Crystal Structure and Types of Crystals

Vocabulary-style flashcards covering crystal structure, unit cells, crystal systems, close packing, ionic/covalent/molecular/metallic crystals, amorphous solids, and common crystal structures.

Crystal structure

Rigid, long-range order in a crystal where atoms, molecules, or ions occupy fixed positions in a periodic lattice.

Unit cell

The basic repeating structural unit that builds up a crystalline solid.

Primitive cubic (SC)

A simple cubic lattice with atoms only at the corners; 1 atom per unit cell.

Tetragonal

A unit cell with a = b ≠ c and α = β = γ = 90°.

Orthorhombic

A unit cell with a ≠ b ≠ c and α = β = γ = 90°.

Rhombohedral (Trigonal)

A unit cell with a = b = c and α = β = γ ≠ 90°.

Monoclinic

A unit cell with a ≠ b ≠ c; α ≠ 90°, β ≈ 90°, γ ≈ 90° (often α = γ = 90°, β ≠ 90°).

Triclinic

A unit cell with a ≠ b ≠ c and α ≠ β ≠ γ; none are 90°.

Hexagonal

A unit cell with a = b ≠ c; α = β = 90°, γ = 120°.

Coordination number (CN)

The number of atoms surrounding an atom in the lattice; indicates how tightly packed the structure is.

Body-centered cubic (BCC)

Cubic lattice with atoms at corners and a central atom; CN = 8; 2 atoms per unit cell.

Face-centered cubic (FCC)

Cubic lattice with atoms at corners and face centers; CN = 12; 4 atoms per unit cell.

Corner atom

An atom located at a corner of the unit cell; contributes 1/8 to the cell.

Face-centered atom

An atom at a face center; contributes 1/2 to the cell.

Edge atom

An atom at an edge; contributes 1/4 to the cell.

Atoms per unit cell (SC, BCC, FCC)

SC contains 1 atom per cell; BCC contains 2; FCC contains 4.

a = 2r (SC)

Edge length a for a simple cubic lattice when atoms touch along the edge.

a = 4r/√3 (BCC)

Edge length a for a body-centered cubic lattice when atoms touch along the body diagonal.

a = 2√2 r (FCC)

Edge length a for a face-centered cubic lattice when atoms touch along the face diagonal.

CsCl-type structure

A CsCl-type crystal with a body-centered arrangement; each ion is 8-fold coordinated (CN = 8).

Zinc blende (ZnS)

Zinc blende structure based on an FCC lattice; 4 Zn2+ and 4 S2− per unit cell with tetrahedral coordination.

Fluorite (CaF2)

Fluorite structure based on FCC; Ca2+ in the lattice and F− in tetrahedral holes; typically 4 Ca and 8 F per cell.

NaCl structure (rock-salt)

FCC lattice with Na+ and Cl− alternating at lattice points; 4 formula units per unit cell.

Ionic crystals

Crystals held together by Coulombic attractions between ions; hard, high melting points, brittle; poor electrical conductors in solid state.

Covalent crystals

Crystals held together by covalent bonds; very hard, high melting points; poor conductors.

Molecular crystals

Crystals held together by intermolecular forces (dispersion, dipole-dipole, hydrogen bonds); soft, low melting points; poor conductors.

Metallic crystals

Crystals with delocalized electrons in a ‘sea’ of electrons; good conductors; metallic bonding.

Delocalized electrons

Electrons free to move throughout the crystal lattice, enabling metallic bonding and conductivity.

Amorphous solids

Solids lacking long-range order; examples include glass; no regular 3D arrangement.

Glass (amorphous solid)

An amorphous solid; SiO2 is a chief component; examples include quartz glass, Pyrex, soda-lime glass.

Quartz vs. glass

Crystalline quartz has long-range order; glass is non-crystalline with no long-range order.

Types of glass (Table 12.5)

Pure quartz glass (100% SiO2); Pyrex (60–80% SiO2, 10–25% B2O3, some Al2O3); Soda-lime (75% SiO2, 15% Na2O, 10% CaO).

Hexagonal close-packed (hcp)

Stacking ABAB…; a site directly over an atom in layer A is part of the arrangement.

Cubic close-packed (ccp)

Stacking ABCABC…; corresponds to an FCC cell; a site directly over A in hcp is not directly over A in ccp.

Close-packed structures relationship

hcp and ccp are two close-packed arrangements; ccp is equivalent to the FCC cell.