**18. Iron**

The atomic number of iron is 26 and its electronic configuration is [Ar]4s2 3d6.. Iron has 14 isotopes. Among them, the mass of iron varies from 52 to 60 Pure iron is chemically reactive and corrodes rapidly, especially in moist air or at elevated temperatures. Iron is vital to plant and animal life. The ionic radius of Fe2+is 0.76 A.U. and that of Fe3+ is 0.64 A.U. The most common oxidation states of iron are +2 and +3. Iron(III) complexes are generally in octahedral in shape, and a very few are in tetrahedral also.

#### **18.1. EPR spectra of iron compounds**

32 Advanced Aspects of Spectroscopy

configuration is 3 1

symmetry (C2v).

indicates an axial compression.

5B1g→5A1g 5B1g→5B2g

5B1g→5Eg

**18. Iron** 

these levels and are independent of Dq.

If ν1 and ν2 are correctly observed and identified in the spectrum, B and C can be calculated. Identification is particularly easy in these cases because of the sharpness of the bands of

2. Manganese(III): This ion has four 3d electrons. The ground state electronic

The transitions in the tetragonal field are described by the following equations:

2 2

2 2

Assignment Localities

distortion (C2h) in montmorillonite are given in Table -14.

5B1g→5A1g 5B1g→5A2g 5B2g 5B1g→5A3g

**Table 14.** Assignment of bands for Mn(III) in montmorillonite

 2 2 1 2 :4 2 6 2 10 *B B Dq Ds Dt Dq Ds Dt Dq g g*

1 1 :6 2 6 6 2 4 5 *B A Dq Ds Dt Dq Ds Dt Ds Dt g g*

<sup>1</sup> : 4 4 6 2 10 3 *g g B E Dq Ds Dt Dq Ds Dt Dq Ds*

In the above equations, Dq is octahedral crystal field and Ds and Dt are tetragonal field parameters. The same sign of Dq and Dt indicates an axial elongation and opposite sign

The optical absorption bands observed for Mn(III) in octahedral coordination with rhombic

D4h C2V (Mexico) (Gumwood Mine) (California)

The atomic number of iron is 26 and its electronic configuration is [Ar]4s2 3d6.. Iron has 14 isotopes. Among them, the mass of iron varies from 52 to 60 Pure iron is chemically reactive and corrodes rapidly, especially in moist air or at elevated temperatures. Iron is vital to plant and animal life. The ionic radius of Fe2+is 0.76 A.U. and that of Fe3+ is 0.64 A.U. The

to one electron transition. This should appear around 20000 cm-1. Mn3+ cation is subject to Jahn-Teller distortion. The distortion decreases the symmetry of the coordination site from octahedral to tetragonal (D4h) or by further lowering the symmetry to rhombic (C2v). Under the tetragonal distortion, the t2g orbital splits into eg and b2g orbitals whereas the eg orbital splits into a1g and b1g orbitals. Hence in a tetragonal site, three absorption bands are observed instead of one. Further distortion splits the eg orbital into singly degenerate a1g and b1g orbitals. Thus four bands are observed for rhombic

<sup>2</sup>*g g t e* . It gives a single spin-allowed transition 5Eg→ 5T2g corresponding

(29)

(30)

(31)

The EPR spectra of powdered Fe3+ compounds may be described by the spin- Hamiltonian,

$$H = \text{gBS} + D\left(S\_z^2 - \frac{1}{3}S\left(S + 1\right) + E\left(S\_X^2 - S\_y^2\right)\right) \tag{32}$$

The second and third terms in the equation (33) represent the effects of axial and rhombic components of the crystal field respectively. When D=E=0, it corresponds to a free ion in the magnetic field, H and if E= 0, it implies a field of axial symmetry. If λ (E/D) increases, it results in the variation of rhombic character. Maximum rhombic character is seen at a value of λ=1/3 and further increase in λ from 1/3 to 1 results in the decrease of rhombic character. When λ =1, the axial field situation is reached. When λ=1/3, the g value is around 4.27 and when λ is less than 1/3, g value is 4. Hence, the resonance is no longer isotropic and the powder spectrum in that region is a triplet corresponding to H along each of the three principle axes. For Fe3+, in fields of high anisotropy, the maximum g value is 9. If g values are limited to 0.80 to 4.30, the Fe3+ ion is under the influence of a strong tetragonal distortion.

