Optical properties of zinc tellurite glasses doped with Cu2 + ions

Optical properties of zinc tellurite glasses doped with Cu2 + ions

Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: w...

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Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol

Optical properties of zinc tellurite glasses doped with Cu2 + ions O.A. Zamyatina,b,⁎, V.G. Plotnichenkoc, M.F. Churbanova,b, E.V. Zamyatinaa, V.V. Karzanova a b c

Lobachevsky State University of Nizhny Novgorod, 23 Gagarin Avenue, Nizhny Novgorod 603950, Russia G.G. Devyatykh Institute of Chemistry of High Purity Substances RAS, 49 Tropinin Str., Nizhny Novgorod 603951, Russia Fiber Optics Research Center of the Russian Academy of Sciences, 38 Vavilov Street, Moscow 119333, Russia



Keywords: ZnO–TeO2 glasses Cu2 + ions Optical properties Specific absorption coefficient Electron paramagnetic resonance.

Copper-containing ZnO–TeO2 glasses were synthesized using orthotelluric acid, zinc nitrate and the formed batch was doped with copper nitrate. EPR and optical spectra in a wide transparency range from were measured for all glass samples. The values of the spin-Hamiltonian parameters indicate that the Cu2 + ions in the glasses are present in octahedral coordination with some trigonal distortion. The molecular orbital bonding parameters have been calculated and were found to be independent of Cu2 + concentration in glasses. The optical transmittance spectra of doped glasses show a single broad band at 820 nm, which was assigned to the overlapping 2 B1g → 2B2g, 2B1g → 2Eg and 2B1g → 2A1g electronic transitions. The spectral dependence of the specific absorption coefficient of Cu2 + ions was calculated for glass samples in the range of 310–2800 nm and was found that the specific absorption coefficient at 820 nm is (103 ± 1) cm− 1/(wt%) and the integral absorption coefficient in the range 0.5–2.8 μm is (63 ± 2) cm− 2/(ppm wt). From the optical data some optical parameters such as the optical energy gap, Urbach energy, metallization criterion and the cut-off wavelength were calculated. It is observed that the optical band gap decreases with an increase in concentration of copper content.

1. Introduction Tellurite glasses have several useful optical and technological properties such as wide transmission window, high linear and nonlinear refractive indices and which find these applications as promising candidates for optical fiber and waveguides, Raman lasers, optical amplifiers, converters of radiation and contact solar cells [1–5]. High rareearth solubility and good chemical stability of the glasses allow the development of linear and non-linear fiber optic elements in a wide spectral region [6–8]. According to data in the literature, the ZnO–TeO2 system is a good basis for multicomponent glasses with possible ultra-low losses [4]. This has been achieved by research of the glass-formation region [9–14], crystallization processes [15–17], optical linear [18,19] and nonlinear [20,21], thermal [15–17,22], elastic [23], structural [24–26] and thermodynamic [27] properties. Moreover, various methods of forming the charge [28] and manufacturing glass preforms for fiber drawing [29] are investigated for this system. These glasses are recognized as promising candidates for LED and optoelectronics due to specific coordination [30,31]. Despite all these facts, the widespread use of zinc-tellurite glasses has been hampered by the high optical losses. In the visible and near IR region the losses are primarily associated with the absorption of

transition elements and the absorption of hydroxyl groups in the middle IR region with [32]. Influence of these elements on the optical properties of tellurite glasses was partially considered in reference [33–37]. The purpose of this paper was to present new results obtained by EPR and optical absorption performed on Cu2 + ions doped the ZnO–TeO2 glasses prepared by using acid and salt as raw materials. 2. Experimental 2.1. Glass preparation The glasses sample with the compositions 30ZnO–70TeO2 containing different concentrations of Cu2 + ions were prepared by a melting quenching method and the details of the glass compositions are given in Table 1. The starting materials used for the preparation of the glasses were orthotelluric acid (H6TeO6), hexahydrate of zinc nitrate (Zn(NO3)2 × 6H2O), and hexahydrate of copper(II) nitrate (Cu (NO3)2 × 6H2O) with a purity of 99.99%. Orthotelluric acid and zinc nitrate were mixed in a molar ratio corresponding to the glass composition 30ZnO–70TeO2, and the different volume of copper nitrate containing 0.80 mmol/L of Cu2 + ions was added. The mixtures in porcelain crucibles were then preheated at 130 °C for 2 h as to remove water content. The dry mixtures were then placed in an electric furnace,

Corresponding author at: Lobachevsky State University of Nizhny Novgorod, 23, Gagarin av., Nizhny Novgorod 603950, Russia. E-mail address: [email protected] (O.A. Zamyatin).

http://dx.doi.org/10.1016/j.jnoncrysol.2017.08.025 Received 23 February 2017; Received in revised form 21 June 2017; Accepted 15 August 2017 0022-3093/ © 2017 Published by Elsevier B.V.

