EPR and Optical Absorption Studies of Cu²⁺ in Boro-Arsenate Glasses

Abstract

Electron paramagnetic resonance (EPR) and optical absorption studies of xMgO- (25-x) Li₂O-50B₂O₃-25As₂O₃ glasses were made by introducing Cu²⁺ as a spin probe. The EPR spectra of all the glass samples recorded at X-band frequencies have similar spectral features. The Cu²⁺ ions are in well-defined axial sites, but subjected to small distortion leading to the broadening of the spectra. The spin-Hamiltonian parameter values indicate that the ground state of Cu²⁺ is d_{x2- y2} and the site symmetry around Cu²⁺ ions is tetragonal distorted octahedral. The optical absorption spectra exhibited a broadband corresponding to the d-d transition bands of Cu²⁺ ion. By correlating EPR and optical data, the bond parameters were evaluated and the values show purely ionic nature for the in-plane σ bonding and in-plane π bonding. The out-of-plane π bonding is moderately covalent.

Keywords:
Glass; spin-Hamiltonian; Optical Absorption; Electron Spin Resonance

1. Introduction

Heavy metal oxide based glasses containing As₂O₃ has received significant attention owing to their interesting optical applications. These glasses have large non-linear optical susceptibility coefficient^1-4 that makes them suitable for potential applications in non-linear optical devices (such as ultra-fast optical switches and power limiters), broad band optical amplifiers operating around 1.5 µm and in a number of solid state ionic devices.

Electron paramagnetic resonance (EPR) spectroscopy is an experimental technique sometimes capable of determining the co-ordination and environment of paramagnetic ions in glasses^5,6. EPR behaviors of oxide glasses doped with transition metal (TM) ions have been extensively studied to obtain information on the glassy network and to identify the site symmetry around the TM ions^7-10. Glasses containing TM ions exhibit memory and photo conducting properties¹¹. Recently, Sumalatha et al.¹², Hayder Khudhair Obayes et al.¹³, Edukondalu et al.¹⁴ studied the local structure around Cu²⁺ ion in oxide glasses by EPR spectra. Copper (II) is the most amenable ion for EPR studies. The main advantage of using Cu²⁺ as the spin probe is that it EPR spectra can easily be recorded at room temperature, the spectrum is simple and the spread of the spectrum is large enough to detect minute changes in the coordination sphere¹⁵.

Optical absorption of transition metal ions in glasses is influenced by the host structure into which the transition metal ions are incorporated. In oxide glasses, the TM ions mostly form coordination complexes with doubly charged oxygen as the ligands. The actual structure will depend on the composition of the glass system¹⁶. By correlating the EPR and optical spectra, one can obtain information regarding the bond parameters which determine the metal-ligand bond in the glasses. The properties of a glass can often be altered by the addition of a network modifier to the basic constituents. The commonly used network modifiers are the alkali and alkaline earth oxides¹⁷. It was observed that the properties of an alkali oxide glass show a non-linear behavior when one kind of alkali is gradually replaced by another. This departure from linearity is called the mixed alkali effect¹⁸. Similar observations were made in the case of mixed alkali-alkaline earth oxide glasses¹⁹. This phenomenon is called mixed oxide effect. In this paper, we report EPR and optical absorption studies of Cu²⁺ spin probe in the quaternary glass system xMgO-(25-x)Li₂O-50B₂O₃-25As₂O₃. The influence of varying the concentrations of Li₂O and MgO, which acts as network modifiers on the spin-Hamiltonian parameters, is discussed.

2. Experimental

Pure and copper (1 mole %) doped glass, samples of composition (table 1)

Table 1
Spin-Hamiltonian and optical absorption and bonding coefficients for Cu²⁺ions in xMgO-(25-x)Li₂O-50B₂O₃-25As₂O₃ glasses.