1. Iron (III): The iron (III) samples exhibit a series of g values ranging from 0 to 9. This is due to the fact that the three Kramers' doublets of S=5/2 are split into S5/2, S3/2 and S1/2 separated by 4D and 2D respectively where D is the zero field splitting parameter. Depending on the relative populations of these doublets, one observes g value ranging from 0 to 9.0. The line widths are larger in low magnetic field when compared to high magnetic field. If the lowest doublet, S1/2 is populated, it gives a g value of 2 to 6 whereas if the middle Kramers' doublet S3/2 is populated, a g value 4.30 is expected. If the third doublet S5/2 is populated, it gives a g value of 2/7 to 30/7. A few systems are known which exhibit resonances from all the three Kramers' doublets.

The iron(III) in the natural sample enters the lattice in various locations which may not correspond to the lowest energy configuration. After heating the sample, the impurity settles in the lowest energy configuration and the EPR spectrum is simplified. Thus, it is observed that heating the sample results in a simplification of the EPR spectrum and gives a g value of around 2.

#### **18.2. Typical examples**


comprises a weak doublet within the strong doublet. The weak doublet also consists of two lines, absorption and dispersion line shapes. The g values of the strong doublet are 4.48 and 3.78 whereas the g values of the weak doublet are 4.22 and 3.96. The data reveal that there are two different centres of Fe(III) which are magnetically distinct.

3. The EPR spectrum of nano iron oxalate recorded at room temperature reveals three sets of four lines in low, medium and high fields corresponding to g1, g2 and g3 respectively. From the positions of the peaks in the EPR spectrum, the following spectroscopic splitting factors are evaluated: g1 = 2.130, g2 = 2.026 and g3 = 1.947. The hyperfine structure constants are A1 = 78 mT, A2 = 46 mT and A3 =26 mT. The EPR spectrum is characteristic of Fe(III) ion or *HCO*<sup>2</sup> or in rhombic symmetry. For the rhombic symmetry, g values follow in the sequence as g1 > g2 > g3. Using the relation, spin-orbit coupling constant, λ is calculated. Resonant value of the magnetic field is given by the relation,

$$H\_{\mathbb{R}}(mT) = \frac{21419.49}{\text{g}\,\lambda(cm)} = \frac{0.07144775}{\text{g}}\,\nu(MHz) \tag{33}$$

λ calculated for each g tensor is 32.18.

For axial symmetry, λ is zero. If rhombic character in the crystal field is increased, it results in the increase of λ upto a maximum of <sup>1</sup> <sup>3</sup> . In the present case, the observed λ is <sup>1</sup> <sup>3</sup> (32.18%). Thus the EPR studies indicate that the iron oxalate nano-crystal is in orthorhombic structure.

#### **18.3. Optical absorption spectra of iron compounds**

#### *18.3.1. Trivalent iron*

Trivalent iron has the electronic configuration of 3d5 which corresponds to a half-filled d sub-shell and is particularly most stable. In crystalline fields, the usual high spin configuration is 3 2 <sup>2</sup>*g g t e* with one unpaired electron in each of the orbitals and the low spin state has the <sup>5</sup> <sup>2</sup>*<sup>g</sup> t* configuration with two pairs of paired electrons and one unpaired electron. The energy level in the crystal field is characterized by the following features. i) The ground state of d5 ion, 6S transforms into 6A1g - a singlet state. It is not split by the effect of crystal field and hence all the transitions are spin forbidden and are of less intensity. ii) In excited state, d5 ion gives rise to quartets (4G, 4F, 4D, 4P) and doublets (2I, 2H, 2G, 2F, 2D, 2P, 2S) . The transitions from the ground to doublet state are forbidden because the spin multiplicity changes by two and hence they are too weak. Thus sextet-quartet forbidden transitions observed are: 6A1g→ 4T1g and 6A1g→ 4T2g. The transitions which are independent of Dq and which result in sharp bands are 6A1g→ 4E(4D) 6A1g→ 4Eg+4E1g etc., iii) The unsplit ground state term behaves alike in both octahedral and tetrahedral symmetries and gives rise to same energy level for octahedral, tetrahedral and cubic coordination with usual difference,

$$\left\lfloor Dq\_{\text{Cette}} : Dq\_{\text{Tetra}} \, \_1Dq\_{\text{Cubic}} = 1 : \frac{4}{9} : \frac{8}{9} \right\rfloor\_1$$