Please cite this article as: Zamyatin, O.A., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.08.025

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Table 1 Optical parameters of the 30ZnO–70TeO2 glasses doped with Cu2+. Parameter











Content of Cu2 +, ppm Cut-off wavelength, λ (nm) Optical band gap, Eopt (eV)

0 360 3.16 3.39 0.318 2.355 14.85 5.890 2.103 1.032 0.876 0.397

10 362 3.15 3.38 0.295 2.358 14.87 5.897 2.107 1.032 0.878 0.397

25 364 3.13 3.37 0.289 2.363 14.89 5.905 2.112 1.03201 0.879 0.396

50 366 3.03 3.34 0.281 2.389 15.04 5.965 2.147 1.03201 0.892 0.389

100 372 2.95 3.29 0.276 2.411 15.19 6.025 2.183 1.03202 0.905 0.384

200 380 2.86 3.22 0.273 2.436 15.33 6.080 2.216 1.03204 0.916 0.378

300 386 2.84 3.18 0.270 2.442 15.35 6.089 2.222 1.03206 0.918 0.377

600 398 2.82 3.10 0.265 2.447 15.38 6.100 2.230 1.03212 0.921 0.375

1200 410 2.73 2.99 0.248 2.474 15.52 6.157 2.267 1.03225 0.933 0.369

2400 424 2.69 2.92 0.226 2.486 15.56 6.174 2.283 1.0325 0.938 0.367

Indirect Direct

Urbach energy, ΔE (eV) Refractive index (n) Molar refraction (Rm) (cm3/mol) Molar polarizability of the materials (αm × 10− 24 cm− 1) Electronic polarizability of oxide ions (αO2 −) (Å3) Theoretical Optical basicity (Λth) Optical basicity (Λ) Metallization criterion (M)

including tellurites and tellurates of zinc [53] and copper [53–56], having the additional ability to retention copper atoms in the divalent state. Such complex oxides, apparently, are the main components of the glass forming melt and are able to manifest themselves during the crystallization of glasses [57]. Note that doping process of the glasses with Cu2 + ion was carried out similarly to [58–62]. This method allows for creation of a homogeneous solution with a molecular dispersion of components which is subsequently able to maintain this quality in the glass-forming melt and glasses. An additional advantage of the method of doping is precise dosing and the possibility of introducing low concentrations without dilution method by repeated glass melting [62,63]. This procedure also allows creating a uniform distribution of copper ions throughout the glass sample ingot that can be easily controlled by the linear dependence of the optical density on the samples length.

where they were slowly heated to 500 °C to remove nitric oxide and oxygen. Then, the solid powders were melted at 800 °C for 15 min. The melts were quenched by pouring them into preheated steel mold which were annealed for about 1 h at 310 °C for 2 h. To provide the transmittance spectra measurements the glass samples with different content of copper ions, being the cylinders with a diameter of 9 mm and a length of 8 cm, were cut into disks of different thickness from 0.6 to 3 mm and then optically polished on two opposite sides (50 samples in total). The thickness of the polished samples was measured using an electronic micrometer with an accuracy of 10− 3 cm. 2.2. Optical transmittance spectra measurements The optical properties of the glass samples were recorded at room temperature in VIS and near IR region on a double beam Shimadzu UV3600 spectrophotometer in the wavelength range 310–2800 nm with a scanning step of 2 nm and a slit width of 8 nm. The cut-off wavelength correlated with the absorption edge of the glasses was taken as the wavelength corresponding to the maximum value in optical absorption spectrum. The IR transmission spectra of the glasses were recorded in the wavenumber range 380–7800 cm− 1 using FTIR spectrometer IRPrestige21 with a resolution of 4 cm− 1.