Parameters	x=0	x=5	x=10	x=15	x=20	x=25
g||	2.455	2.442	2.430	2.434	2.439	2.446
g⊥	2.057	2.057	2.057	2.058	2.058	2.058
A||	139	144	156	162	159	153
A⊥	18	13	25	13	13	25
λ	722	740	758	730	769	770
G	8.276	8.038	7.819	7.751	7.840	7.966
∆Exy	12953	13513	13192	13698	13004	12987
α2	0.901	0.906	0.928	0.970	0.964	0.933
β12	0.981	0.989	0.916	0.919	0.887	0.931
β2	0.657	0.653	0.638	0.610	0.614	0.634
Γσ (%)	21	20	16	6	18	15
ΓΠ (%)	4	2	17	16	23	14

xMgO-(25-x)Li₂O-50B₂O₃-25As₂O₃ (0 ≤ x ≤ 25) were prepared using the conventional melt-quench technique. Glasses were prepared by mixing the required proportions of the reagent grade Li₂CO₃, H₃BO₃, As₂O₃ (May and Baker), MgO (Fluka) and CuO in an electrical furnace using silica crucibles. The furnace temperature is varied from 1000- 1150 ºC depending on the glass composition. For samples taken from different regions of the bulk specimen, the absence of any Bragg peaks in the X-ray diffraction pattern confirmed that the glasses are amorphous and homogeneous.

The room temperature EPR spectra of powdered glass samples were recorded using a JOEL PE-3X EPR spectrometer operating in the X-band and employing a field modulation of 100 kHz. DPPH was used as the standard g marker for the determination of magnetic field. The optical absorption spectra of the present glasses in 200 - 800 nm region was recorded by using a Shimadzu spectrometer (model UV-3100) at room temperature.

3. Results and Discussion

3.1. EPR spectra

The room temperature Cu²⁺ ion doped EPR spectra of the present glasses were shown in Figure 1. The EPR spectra of Cu²⁺ ions in the remaining glasses exhibited the same spectral features. In all the EPR spectra recorded in the present investigation three parallel components were observed in the low field region. However, the perpendicular components are not resolved, leading to an intense line in the high field region.

Figure 1
EPR spectra of Cu²⁺ in the present glass system.

Spectroscopic splitting (g) and hyperfine (A) tensors with axial symmetry have been assumed in the analysis of EPR spectra of oxide glasses^20,21. The Jahn-Teller effect causes predominantly an elongated octahedral coordination with four short in-plane bond lengths. Therefore an axial spin-Hamiltonian can be employed in the analysis of the EPR spectra which is given below:

(1) $\begin{array}{c}\mathit{H}=g_{\mathrm{||}}\beta H_z{S}_z+{\mathit{g}}_{\perp }\beta \left(H_x{S}_x+H_y{S}_y\right)+\hfill \\ A_{\mathrm{||}}{I}_z{S}_z+A_{\perp }\left(I_x{S}_x+I_y{S}_y\right)\hfill \end{array}$

where the symbols have their usual meanings²². The nuclear quadrupole contribution is neglected. The calculated spin-Hamiltonian parameters are given in Table 1.

The observed g values are characteristic of Cu²⁺ coordinated by six ligands, which form an octahedron elongated along the z-axis. The general nature of the ligand coordination can be obtained²³ from the fact that g_||, g_⊥, A_|| and A_⊥ g|| > g_⊥ > 2.0023. Only an environment elongated along one of the cube axis can yield this result. In the present investigation, it is observed that g|| > g_⊥ > g_e. Therefore from the g values and shape of the EPR spectra, it can be concluded that the ground state of the Cu²⁺ is d_x2_-y2 orbital, the Cu²⁺ ions being located in tetragonally distorted octahedral sites^12,20,23,24.