#### *18.3.2. Divalent iron*

34 Advanced Aspects of Spectroscopy

relation,

*18.3.1. Trivalent iron* 

configuration is 3 2

coordination with usual difference,

state has the <sup>5</sup>

characteristic of Fe(III) ion or *HCO*<sup>2</sup>

λ calculated for each g tensor is 32.18.

in the increase of λ upto a maximum of <sup>1</sup>

**18.3. Optical absorption spectra of iron compounds** 

comprises a weak doublet within the strong doublet. The weak doublet also consists of two lines, absorption and dispersion line shapes. The g values of the strong doublet are 4.48 and 3.78 whereas the g values of the weak doublet are 4.22 and 3.96. The data reveal that there are two different centres of Fe(III) which are magnetically distinct. 3. The EPR spectrum of nano iron oxalate recorded at room temperature reveals three sets of four lines in low, medium and high fields corresponding to g1, g2 and g3 respectively. From the positions of the peaks in the EPR spectrum, the following spectroscopic splitting factors are evaluated: g1 = 2.130, g2 = 2.026 and g3 = 1.947. The hyperfine structure constants are A1 = 78 mT, A2 = 46 mT and A3 =26 mT. The EPR spectrum is

symmetry, g values follow in the sequence as g1 > g2 > g3. Using the relation, spin-orbit coupling constant, λ is calculated. Resonant value of the magnetic field is given by the

> 21419.49 0.07144775 ( ) ( ) ( ) *H mT <sup>R</sup> MHz g cm g*

For axial symmetry, λ is zero. If rhombic character in the crystal field is increased, it results

Thus the EPR studies indicate that the iron oxalate nano-crystal is in orthorhombic structure.

Trivalent iron has the electronic configuration of 3d5 which corresponds to a half-filled d sub-shell and is particularly most stable. In crystalline fields, the usual high spin

electron. The energy level in the crystal field is characterized by the following features. i) The ground state of d5 ion, 6S transforms into 6A1g - a singlet state. It is not split by the effect of crystal field and hence all the transitions are spin forbidden and are of less intensity. ii) In excited state, d5 ion gives rise to quartets (4G, 4F, 4D, 4P) and doublets (2I, 2H, 2G, 2F, 2D, 2P, 2S) . The transitions from the ground to doublet state are forbidden because the spin multiplicity changes by two and hence they are too weak. Thus sextet-quartet forbidden transitions observed are: 6A1g→ 4T1g and 6A1g→ 4T2g. The transitions which are independent of Dq and which result in sharp bands are 6A1g→ 4E(4D) 6A1g→ 4Eg+4E1g etc., iii) The unsplit ground state term behaves alike in both octahedral and tetrahedral symmetries and gives rise to same energy level for octahedral, tetrahedral and cubic

<sup>2</sup>*g g t e* with one unpaired electron in each of the orbitals and the low spin

<sup>2</sup>*<sup>g</sup> t* configuration with two pairs of paired electrons and one unpaired

(33)

<sup>3</sup> . In the present case, the observed λ is <sup>1</sup>

<sup>3</sup> (32.18%).

or in rhombic symmetry. For the rhombic

In divalent iron (d6), the free ion ground term is 5D and the excited terms are triplet states (3H, 3P, 3F, 3G, 3D) and singlet states (1I, 1D). In an octahedral field, the 5D term splits into an upper 5Eg level and a lower 5T2g level of which the latter forms the ground state. The only allowed transition is 5T2g → 5Eg which gives an intense broad absorption band. This band splits into two bands due to Jahn-Teller effect. The average of these two bands is to be taken as 10Dq band. The transitions arising from the excited triplet states are spin forbidden and hence are weaker than the 10Dq band.

#### *18.3.3. Typical examples*



**Table 15.** Band headed data with assignments for Fe(III) in various compounds