3.2. Optical absorption Fig. 1 shows the optical transmittance spectra of the ZnO–TeO2 parent and codoped glasses with different Cu2 + concentration from 10 to 2400 ppm wt. In the visible and near infrared region the spectra show a strong absorption band with the maximum at 820 nm which can be interpreted as the overlap of 2B1g → 2B2g [64–67], 2B1g → 2Eg [68,69] and 2B1g → 2A1g [70,71] electronic transitions of Cu2 + ions. The estimated value of the crystal field parameter of this ion is 1220 cm− 1. The intensity of the band is found to increase and the absorption edge has a red shift toward the higher wavelength with an increasing concentration of Cu2 + [69] (Table 1). The position and

2.3. EPR studies The EPR spectra of glass samples were recorded at room temperature on an automated spectrometer Bruker EMX + in the X-band frequency 9.4435 GHz and employing a field modulation of 100 kHz. The magnetic field was scanned from 2300 to 4300 G. 3. Result and discussion 3.1. Glass preparation method The traditional method for preparing tellurite glasses based on melting a mixture of tellurium and zinc oxides at high temperature in a controlled gaseous atmosphere with subsequent curing in glass [4]. The introduction of Cu2 + ions into the glasses is also carried out in the oxide form and can lead to a partial reduction to Cu+ [38–40]. This process might be suppressed using various external influences ensuring the preservation of the maximum number of copper atoms in the oxidation state +2 [41–44]. In this study, the copper atoms were stabilized in the oxidation state of +2 due to good oxidizing reagents (nitric oxide and oxygen) released during the decomposition of zinc [45,46] and copper [47,48] nitrates at temperatures of about 400°С. At a higher synthesis temperature the oxidizing effect is provided by orthotelluric acid decomposing with the evolution of oxygen [49–52]. The final decomposition products of these individual starting compounds are the binary oxides (ZnO, TeO2 and CuO). Besides, all these substances are capable of reacting with each other to form a variety of compounds,

Fig. 1. Optical transmittance of the 30ZnO–70TeO2 glasses doped with Cu2 + ions (l = 1 cm).


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Fig. 2. Dependence of the optical density for 30ZnO–70TeO2 glasses containing 0.12 wt% Cu2 + at 820 nm on the sample length and its linear approximation.

shape of the absorption peak are preserved for all investigated compositions. This indicates that the coordination environment of copper ions in the glass network depends weakly on concentration of Cu2 +. The wavelength values of the absorption band peak is close to those for other tellurite [33] and tellurite-molybdate glasses [33,60]. The absorption coefficient (α) of Cu2 + ions impurity for the glass samples was determined from the dependence of optical density (− lnT) on the glass thickness as the slope of the straight line (Fig. 2). The intercept on the ordinate gives the losses due to reflection and scattering inside the sample and depends on the heterogeneity and asymmetry of glass. The square of the correlation coefficient indirectly indicates a relatively uniform distribution of copper ions throughout the glass [60]. Fig. 3 shows the dependence of the absorption coefficient at 820 nm band (A), and the integral absorption coefficient in the range from 0.5 to 2.8 μm (B) on the Cu2 + content. The linear and integral absorption coefficient of copper ions in the 30ZnO–70TeO2 glasses were found from the slope of the lines and the values were calculated to be (103 ± 1) cm− 1/(wt%) and (63 ± 2) cm− 2/ppm wt, respectively. Comparison of the peak position of absorption band for Cu2 + ions and the value of the specific absorption coefficient for different glasses are given in Table 2. The maximum of band indicates that the presence of heavy atoms such as molybdenum, tungsten, zinc in oxide glasses as compared to silicate glasses shifts the peak toward longer wavelengths. It is noteworthy that the specific absorption coefficient for glasses of different composition has a fairly wide range of values. At the same time for glasses containing the heavy metal oxides, the values of specific absorption coefficient are the largest ones. The spectral dependence of the specific absorption coefficient over a wide range of wavelengths was calculated and is shown in Fig. 4. Based on the specific absorption coefficient the maximum permissible concentration of Cu2 + ions to achieve predetermined optical losses due to the impurity of copper atoms in tellurite glasses and fibers was estimated. In particular, the optical losses equal 100 dB/km in the spectral range of 0.5–2.8 μm may be achieved with the copper content approximately 20 ppbwt. In this regard, Cu2 + ions should be attributed to a very strong absorbance impurity for tellurite glasses and its contents should be carefully monitored when producing glasses for optical fibers.

