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Dr. Dmitri Kopeliovich
The following factors affect structure of ceramics:
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.
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 |
Tetrahedral silica block (SiO4-4) may form various silicate structures:
Single or two silica blocks, containing other cations, form Island (olivine) or Double Island Silicates (hemimorphite).
Several (3,4,5,6) silica units join each other, forming a chain (orthopyroxenes, clinopyroxenes, asbestos) or closed ring (beryl).
Silica units connect to each other, forming infinite sheet (micas, serpentine, chlorite, talc).
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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