The g_|| and A_|| values are found to be dependent on the glass composition while g_⊥ and A_⊥ values are essentially constant. The variation of g_|| and A_|| with MgO content is illustrated in Figure 2. g_|| and A_|| varies non-linearly as the MgO content increases: g_|| decreases and then increases whereas A_|| increases and then decreases indicating the change in the tetragonal distortion of Cu²⁺ ions²⁵. The variation in g_|| and A_|| values may be associated with the change in the environment of Cu²⁺. In the B₂O₃ glasses, the addition of network modifiers (MgO and Li₂O) leads to an increase in the coordination number of a certain portion of the boron atoms from 3 to 4. It is assumed that the resulting glass is composed of both triangular and tetrahedral units which form a relatively open network with holes between the oxygen atoms of sufficient size to accommodate the Li and Mg ions¹⁵. As a doubly charged cation, Mg²⁺ is sufficiently strong to split the network. Thus, sufficient non-bridging oxygen's are available for good coordination in the broken network. The alkali oxide Li₂O makes available additional weakly bonded O^2- for each Mg²⁺, i.e. Mg²⁺captures the O^2- from Li₂O and this happens at the expense of Li₂O coordination. Li⁺ should remain in the neighborhood of the next stronger Mg²⁺ than that it should be incorporated separately into the rigid network. The configuration Li-O-Mg may energetically favored. The solubility of the Cu²⁺ increases with the addition of the network modifiers presumably due to the coordination of the metal ion by the extra oxygen ions. Thus, incorporation of MgO into the glass will influence the field at the site of Cu²⁺, which in turn will reflect in the non-linear variation of the spin Hamiltonian parameters as observed in the present study.

Figure 2
Variation of g|| and A|| with MgO content in the present study.

The line width of the parallel hyperfine components was found to increase with increasing values of the nuclear spin quantum number (ml), which may be due to fluctuations in both the ligand fields and bond covalencies from one copper (II) complex to the next, giving rise to a narrow distribution in g²⁴. The mobility of the paramagnetic species in the glass medium may also affect the EPR line shape.

3.2. Optical absorption spectra

In a regular octahedral field, the 3d⁹ configuration would result in the adegenrate ground state (²E_g). In glasses, it is assumed that no site is perfect cubic due to the disordered vitreous state. Hence, the tetragonal distortion is endemic to the vitreous state that leads to splitting of energy levels. It is observed that the elongated structures are usually more energetically favored than the compressed ones. For Cu²⁺ in an elongated octahedral symmetry , more than one band is observed¹. Figure 3 presents the optical absorption spectrum of Cu²⁺ ions in the present glasses.

Figure 3
Optical absorption spectra of present glass system.

The optical absorption co-efficient α(ν), near the fundamental absorption edge of the curve in the above figure was determined from the relation

(2) $\mathit{\alpha }\left(\mathit{v}\right)=\left(1/d\right)\log \left(I_t/I_0\right)$

where I₀ and I_t are the intensities of the incident and transmitted beams, respectively, and d is thickness of the glass sample. The factor log(I₀/I_t) corresponds to absorbance. Davis and Mott²⁶ and Tauc and Menth²⁷ relate this data to the optical band gap, E_opt through the following general relation proposed for amorphous materials

(3) $\mathit{\alpha }\left(\mathit{v}\right)=B{\left(h\mathit{v}-E_{\mathrm{opt}}\right)}^n/h\mathit{v}$

where B is a constant related to the extent of the band tailing and hν is incident photon energy. The index n determines the type of electronic transitions causing the absorption, and takes the values 1/2, 2, 3/2 and 3 for direct allowed, indirect allowed, direct forbidden, and indirect forbidden transitions. By plotting (αhν)^1/2 and (αhν)² as a function of photon energy hν, one can find the optical energy band gap E_opt for indirect and direct transitions, respectively. The respective values of E_opt are obtained by extrapolating to ( αhν)^1/2 = 0 for indirect transition and (αhν)² = 0 for direct transition. Figure 4 represents the Tauc plots {(αhν)^1/2 vs hν} for different glass samples. The values of the all optical energy gap, E_opt, thus obtained from the extrapolation of the linear region of Tauc plots are presented in Table 2. In the present glasses, the indirect band gap energy varies from 2.34 to 2.5 eV and dirct band gap energy vary between 2.55 to 2.68 eV respectively. In the present glasses, the compositional dependence of direct and indirect optical band gap energies is illustrated in Figure 5. It is observed that the band gap energies vary non-linearly with compositional parameter, indicating the existence of mixed alkali effect.