Fig. 3. Dependence of the absorption coefficient at 820 nm (A) and the peak area of this band (B) on the copper content in the 30ZnO–70TeO2 glasses.

Table 2 Peak position and the specific absorption coefficient for the absorption band of Cu2 + ions for different glass systems.

3.3. Optical band gap and Urbach energy Transmittance spectroscopy is an important technique which permits to determine various optical parameters of materials such as the 3

Glass composition

Specific absorption coefficient at maximum absorption, dB/ (km ppm)

Maximum of absorption band, nm


SiO2 (Na2O)0.22(CaO)0.03(SiO2)0.75 (GeO2)0.29(Al2O3)0.02(SiO2)0.44 (Na2O)0.14(CaO)0.1(K2O)0.01 TeO2–GeO2–WO3 TeO2–Ag2O–WO3 Albite 50% Albite-50% Diopside ZrF4–BaF2–LaF3–AlF3–NaF–PbF2 (MoO3)0.20(TeO2)0.80 (ZnO)0.30(TeO2)0.70

250 ≈ 640 ≈ 250

500 800 780

[35] [69] [70]

– – ≈ 2250 – 500 4700 ± 30 4470 ± 40

806–839 794–808 790 780 ≈ 1000 830 820

[68] [62] [71] [41] [57] [Present paper]

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Fig. 4. Spectral dependence of the specific absorption coefficient for Cu2 + ions in the 30ZnO–70TeO2 glasses.

valence state of atoms, location of electronic levels, fundamental absorption edge, band gap, Urbach energy et al. According to [72,73] absorption at the short-wave edge of the glasses is determined by direct and indirect electronic transitions for which the dependence on photon energy (hv) is as follows:

αhν = A (hν − Eopt )r ,


where A is a constant, Eopt is the energy of optical band gap and the exponent r depends on the type of transition and have values 1/2 and 2 corresponding to the allowed direct and indirect transitions respectively. The dependence of (αhv)1/2 and (αhv)2 on energy was calculated from the absorption spectra and is shown in Fig. 5. The optical band gap energy was obtained by extrapolating the linear region to the (hv) axis at (αhv)1/2 = 0 and (αhv)2 = 0. The calculated values of the optical band gap energy are listed in Table 1 and the behavior as the function of Cu2 + concentration is shown in Fig. 6. The energy of indirect transitions decreases from 3.16 to 2.89 eV with the increasing Cu2 + content in the glasses. This tendency was also observed for fluoride glasses containing CuO [74]. The decrease of optical band gap and the shift of the cut-off wavelength to a higher wavelength with an increase in Cu2 + could be understood in terms of the variation in non-bridging oxygen (NBO) ion concentrations. From the physical point of view, the valence-band maximum state mainly consists of the oxygen p orbitals with small mixing of the cation p and d orbitals, which gives only small perturbations at the valence-band maximum [75]. The conduction band minimum mainly consists of Metal (ns) orbital [76]. The NBO ions contribute to the VBM and bind excited electrons less tightly than bridging oxygen (BO) content [77]. When a metal–oxygen bond is broken, the bond energy is released. The non-bridging orbitals have higher energies than bonding orbitals. Increase in concentration of the NBO ions results in the shifting of the valence-band maximum to higher energies and reduces the band gap [4]. Thus, the increasing in the Cu2 + content leads that non-bridging oxygen ion concentration in the glasses increases, which lowers the band gap energy. Another reason of the decrease of Eopt could be that at increasing of Cu2 + concentration, the broadening of the impurity band and the formation of band tails on the edges of the conduction and valence bands would lead to a reduction in Eopt [78]. The obtained energy values of indirect transitions vary from 3.39 to 2.92 eV with the increasing Cu2 + concentration and these are close to those for zincate-tellurite glasses [18,20,79–81]. For lower values of photon energy the absorption at the short-wave

Fig. 5. Plots of (αhv)2 for direct (A) and of (αhv)1/2 for indirect (B) band gap versus photon energy for Cu2 + doped 30ZnO–70TeO2 glasses.

Fig. 6. Concentration dependence of the direct and indirect optical band gap, Urbach energy and cut-off wavelength for the 30ZnO–70TeO2 glasses.


