed as FCC with an APF of 0.74 , but contains 6 atoms per unit cell

Example: Titanium

COORDINATION NUMBER (CN)

The number of nearest neighbor atoms

NUMBER OF ATOMS PER UNIT CELL (n)

The total equivalent atoms contained entirely within one unit cell

ATOMIC PACKING FACTOR (APF)

The fraction of space filled by spherical atoms. It is the ratio of the volume occupied by the atoms to the total available volume of the unit cell

SIMPLE CUBIC CN

CN = 6

BCC CN

CN = 8

FCC CN

CN = 12

HCP CN

CN = 12