Table 2
Optical parameters of the present glass system

Parameter	x = 0	x = 5	x = 10	x = 15	x = 20	x = 25
Optical band gap energies (Eopt ) (eV) (± 0.05)
Indirect allowed (n=1/2)	2.50	2.39	2.42	2.34	2.38	2.41
Indirect forbidden (n=1/3)	2.42	3.35	2.37	3.32	2.37	2.40
Direct allowed (n=2)	2.66	2.59	2.68	2.62	2.55	2.64
Direct forbidden (n=2/3)	2.65	2.61	2.70	2.64	2.66	2.67
Urbach energy ∆E (eV) (±0.001)	0.426	0.421	0.429	0.399	0.428	0.423

Figure 4
Tauc's plots (n=1/2, 2, 2/3 and 1/3 ) for the present glass system.

Figure 5
Optical band gap energy (for all transitions) as a function of composition (mol%) in present glasses.

The main feature of the absorption edge of amorphous materials is an exponential increase of absorption coefficient α(ν) with photon energy hν, which is given by the Urbach rule²⁸

(4) $\mathit{\alpha }\left(\mathit{v}\right)=C\exp \left(h\mathit{v}/\Delta E\right)$

where C is a constant, and ΔE is the Urbach energy which is a measure of band tailing. The values of Urbach energy (ΔE) were determined by taking the reciprocals of slopes of the linear portion of ln(α) vs hν curves. The Urbach energy values of the present glass samples are presented in Table 2. The compositional dependence of Urbach energy ΔE is illustrated in Figure 6. Urbach energy is varying non-linearly with a compositional parameter indicating the existence of mixed alkali effect in present glasses.

Figure 6
Compositional dependence of Urbach energy (ΔE) in the present glasses.

The optical absorption spectra of all glasses studied reveal only a broad absorption band. The optical absorption peak position values are determined by peak pick facility of the spectrometer and are presented in Table 1 and shown in Figure 7.

Figure 7
The compositional mole% as a function of peak position (λ).

The stable state of copper are Cu⁺ and Cu²⁺. The Cu⁺ ion has 3d¹⁰ configuration and consequently is not expected to have any ligand field band. The Cu²⁺ ion has the 3d⁹ configuration and the spectra associated with the Cu²⁺ ion are d-d transitions which can be described in terms of the ligand field theory²⁹. In a regular octahedral field, the 3d⁹ configuration would result in a degenerate ground state (²E_g) and the Jahn-Teller effect gives a marked tetragonal distortion which leads to splitting of energy levels. In glasses it is assumed that due to disordered vitreous state no site is perfectly cubic. Therefore, tetragonal distortions are endemic to the vitreous state which leads to splitting of energy levels. It is observed that the elongated structures are usually more energetically favored than the compressed ones and the cupric ion exists in solution, solids and glasses in octahedral symmetry with a tetragonal distortion³⁰. For Cu²⁺ in elongated octahedral symmetry more than one band will be observed¹². But only a single optical absorption maximum was observed in most of the cases^31,32. This single optical band was interpreted as the overlap of all the three transitions. Hence, in the present investigation the observed asymmetric band around 13,192 cm^-1 is due to the overlap of ²B_1g →²A_1g and ²B_1g →²B_2g transitions. Most of the authors^33,34 assigned the observed optical peak to the ²B_1g→²B_2g transition (ΔE_xy) and have used this value in the evaluation of the bond parameters.

3.3. Cu²⁺ ligand bond nature

The EPR and optical absorption data can be related to evaluate the bonding coefficients of Cu²⁺. The bonding between the Cu²⁺ ion and its ligands can be described in terms of the covalency parameters α², β² and β₁ ² where α² describes the in-plane σ bonding with the copper d_x2_-y2 orbital, β² describes the out-of-plane π bonding with the d_xz and d_yz orbital and the β₁ ² parameter is a measure of the in-plane π bonding with the d_xy orbital. The values of α² lie between 0.5 and 1, the limits of pure covalent and pure ionic bonding, respectively. The terms β² and β₁² can be interpreted similarly.