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3.5. Electronic polarizability and optical basicity 3.5.1. Electronic polarizability The average molar refraction (Rm) for the glasses were calculated by using Lorentz-Lorenz equation [87,88]

Rm =

(n2 − 1) M (n2 − 1) ⎛ ⎞= Vm, 2 (n + 2) ⎝ d ⎠ (n2 + 2)


where n is the calculated refractive index, M is the molecular weight, d is the density and Vm is the molar volume of glass. Molar refraction is related to the glass structure [89] and is proportional to the molar electronic polarizability αm of the constituent ions of the material (expressed in cm3 × 10− 24) according to Clasius-Mosotti equation

3 ⎞ Rm , αm = ⎛ ⎝ 4πN ⎠

where N is the number of polarized ions per mole of a substance equal to Avogadro's number (NA). This equation can be shortly written as Rm = 2.25αm, if αm is expressed in Å3. The calculated values of the molar polarizability are listed in Table 1. The results show that the refractive index and molar refraction values are increasing with an increase of polarizability. The average value of polarizability of the oxide ion (α(O2 −)) was calculated from Dimitrov and Sakka equation [84,90], based on the energy of the optical band gap

Fig. 7. Plot of lnα versus photon energy for the 30ZnO–70TeO2 glasses doped with Cu2 +.

edge of the glasses is usually well extrapolated by the exponential Urbach dependence [82]

hv ⎞ α (ν ) = α 0 exp ⎛ , ⎝ ΔE ⎠

⎡ V α O2 − (Eopt ) = ⎢ ⎛ m ⎞ ⎜⎛1 − 2.52 ⎠ ⎝ ⎣⎝


where α0 is a constant, ΔE is the Urbach's energy defined as the energy gap between localized tail states in the forbidden band gap. This expression can be also rewritten in logarithmic form

ln α (ν ) =

hv + ln α 0. ΔE


Λth = XZnO ΛZnO + XTeO2 ΛTeO2 + XCuO ΛCuO ,




where Xi are the mole fractions of oxides in glass, Λi are the optical basicity value of individual oxides taken from [92] (ΛZnO = 1.13, ΛTeO2 = 0.99, ΛCuO = 1.11). The optical basicity of glass is related to its polarizability by the equation [93]

The refractive index of the glass samples was evaluated from the optical band gap values using the following equation [84]


∑ αi ⎤⎥ (NO2−)−1,

3.5.2. Optical basicity Interaction of oxide components in glass may be considered from the viewpoint of acid-base interaction. Oxide anions can be considered as Lewis acids which are able to partially transfer the negative charge on cations. The greatest ability to transfer negative charge manifests itself in a weak environment. Duffy [91] proposed the concept of optical basicity based on the shift of UV spectrum due to the introduction of various oxides. The theoretical optical basicity (Λth) for the investigated glass samples was calculated from the expression:

3.4. Refractive index


Eopt ⎞ ⎟ − 20 ⎠

where ∑αi is the sum of molar cationic polarizabilities (for the glass 30ZnO–70TeO2, ∑αi = 0.3αZn2 + + 2(0.7αTe4 +) and NO2 − is the number of oxide ions in the chemical formula (for 30ZnO–70TeO2 glass, NO2 − = 1(0.3) + 2(0.7)). The molar cation polarizability was taken from [81] having the following values: αZn2 + = 0.283 Å3, αTe4 + = 1.595 Å3, αCu2 + = 0.437 Å3. The results of calculation of molar polarizability of oxide ion are also given in Table 1. Attention is drawn to a symbate increase in the refractive index and molar polarizability of oxygen ion with increasing of copper content in the glasses.

The plot of this dependence for all studied samples is given in Fig. 7. The values of Urbach energy (ΔE) were calculated from the reciprocal of the slope of the straight line from the plots of ln(α) vs hν and the data are listed in Table 1. The ΔE values decreases from 0.318 to 0.226 eV due to increase of Cu2 + content (Fig. 6). This can be explained by the addition of copper ions in the glasses decreases the degree of disordering the fragments of the glass network. Smaller values of Urbach energy as compared to the data for glasses of the same system [18,33,81] indicate that these glasses possess minimum number of defects [83] and samples are homogeneitic and stabile [33]. This advantage indicates that the used method of batch forming compared to the traditional one gives the glass samples a higher quality of homogeneity.

