**16. Chromium**

Chromium is the 6th most abundant transition metal. Chromium is used in the manufacture of stainless steel and alloys. The ground state electronic configuration is [Ar] 3d44s2. It exhibits +2 to +6 oxidation states. Most stable oxidation state are +2 (CrO), +3 (Cr 2 O 3) and +6 (K 2 Cr 2 O 7).

#### **16.1. Optical spectra**

a. Divalent chromium*(d2)*

Cr2+ has a d4 configuration and forms high spin complexes only for crystal fields less than 2000 cm-1. The ground state term in an octahedral crystal field is 5Eg belonging to the 3 1 <sup>2</sup>*g g t e* configuration. The excited state 5T2g corresponds to promotion of one single electron to give 2 2 <sup>2</sup>*g g t e* configuration. The d4 electron is susceptible to Jahn-Teller distortion and hence Cr2+ compounds usually are of low symmetry. In lower symmetry, the excited quintet state of Cr2+ splits into three levels and the ground level quintet state splits into two levels. In the case of Cr2+(H2O)6, the value of Dq is 1400 cm-1. In spinels, Cr2+is in the tetrahedral environment and Dq is about 667 cm-1only.

b. Trivalent chromium(d3):

In octahedral symmetry, the three unpaired electrons are in <sup>3</sup> <sup>2</sup>*<sup>g</sup> t* orbitals which give rise to 4A2g, 2Eg, 2T1g and 2T2g states. Of these 4A2g is the ground state. If one electron is excited, the configuration is 2 1 <sup>2</sup>*g g t e* which gives two quartet states 4T1g and 4T2g and a number of doublet states. When the next electron is also excited, the configuration is 1 2 <sup>2</sup>*<sup>g</sup> <sup>g</sup> t e* which gives rise to one quartet state 4T1g and some doublet states.

$$\begin{aligned} \,^4F &\to \,^4A\_{2\_{\mathcal{S}}}\left(F\right), \,^4T\_{1\_{\mathcal{S}}}\left(F\right), \,^4T\_{2\_{\mathcal{S}}}\left(F\right), \\\\ &\,^4P \to \,^4T\_{1\_{\mathcal{S}}}\left(P\right) \end{aligned}$$

$$\begin{aligned} \,^2\mathrm{G} &\rightarrow \,^2A\_{1\_{\mathbb{X}}}\left(\mathrm{G}\right)\_{\prime}\,^2T\_{1\_{\mathbb{X}}}\left(\mathrm{G}\right)\_{\prime}\,^2T\_{2\_{\mathbb{X}}}\left(\mathrm{G}\right)\_{\prime}\,^2E\_{\mathbb{X}}\left(\mathrm{G}\right)\_{\prime} \\\\ \,^2H &\rightarrow \,^2E\_{\mathbb{X}}\left(H\right)\_{\prime}\,\mathrm{2}\,^2T\_{1\_{\mathbb{X}}}\left(H\right)\_{\prime}\,^2T\_{2\_{\mathbb{X}}}\left(H\right) \end{aligned}$$

In both fields, 4A2g,(F) represents the ground state. Hence, three spin allowed transitions are observed in high spin state 4A2g(F) 4T2g(F) (1), 4A2g(F) 4T1g(F) (2) and 4A2g(F) 4T1g(P) (3). These spin allowed bands split into two components when the symmetry of Cr3+ ion is lowered from octahedral to C4V or C3V. Generally, 4A2g(F) 4T1g(P) occurs in the UV-Vis region.

The strong field electronic configurations for the ground state and their terms are given as follows:

