Topics in Current Chemistry (2020) 378:6
1 3
1 Ferrites: Structure, Synthesis and Properties
1.1 Structure of Ferrites
Ferrites—whose etymology is from the Latin word ferrum meaning iron—are a
large class of oxides containing Fe
3+
and at least another metal cation that have been
investigated and applied as powder, films or ceramic bodies for the last 50 years.
They are classified according to their crystal structure and the way the oxygen
anions are arranged around the metal cations, as spinel, garnet, magnetoplumbite
and orthoferrite. While spinel and garnet ferrites both crystallize within a cubic
structure, magnetoplumbites and orthoferrites crystallize within a hexagonal and
orthorhombic structure, respectively.
1.1.1 Spinel Ferrites
Spinel ferrites are compounds with a general chemical formula MFe 2 O 4 , where M
refers to a divalent metal cation, and that crystallize in a crystallographic structure
isomorphic with that of the naturally occurring mineral spinel MgAl 2 O 4 (AB 2 O 4
as general composition). They crystallize in a cubic symmetry structure containing
eight formula units A[B] 2 O 4 , with a rigid sublattice of 32 closely packed oxygen anions giving rise to 64 tetrahedral (A)-type and 32 octahedral [B]-type interstitial sites
partially occupied by 24 M
2+
and Fe
3+
metal cations (Fig. 1). Depending on the synthetic conditions, tetragonal unit cells can be obtained as well [1]. In the spinel ferrites, the electrical neutrality is maintained by M
2+
and Fe
3+
cations occupying both
tetrahedral and octahedral interstitial sites. The interstitial fourfold and sixfold sites
have r t and r o radii in the 0.055 nm < r t < 0.067 nm and 0.070 nm < r o < 0.075 nm
range, so that they can be occupied by many transition metal cations with d
0
to d
10
electronic configurations.
The octahedral vs. tetrahedral site occupation—and therefore the spinel ferrite
structure—is reported to be driven by the electrostatic contribution to the lattice
energy, the cation radii, the cation charge and the crystal field effect. The reader can
refer to the work of Sickafus et al. [2]. According to the M
2+
and Fe
3+
metal cation
balance between both tetrahedral and octahedral sites, the spinel ferrites are categorized as follows:
• Normal spinels (M
2+ )
Td [Fe
3+ ]
Oh
2
O 4 , in which the smaller divalent M
2+
cations
occupy only the tetrahedral A-sites and the trivalent Fe
3+
cations occupy the
octahedral B-sites, e.g. ZnFe 2 O 4 or CdFe 2 O 4 .
• Inverse spinels (Fe
3+ )
Td [M
2+ Fe
3+ ]
Oh O 4 in which the divalent M
2+
cations are
located in the octahedral B-sites, while the trivalent Fe
3+
cations are equally distributed between both tetrahedral A-sites and octahedral B-sites, e.g. MgFe 2 O 4 ,
CoFe 2 O 4 , CuFe 2 O 4 , NiFe 2 O 4 , NiCoFe 2 O 4 , CuMnFe 2 O 4 or MgCuFe 2 O 4 .
• Mixed spinels (M
2+
x
Fe
3+
y
)
Td [M
2+
1−x
Fe
3+
2−y
]
Oh O 4 , in which both the divalent M
2+
and
trivalent Fe
3+
cations are distributed between both tetrahedral A-sites and octa108
Reprinted from the journal
1 3
1 Ferrites: Structure, Synthesis and Properties
1.1 Structure of Ferrites
Ferrites—whose etymology is from the Latin word ferrum meaning iron—are a
large class of oxides containing Fe
3+
and at least another metal cation that have been
investigated and applied as powder, films or ceramic bodies for the last 50 years.
They are classified according to their crystal structure and the way the oxygen
anions are arranged around the metal cations, as spinel, garnet, magnetoplumbite
and orthoferrite. While spinel and garnet ferrites both crystallize within a cubic
structure, magnetoplumbites and orthoferrites crystallize within a hexagonal and
orthorhombic structure, respectively.
1.1.1 Spinel Ferrites
Spinel ferrites are compounds with a general chemical formula MFe 2 O 4 , where M
refers to a divalent metal cation, and that crystallize in a crystallographic structure
isomorphic with that of the naturally occurring mineral spinel MgAl 2 O 4 (AB 2 O 4
as general composition). They crystallize in a cubic symmetry structure containing
eight formula units A[B] 2 O 4 , with a rigid sublattice of 32 closely packed oxygen anions giving rise to 64 tetrahedral (A)-type and 32 octahedral [B]-type interstitial sites
partially occupied by 24 M
2+
and Fe
3+
metal cations (Fig. 1). Depending on the synthetic conditions, tetragonal unit cells can be obtained as well [1]. In the spinel ferrites, the electrical neutrality is maintained by M
2+
and Fe
3+
cations occupying both
tetrahedral and octahedral interstitial sites. The interstitial fourfold and sixfold sites
have r t and r o radii in the 0.055 nm < r t < 0.067 nm and 0.070 nm < r o < 0.075 nm
range, so that they can be occupied by many transition metal cations with d
0
to d
10
electronic configurations.
The octahedral vs. tetrahedral site occupation—and therefore the spinel ferrite
structure—is reported to be driven by the electrostatic contribution to the lattice
energy, the cation radii, the cation charge and the crystal field effect. The reader can
refer to the work of Sickafus et al. [2]. According to the M
2+
and Fe
3+
metal cation
balance between both tetrahedral and octahedral sites, the spinel ferrites are categorized as follows:
• Normal spinels (M
2+ )
Td [Fe
3+ ]
Oh
2
O 4 , in which the smaller divalent M
2+
cations
occupy only the tetrahedral A-sites and the trivalent Fe
3+
cations occupy the
octahedral B-sites, e.g. ZnFe 2 O 4 or CdFe 2 O 4 .
• Inverse spinels (Fe
3+ )
Td [M
2+ Fe
3+ ]
Oh O 4 in which the divalent M
2+
cations are
located in the octahedral B-sites, while the trivalent Fe
3+
cations are equally distributed between both tetrahedral A-sites and octahedral B-sites, e.g. MgFe 2 O 4 ,
CoFe 2 O 4 , CuFe 2 O 4 , NiFe 2 O 4 , NiCoFe 2 O 4 , CuMnFe 2 O 4 or MgCuFe 2 O 4 .
• Mixed spinels (M
2+
x
Fe
3+
y
)
Td [M
2+
1−x
Fe
3+
2−y
]
Oh O 4 , in which both the divalent M
2+
and
trivalent Fe
3+
cations are distributed between both tetrahedral A-sites and octa108
Reprinted from the journal