**2.1 Garnets ferrites**

The general formula for garnets is Me3Fe5O12, where Me is one of the rare earth metal ions, including Y, La and Gd. The cubic unit cell contains 8 formula units or 160 atoms, which can be described as a spatial arrangement of 96 O2- with interstitial cations. Yttrium iron garnet Y3Fe5O12 (YIG) is a well-known garnet. The coordination of the cations is considerably more complex than spinels, with 24 Y3+ in dodecahedral sites, 24 Fe3+ ions in tetrahedral sites and 16 remaining Fe3+ in octahedral sites. Similar to spinels and hexagonal ferrites, a wide range of transition metal cations can substitute Y3+ or Fe3+; especially rare earth ions may replace the ions on octahedral and dodecahedral sites. Each type of lattice site will accept other metal ions at dodecahedral sites, octahedral sites and at tetrahedral sites. Thus pentavalent ions such as V5+ and As5+ can occupy tetrahedral sites, while Ca2+ substitute ions on dodecahedral sites [7].