$$\begin{aligned} \left(\boldsymbol{t}\_{2\_{\mathcal{S}}}\right)^{3} \left(\boldsymbol{e}\_{\boldsymbol{s}}\right)^{0} &: \, ^{4}A\_{2\_{\mathcal{S}}}\left(\boldsymbol{F}\right) \, ^{2}E\_{\boldsymbol{s}}\left(\boldsymbol{G}\right) \, ^{2}T\_{1\_{\mathcal{S}}}\left(\boldsymbol{G}\right) \, ^{2}T\_{2\_{\mathcal{S}}}\left(\boldsymbol{G}\right) \\\\ \left(\boldsymbol{t}\_{2\_{\mathcal{S}}}\right)^{2} \left(\boldsymbol{e}\_{\boldsymbol{s}}\right)^{1} &: \, ^{4}T\_{1\_{\mathcal{S}}}\left(\boldsymbol{F}\right) \, ^{4}T\_{2\_{\mathcal{S}}}\left(\boldsymbol{F}\right) \, ^{2}T\_{2\_{\mathcal{S}}}\left(\boldsymbol{H}\right) \\\\ \left(\boldsymbol{t}\_{2\_{\mathcal{S}}}\right)^{1} \left(\boldsymbol{e}\_{\boldsymbol{s}}\right)^{2} &: \, ^{4}T\_{1\_{\mathcal{S}}}\left(\boldsymbol{P}\right) \end{aligned}$$

Racah parameter, B, is calculated with spin allowed transitions using equation (17)

$$B = \begin{pmatrix} 2\nu\_1^2 + \nu\_2^2 - 3\nu\_1\nu\_2 \\ \end{pmatrix} \Bigg/ \begin{pmatrix} 15\nu\_2 - 27\nu\_1 \\ \end{pmatrix} \tag{17}$$

The octahedral crystal field parameter Dq is characteristic of the metal ion and the ligands. The Racah parameter, B depends on the size of the 3d orbital; B is inversely proportional to covalency in the crystal.

#### **16.2. EPR spectra of chromium compounds**

24 Advanced Aspects of Spectroscopy

transfer transitions.

**16. Chromium** 

+3 (Cr 2 O 3) and +6 (K 2 Cr 2 O 7).

environment and Dq is about 667 cm-1only.

one quartet state 4T1g and some doublet states.

In octahedral symmetry, the three unpaired electrons are in <sup>3</sup>

states. When the next electron is also excited, the configuration is 1 2

**16.1. Optical spectra** 

2 2

a. Divalent chromium*(d2)*

b. Trivalent chromium(d3):

configuration is 2 1

. The minimum value of 10Dq for <sup>3</sup> *VO*<sup>4</sup>

energy separation (8000 cm-1) observed for tetrahedral <sup>3</sup> *VO*<sup>4</sup>

of <sup>3</sup> *VO*<sup>4</sup>

Vanadium doped silica gel also shows sharp band at 41520 cm-1 and shoulders at 45450 and 34480 cm-1. These are also assigned to charge transfer transitions in tetrahedral environment

geometry. This is expected because the two bands at 34480 and 45450 cm-1 are from the ligand orbitals to two vacant d orbitals which are 10Dq apart. This would be about twice the

satisfy the assignment of bands to d-d transitions. Therefore the bands are due to charge

Chromium is the 6th most abundant transition metal. Chromium is used in the manufacture of stainless steel and alloys. The ground state electronic configuration is [Ar] 3d44s2. It exhibits +2 to +6 oxidation states. Most stable oxidation state are +2 (CrO),

Cr2+ has a d4 configuration and forms high spin complexes only for crystal fields less than 2000 cm-1. The ground state term in an octahedral crystal field is 5Eg belonging to the 3 1

configuration. The excited state 5T2g corresponds to promotion of one single electron to give

<sup>2</sup>*g g t e* configuration. The d4 electron is susceptible to Jahn-Teller distortion and hence Cr2+ compounds usually are of low symmetry. In lower symmetry, the excited quintet state of Cr2+ splits into three levels and the ground level quintet state splits into two levels. In the case of Cr2+(H2O)6, the value of Dq is 1400 cm-1. In spinels, Cr2+is in the tetrahedral

4A2g, 2Eg, 2T1g and 2T2g states. Of these 4A2g is the ground state. If one electron is excited, the

