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Structure of ceramic materials

Dr. Dmitri Kopeliovich

The following factors affect structure of ceramics:

  • Balance of electrical charges of anions and cations
  • Radius Ratio (rc/ra)

Where

rc – radius of cation;

ra – radius of anion.

Radius Ratio determines Coordination Number (CN)– the maximum number of anion nearest neighbors for a cation.The anion neighbors do not touch each other.

rc/ra = 0.225…0.414(SiO2) CN = 4

rc/ra = 0.414…0.732(SnO2, PbO2) CN = 6

rc/ra = 0.732…1.0(ThO2) CN = 8

Covalent bonding component, which tends to form tetrahedral coordination, may affect the Coordination Number.

  • Ions are packed with maximum density, providing minimum energy of the structure.


Ceramic structures are classified and designated according to the pattern structures of several natural minerals:

Mineral Name Formula Coordination Number Structure Characterization
Rocksalt NaCl 6 Octahedral unit cell, cubic appearance
Zincblende ZnS 4 FCC unit cell with S2- anions at 4 tetrahedral sites
Fluorite CaF2 8-cation CN
4-anion CN
FCC unit cell with F- anions at 8 tetrahedral sites
Corundum Al2O3 6-cation CN
4-anion CN
HCP unit cell with O2- anions at the lattice sites and Al3+ at interstitial sites
Perovskite CaTiO3 6-cation(Ti) CN
2-anion(O) CN
Cubic unit cell with Ti4+ cations coordinated octahedrally among six oxygen anions
Silicate Combination of SiO44- blocks 4 Tetrahedral arrangement with Si4+ cations at the center bonded to O2- anions at the apices of the tetrahedron

Examples of some ceramic structures

Zincblende and chloride.png

Silicate block.png

Tetrahedral silica block (SiO4-4) may form various silicate structures:

  • Island and DoubleIsland Silicates

Single or two silica blocks, containing other cations, form Island (olivine) or Double Island Silicates (hemimorphite).

  • Ring and Chain Structures

Several (3,4,5,6) silica units join each other, forming a chain (orthopyroxenes, clinopyroxenes, asbestos) or closed ring (beryl).

  • Sheet (layer) structure

Silica units connect to each other, forming infinite sheet (micas, serpentine, chlorite, talc).

  • Framework silicate

Quartz, cristobalite, and tridymite minerals are based on the framework silicate structure.

Silicates exist in two forms: crystalline and amorphous (glasses).

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structure_of_ceramic_materials.txt · Last modified: 2014/08/24 by dmitri_kopeliovich
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