EPR results give rise to a new parameter (G), which is defined as

(5) $G=\left(g\mathit{ll}-g_e\right)/\left(g_{\perp }-g_e\right)$

If the G value falls in between 3 and 5, the unit cell contains magnetically equivalent ions. If the G value is less than 3, the exchange coupling among the magnetically non- equivalent Cu(II) ions in the unit cell is not very strong. If G is greater than 5, a strong exchange coupling takes place among the magnetically non -equivalent Cu(II) ions in the unit cell. Truly compressed structures are relatively rare when compared to elongated structures³⁵.

The bonding parameters were evaluated using the equations given below³⁶

(6) ${\mathit{\alpha }}^2=-\left({\mathit{A}}_{\mathrm{||}}P\right)+\left({\mathit{g}}_{\mathrm{||}}-2\right)+3/7\left({\mathit{g}}_{\perp }-2\right)+0.04$

(7) $\beta _1{}^2=\left[\left(g_{\perp }/g_e\right)-1\right]\Delta E_{\mathrm{xy}}/3312{\alpha }^2$

(8) ${\beta }^2=\left[\left(g_{\perp }/g_e\right)-1\right]\Delta E_{\mathrm{xz},\mathrm{yz}}/828{\alpha }^2$

where P is the dipolar hyperfine coupling parameter. ΔE_xy is the energy corresponding to the transition ²B_1g → ²B_2g and λ is the spin-orbit coupling constant (λ= -828 cm^-1).

The corresponding value of ΔE_xz,yz was calculated using the approximation¹⁷

(9) $\Delta E_{\mathrm{xz},\mathrm{yz}}=1656K^2/\left(g_{\perp }/g_e\right)$

(10) $\begin{array}{c}{\Gamma }_{\mathit{\sigma }}=200\left(1-S\right)\left(1-{\alpha }^2\right)/\left(1-2S\right)\%\mathrm{and}\hfill \\ {\Gamma }_{\mathit{\pi }}=200\left(1-\beta _1{}^2\right)\%\hfill \end{array}$

where K² is the orbital reduction factor (= 0.77).

The normalized covalencies of Cu (II)-O in-plane bonds of σ and π symmetry are expressed³⁷ in terms of bonding coefficients α² and β₁ ² as follows. Where S is the overlap integral (S_oxy = 0.076). The calculated values of α², β₁ ², β², are presented in Table 1. are illustrated in Figure 8. The normalized covalency of Cu (II)-O in-plane bonding of π symmetry (Γ_π) indicates the basicity of the oxide ion. The values of α² andβ₁ ² indicate pure ionic for the in-plane σ bonding and in-plane π bonds, while β² out of the plane. Π bonding seems to be mostly covalent nature in all the glass systems.

Figure 8
The mole% as a function of Γ_σ and Γ_π

4. Conclusions

The quaternary glass system of xMgO-(25-x)Li₂O-50B₂O₃-25As₂O₃ (0 ≤ x ≤ 25) were prepared, and their optical and EPR measurements have been studied. The following conclusions were made: From EPR and optical measurements it is clear that Cu²⁺ ions are present in all the glasses investigated and they exist on tetragonally distorted octahedral sites with d_x2_-y2. The spin-Hamiltonian parameters are influenced by the composition of glass which may be attributed to the change of ligand field strength around Cu²⁺. With increasing MgO content the ligand field strength is reduced around the Cu²⁺ ion due to the modification of the boron-oxygen network. The bond parameter values show purely ionic nature of the in-plane σ bonding and in-plane π bonding. The out-of-plane π bonding is moderately covalent.

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Publication Dates

• Publication in this collection
  01 Dec 2016
• Date of issue
  Jan-Feb 2017

History

• Received
  20 Jan 2016
• Reviewed
  06 June 2016
• Accepted
  03 Nov 2016