n2 − 1 =1− n2 + 2



1 ⎞ Λ = 1.67 ⎜⎛1 − ⎟, α O2 − ⎠ ⎝


The calculated values of refractive index of the prepared Cu doped glasses are given in Table 1. It was established that the addition of copper to the glasses increases the refractive index values and this dependence follows the general tendency for the family of glass compositions [85,86]. A slight increase in the refractive index value indicates that no significant structural changes might have occurred in the glass network.


which permits to evaluate the effect of impurity copper ions on the optical basicity of glasses. The calculated optical basicity values are shown in Table 1. The optical basicity increases with increasing the copper content in the glasses. This is apparently due to a greater contribution of copper oxide into the total basicity as compared to the basicity of zinc and tellurium oxides which leads to the formation of non-bridging oxygens at the 5

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Fig. 9. The EPR spectra of Cu2 + ions in the ZnO–TeO2 glasses. Fig. 8. Middle IR transmittance spectra of the 30ZnO–70TeO2 glasses containing Cu2 + ions (l = 2.5 mm).

3.8. EPR studies Fig. 9 shows the EPR spectra of ZTC3-ZTC10 glass samples containing copper. No EPR signal is detected in the spectra of undoped glasses. Those spectra of all the glass samples are similar to those reported for Cu2 + ions in other glass systems [33,39,71,97,98]. The increase in EPR signal intensity with an increase in concentration of Cu2 + are due to increase in the number of free electrons in the glass network. For Cu2 + ion with electron spin S = 1/2 and nuclear spin I = 3/2, (2I + 1) bands should be manifested in EPR spectrum for the two natural isotopes 63Cu (natural abundance 69%) and 65Cu (natural abundance 31%): four parallel and four perpendicular hyperfine components would be expected. In the observed EPR spectra three parallel components are observed in the lower field region and the fourth parallel component is overlapped with the perpendicular component. The four perpendicular components in the high field region were not resolved. The spin Hamiltonian parameters (g factors and the hyperfine structure constants A┴, A‖‖) for the Cu2 + ion were estimated and the data are given in Table 3. The estimated values of anisotropic g factors satisfy the relationship g|| > g⊥ > ge (ge = 2.0023 for free electron) characteristic of copper ions coordinated with six ligand atoms in a distorted octahedron, elongated along one axis (D4h symmetry) and the ground state for unpaired electrons is 2B1g (dx2 − y2) [99].

expense of bridging oxygens.

3.6. Metallization criterion Another parameter that can be obtained from the absorption spectra of the glasses near their short-wave transmittance edge is the “metallization criterion” M determined by the expression [94]


n2 − 1 = n2 + 2

E ⎛ opt ⎞ , ⎝ 20 ⎠ ⎜


predicting the metallic and non-metallic nature of glasses. The materials for which the metallization criterion is close to 1 are called insulators. The calculated values of the metallization criterion for the studied glasses are shown in Table 1. The data obtained indicate that doping the glass with cupric ions causes a decrease in metallization parameter. This means that the glass sample is metalizing, apparently due to an increase in the refractive index of glass. On the other hand, the metallization criterion increases with the increasing of band gap energy band gap. This is due to the increasing value of the band gap in which decreases the conduction band.

3.9. Cu2 +-Ligand bond nature 3.7. FTIR spectroscopy By correlating the EPR and optical absorption spectra the bond parameters that determine the metal-ligand bond in the 30ZnO–70TeO2 glasses were evaluated by using the equations

Fig. 8 shows the transmittance spectra for the glass samples with thickness of 2.5 mm in the middle infrared range. Two broad absorption bands with the peaks at about 3.3 and 4.4 μm are found at all concentrations of doped copper impurities and are caused by the stretching vibrations of free OH groups hydrogen-bonded with glass network [95]. The presence of these bands is due to the fact that in the preparation of samples no special techniques such as adding to the glasses of fluorides of the main components [17] or passing through the melt of dry oxygen [96] were used. Two small narrow bands with the peaks around 3.5 and 3.4 μm are due to the vibrations of CH groups, resulting from the polishing process of the samples and the use of various organic reagents. The long-wavelength edge for the samples of equal thickness and different concentrations of copper ions within the measurement accuracy appeared to be the same.