 44 4 4 <sup>212</sup> , , *ggg F A F TF TF*

> 4 4 <sup>1</sup>*<sup>g</sup> P TP*

<sup>2</sup>*g g t e* which gives two quartet states 4T1g and 4T2g and a number of doublet

is expected at about 16000 cm-1 in octahedral

.Hence the evidence does not

<sup>2</sup>*<sup>g</sup> t* orbitals which give rise to

<sup>2</sup>*<sup>g</sup> <sup>g</sup> t e* which gives rise to

<sup>2</sup>*g g t e*

Cr3+ ion, splits into |1/2 and |3/2 Kramers' doublets in the absence of magnetic field, separated by 2D, D being the zero-field splitting parameter. This degeneracy can be lifted only by an external magnetic field. In such a case, three resonances are observed corresponding to the transitions, |-3/2 |-1/2, |-1/2 |1/2 and |1/2 |3/2 at gB – 2D, gB and gB + 2D respectively. In a powder spectrum, mainly the perpendicular component is visible. If all the three transitions are observed, the separation between the extreme sets of lines is 4D [gB + 2D –(gB - 2D) = 4D]. If D is equal to zero, a single resonance line appears with g ~ 1.98. If D is very large compared to microwave frequency, a single line is seen around g = 4.0.

#### **16.3. Relation between EPR and optical absorption spectra**

A comparison is made between the observed geff from EPR results and the calculated one from the optical spectrum. For Cr3+, EPR and optical results are related by,

$$\mathbf{g}\_{11} = \mathbf{g}\_{\epsilon} - \frac{8\mathcal{X}}{\Delta E \left( {}^{4}T\_{1\_{\mathcal{X}}} \left( F \right) \right)}\tag{18}$$

$$\mathbf{g}\_1 = \mathbf{g}\_\iota - \frac{8\mathcal{A}}{\Delta E \left( ^4T\_{\mathbf{z}\_\mathcal{S}} \left( F \right) \right)}\tag{19}$$

Here g11 and g are the spectroscopic splitting factors parallel and perpendicular to the magnetic field direction, g , the free electron value ge, is 2.0023. These values give,

$$\mathcal{g}\_{\mathcal{eff}} = \frac{1}{3} (\mathcal{g}\_{11} + \mathcal{g}\_1) \,. \tag{20}$$

The value of D can also be estimated from the optical absorption spectrum. The 4A2g(F) →4T2g(F) component in the optical spectrum is due to the lowering of symmetry which also includes the D term.

$$D = \left(\frac{2\mathcal{A}}{10Dq}\right)^2 \left(\Lambda\_z - \Lambda\_\chi\right). \tag{21}$$

 The spin-orbit splitting parameter, [for free ion, Cr3+ is 92 cm-1] is related to Racah parameter (B) by the equation,

$$
\lambda = 0.11 \text{(B+1.08)}^2 + 0.0062 \tag{22}
$$

#### **16.4. Typical examples**

The data chosen from the literature are typical for each sample. The data should be considered as representative only. For more complete information on specific examples, the original references are to be consulted. X-band spectra and optical absorption spectra of the powdered sample are recorded at room temperature (RT).

1. Trivalent chromium [*d3*]: The optical absorption spectrum of fuchsite recorded in the mull form at room temperature shows bands at 14925, 15070, 15715, 16400, 17730 and 21740 cm-1. The two broad bands at 16400 and 21740 cm-1 are due to spin-allowed transitions, 4A2g(F) 4T2g(F) and 4T1g(F) respectively. The band at 17730 cm-1 is the split component of the 4T2g(F) band. This indicates that the site symmetry of Cr3+ is C4v or C3v. The bands at 16400 and 21700 cm-1 are responsible for the green color of the mineral. The additional weak features observed for the 1 band at 15715 and 15070 cm-1 are attributed to the spin-forbidden transitions, 4A2g 2T1g(G) and 4A2g 2Eg(G). Using equation (17), Racah parameter, B, is calculated (507 cm-1). Substituting Dq and B values and using T-S diagrams for d3 configuration and solving the cubic field energy matrices ,another Racah parameter, C is evaluated (2155 cm-1) which is less than the free ion value [C =3850 cm-1].

Several examples are available in the literature. Some of them are given in the Table-12.

