Electron microscopy
 
Garnets
- Practical Electron Microscopy and Database -
- An Online Book -
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Garnets consist of a large family of complex natural and synthetic metal-oxide phases and have a general formula A3B2C3O12 (space group Ia-3d) formed with linked AO8, BO4 and MO6 polyhedra:
        i) A and B are normally di- and tri-positive cations.
        ii) A is usually a relatively large cation with coordination number 8 in distorted cubic coordination.
        iii) The smaller B and C ions often occupy octahedral and tetrahedral sites, respectively.

Table 1898. Atomic structure of Ca3Al2Si3O12.

Ion
Ga2+
Al3+
Si4+
O2- coordination
8 6 4

O bond strength, S

2/8 = 1/4 3/6 = 1/2 4/4 = 1

Typical examples of garnets are grossular Ca3Al2Si3O12, uvarovite Ca3Cr2Si3O12, andradite Ca3Fe2Si3O12, Y3Al5O12 (YAG; yttrium aluminum garnet), Gadolinium Gallium Garnet (GGG, Gd3Ga5O12). All species of garnets have similar physical properties and crystal forms, but magnetically interesting garnets can be obtained if the A site is occupied by a rare earth element and the B and C sites by iron.

 

 

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