g = 2.0023(1 − 4λα 2β12 )ΔExy ,


g⊥ = 2.0023(1 − λα 2β 2)ΔExz, yz ,


where ΔExy and ΔExz,yz are the energies corresponding to electronic transitions 2B1g → 2B2g and 2B1g → 2Eg respectively, λ is the spin-orbit coupling constant (= − 828 cm− 1), α2 characterizes in-plane σ-bond of the ligand with dx2 − y2orbital, β12 characterizes in-plane π-bond with dxy orbital, and β2 characterizes in-plane π-bond with dyz orbital. The values of α2 lie between 0.5 and 1 within the limits of pure covalent and ionic bonds respectively. These parameters are calculated by using the equations [100]. 6

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Table 3 The spin-Hamiltonian parameters and molecular orbital bonding coefficients of Cu2 + doped glasses. Sample



A|| (10− 4 cm− 1)

A⊥ (10− 4 cm− 1)




ΔExz,yz, cm− 1

Гσ, %

Гπ %


2.345 2.343 2.354 2.362 2.354 2.356 2.362 2.365

2.066 2.068 2.066 2.065 2.064 2.067 2.066 2.068

135 138 132 125 130 130 129 120

10 11 11 7 8 11 12 15

0.788 0.796 0.789 0.777 0.782 0.786 0.789 0.767

0.725 0.719 0.725 0.735 0.730 0.728 0.725 0.746

0.799 0.787 0.820 0.851 0.827 0.828 0.838 0.870

20,018 19,408 20,018 20,337 20,666 19,708 20,018 19,408

46.2 44.5 46.0 48.6 47.5 46.6 46.0 50.8

40.2 42.6 36.0 29.8 34.6 34.4 32.4 26.0

α 2 = (A P ) + (g − 2) + 3 7(g⊥ − 2) + 0.04,



β12 = [(g ge ) − 1]ΔExy 4λα 2,


The reported study was funded by RFBR according to the project no. 16-33-50136.

β 2 = [(g⊥ ge ) − 1]ΔExz, yz λα 2,



where P is the dipolar hyperfine coupling parameter (P = 0.036 cm− 1), λ is the spin-orbit coupling constant. The values of the electronic transition energy were calculated according to the equation

2K 2λ , ge − g⊥

ΔExz, yz = 2

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(16) 2

where K is the orbital reduction factor (K = 0.77). The normalized covalency of CueO in-plane bond of σ and π symmetry is expressed by equations

Γσ =

200(1 − S )(1 − α 2) , (1 − 2S )

Γπ = 200(1 − β12),

(17) (18)

where S is the overlap integral between the copper 3d orbitals and the normalized ligand orbitals (S0xy = 0.076). Calculated α2 values for present glasses (the range 0.767–0.796) suggest that the in-plane σ-bonding in the glasses is moderately covalent in nature whereas the values of β12 values (from 0.787 to 0.870) indicate that the in-plane π-bonding is significantly ionic in nature. The normalized covalency (Γπ) of Cu2 +eO bonding of π symmetry indicates the basicity of the oxide ion. In general, covalency of in-plane π bonding (Γπ) decreases while covalency of the in-plane σ bonding (Γσ) varies non-linearly with the Cu2 + content. 4. Conclusion From the optical absorption spectra of the 30ZnO–70TeO2 glasses doped with Cu2 + ions a single broad band corresponding to overlap B1g → 2B2g, 2B1g → 2Eg and 2B1g → 2A1g electronic transitions of copper ions in an axially distorted octahedral sites was established. The spectral dependence of the specific absorption coefficient in the spectral range 0.5–2.8 μm for copper ions have been calculated and the maximum permissible concentration of impurity Cu2 + in the glasses has been estimated. It has been evaluated that the optical losses less 100 dB/km must not exceed 20 ppbwt of Cu2 + content. By correlating the EPR and optical data the spin-Hamiltonian parameters and molecular orbital bonding coefficients were calculated. The value show that the in-plane σ bonding is moderately covalent and the in-plane π bonding is significantly ionic in nature. The optical band gap, Urbach energy, molar refraction and metallization criterion have been determined and their variations have been discussed in terms of the glass structure. It was found that optical band gap energies slightly decrease with the addition of copper content and Urbach energy decreases from 0.318 to 0.226 due to decreasing of the possible structural defects in the glasses. 7

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