Electronic (Absorption) Spectra of 3d Transition Metal Complexes 27


1. The EPR spectrum of fuchsite recorded at room temperature (RT) clearly indicates a strong resonance line with a few weak resonances on either side of it. The g value for this centrally located strong line is 1.98. This is due to the main transition |-1/2 |1/2 of Cr3+. The calculated value of D is around 270 G. For weak lines, D is around 160 G. Since the lines are equally spaced on either side of the strong resonance, E is zero. The strong line at g (1.98) value is observed indicating a high concentration of chromium.

2. The EPR spectrum of chromate shows a broad EPR signal with g value of 1.903 which may be due to Cr3+ which is in high concentration in the mineral. The chromium ion is in octahedral coordination.

3. EPR spectrum of zoisite at LNT givesa g and D values of 1.99 and 42.5 mT respectively which are due to Cr3+ in octahedral environment.

4. EPR spectrum of chromium containing fuchsite quartz shows a g value of 1.996 which may due to Cr3+ which is in octahedral environment.

5. EPR spectrum of blue sapphire shows four Cr3+ sites with the same g value of 1.98 having different D values (130,105,65 and 34 mT) . Green sapphire also has the same g value but different D values (132,114, 94 and 35 mT). The results suggest that chromium content is slightly different in different sapphires.

**Table 12.** Assignment of bands for Cr(III) with 4A2g(F) ground state. All values are given in cm-1

Several examples are given in the literature. Some of them are presented in the Table-13.


**Table 13.** EPR parameters of Cr3+ compounds.

26 Advanced Aspects of Spectroscopy

which also includes the D term.

parameter (B) by the equation,

**16.4. Typical examples** 

value [C =3850 cm-1].

 <sup>11</sup> <sup>4</sup> 1 8

 <sup>1</sup> <sup>4</sup> 2 8

Here g11 and g are the spectroscopic splitting factors parallel and perpendicular to the

The value of D can also be estimated from the optical absorption spectrum. The 4A2g(F) →4T2g(F) component in the optical spectrum is due to the lowering of symmetry

2

The spin-orbit splitting parameter, [for free ion, Cr3+ is 92 cm-1] is related to Racah

<sup>2</sup>

The data chosen from the literature are typical for each sample. The data should be considered as representative only. For more complete information on specific examples, the original references are to be consulted. X-band spectra and optical absorption spectra of the

1. Trivalent chromium [*d3*]: The optical absorption spectrum of fuchsite recorded in the mull form at room temperature shows bands at 14925, 15070, 15715, 16400, 17730 and 21740 cm-1. The two broad bands at 16400 and 21740 cm-1 are due to spin-allowed transitions, 4A2g(F) 4T2g(F) and 4T1g(F) respectively. The band at 17730 cm-1 is the split component of the 4T2g(F) band. This indicates that the site symmetry of Cr3+ is C4v or C3v. The bands at 16400 and 21700 cm-1 are responsible for the green color of the mineral. The additional weak features observed for the 1 band at 15715 and 15070 cm-1 are attributed to the spin-forbidden transitions, 4A2g 2T1g(G) and 4A2g 2Eg(G). Using equation (17), Racah parameter, B, is calculated (507 cm-1). Substituting Dq and B values and using T-S diagrams for d3 configuration and solving the cubic field energy matrices ,another Racah parameter, C is evaluated (2155 cm-1) which is less than the free ion

Several examples are available in the literature. Some of them are given in the Table-12.

2 10 *<sup>D</sup> z x Dq* 

powdered sample are recorded at room temperature (RT).

1

11 1

*g g ET F*

*g g ET F*

*g*

*g*

(19)

<sup>3</sup> *eff <sup>g</sup> g g* . (20)

0.11 1.08 0.0062 *B* (22)

. (21)

(18)

*e*

*e*

magnetic field direction, g , the free electron value ge, is 2.0023. These values give,

2. Tetravalent chromium (d2):

Absorption spectra of Cr4+ in forsterite and garnet show the absorption band at 9460 cm-1 which is the typical of Cr4+ ions. It is attributed to the 3A2g 3T2g transition. The absorption band at 19590 cm-1 is also attributed to 3A2g 3T1g transition. The absorption band at 19590 cm-1overlaps with the bands at 16130 and 23065 cm-1.
