Journal of Materials Research and Technology Journal of Materials Research and Technology
Original Article
Spectroscopic studies of ZnO borate–tellurite glass doped with Eu2O3
Ali A. Alia,, , Hany M. Shaabanb, Amany Abdallahc
a Glass Research Department, National Research Centre, Dokki, 12622 Cairo, Egypt
b Chemistry Department, Faculty of Science, Tanta University, Tanta, Egypt
c Physics Department, Faculty of Science, Ain Shams University, Abbassia, 11556 Cairo, Egypt
Received 24 February 2017, Accepted 27 June 2017
Abstract

Optical, physical and thermal properties of glasses with composition 24B2O3(25x) ZnO51TeO2, x Eu2O3, mol% (where x=0.0, 0.5, 1 and 3%) were studied. The density, crystallization temperature, oxygen packing density and glassy temperature were found to increase with the addition of Eu2O3. The molar volume, optical band gap (Eg) and refractive index values were found to decrease with increasing Eu2O3 content. The FTIR measurements indicated that the continuous increase of europium concentration helped conversion of [BO3] to [BO4] groups in the base glass ZnO TeO2–B2O3. Thermal analysis reveals that the glass transition temperature Tg, the glass thermal stability ΔT and the glass forming ability Kg increased by the addition of Eu2O3. The transmission and absorption spectra of all glasses indicated that there is a decrease in the optical band gap Eg values due to the enhancement of the number of non-bridging oxygen (NBO) atoms with the addition of Eu2O3. The studied glasses have high values of nonlinear refractive index n2 and nonlinear optical susceptibility χ(3) with a good thermal stability. Thus, such glasses have promised applications in nonlinear optical devices.

Keywords
Tellurite glasses, Europium glasses, Refractive index and nonlinear optical devices
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1Introduction

Glasses with tellurite offer distinct properties when compared with other glasses, such as good chemical durability and chemical resistance, high linear and non-linear refractive index, high transmittance especially in near infrared (NIR) to middle infrared (MIR) regions and high electrical conductivity [1,2]. They also are suitable candidates for use in fiber optics, optical amplifiers, and laser, in addition to being widely used as photonic crystal fibers (PCFs); therefore, these glasses are considered as potential nonlinear materials [3–5]. Extensive studies were performed on B2O3 glasses for the past few decades because of their promised properties, which can be summarized as follows: 1 – glasses with low melting point, which can help save energy, 2 – high transparency, which is useful in optics, 3 – glasses with high thermal stability, 4 – easy to prepare and can dissolve a high concentration of rare earth ions, which means that these glasses are more suitable for optical device fabrication [6,7]. Formation of glass with two glass formers was known previously and is of scientific interest. The addition of TeO2 into the borate network, which means the formation of glass with two glass formers, improves the transparency of the glass and its refractive index. Boro-tellurite glasses containing ZnO have a wide glass forming possibility and low ability to crystallize [8–10]. Doped glasses with rare earth ions have superior technological applications such as in fiber amplifiers, planar waveguides and display monitors of wavelength-converting devices [11–14]. It is known that europium is an element with optical active properties. The optical properties of trivalent europium ion are very sensitive to the surrounded atoms inside the glass, i.e. these properties are dependent on the glass composition. The selection of Eu3+ ions to add to the glass matrix is useful for the study of disordered materials, because europium has an energy level with simple structure and non-degenerate ground 7F0 and emitting 5D0 states [15,16]. We think that there is insufficient information regarding the presence of Eu3+ in the glasses of borate–tellurite composition. Therefore, it is very interesting to discover the relation between the change in the glass structural units and the linear and nonlinear properties of Eu3+ ions in borate–tellurite glasses.

2Experimental2.1Glass preparation

In the present study, glasses having the composition 24B2O3(25x) ZnO51TeO2, x Eu2O3, mol% (where x=0.0, 0.5, 1 and 3%) were prepared using pure reagent grade H3BO3, ZnO, TeO2 and Eu2O3 as starting materials. For each composition, the raw materials in the powder form were weighted accurately; then, they were mixed together using agate mortar and were melted in alumina crucibles at about 800°C for 30min in air. The glass melts were stirred occasionally with an alumina rod to achieve good homogeneity. The highly viscous melt was cast into a cylindrically shaped split mold of mild steel, and the produced glass was annealed at 450°C in another furnace for 1h, after which the furnace was switched off and the glass was allowed to cool gradually in situ for 24h.

2.2Density

The densities of these glasses (ρ) were determined by using Automatic Gas Pycnometers for True Density, Ultrapyc 1200e, and apparatus with helium gas.

2.3FTIR

FTIR spectra of the glass samples were recorded at room temperature in the range of 400–4000cm−1 using Shimadzu FTIR 8400S spectrophotometer [resolution of 0.85cm−1 by KBr pellet technique]. The powdered samples were thoroughly mixed with dry KBr in the ratio of 1:20 by weight, and then the pellets were formed under a pressure of 9–10tons.

2.4DTA measurement

The thermal behaviors (differential thermal analysis DTA) of the finely powdered quenched samples were examined using SEATRAM Instrumentation Regulation, Labsys TM TG-DSC16 (Setaram, Caluire, France) under inert gas. The powder was heated in Pt-holder with another Pt-holder containing Al2O3 as a reference material. The results obtained were used as a guide for determining the required heat-treatment temperatures needed to induce crystallization in the samples, as will be shown later.

2.5Optical absorption

The absorption and transmission spectra of the polished samples of 2-mm thickness at room temperature were measured in the range of 200–1800nm using a recording spectrophotometer (JSCO Corp., V-570, Rev. 1.00).

3Results and discussion3.1Density and molar volume

Although the measurements of the density (ρ) and the molar volume (Vm) are so simple tools, they are enough to tell us about the structural changes that occur in the studied glasses. Both of them depend on softening or compactness of structure of the studied sample. The molar volume Vm can give us a good idea about spatial distribution of the oxygen in the glass sample. It is know that the molar volume is a very perfect parameter to determine the compaction or expansion occurring in the glass structure. Molar volume is a bit better in the investigation of the structure and structural information than density. Therefore, we calculate the molar volume, Vm, for all studied glasses as follows:

where Xi is molar fraction of oxides used in the glass, Mi is molecular weight of oxides used and ρ is the density. As shown in Table 1, there is a decrease in the values of calculated molar volume with increase the amount of Eu ions added. The observed increase in ρ of the glass with increase in the contents of Eu2O3 may be attributed to the change in the coordination of boron atom in the glass sample. Replacing ZnO of less molar mass [ZnO=81.39amu] by Eu2O3 of larger molar mass content [Eu2O3=351.93amu] leads to formation a large number of oxygen ions, which are available in glass structure. An observed decrease in the molar volume may be explained by the decrease in the bond length or the decrease in inter-atomic spacing among the atoms of glass network, which leads to a compact structure of the glass. The average distance between two boron atoms {dB–B} is calculated to estimate the change in the glass structure by addition of Eu2O3. The volume containing one mole of boron can be calculated with the given formula (Vmz):
where Vm is molar volume and Xz is the mole fraction of B2O3
where NA is the Avogadro number.

Table 1.

The physical properties of 24B2O3(25x) ZnO51TeO2, x Eu2O3 glasses.

Glass sample  D (gcm−3Vm (cm3/mol)  Vc (cm3/mol)  Vo (cm3/mol)  dB–B (nm)  (OPD) (mol/l) 
4.1  28.89  24.77  4.123  0.316  54.135 
0.5  4.15  28.86  24.93  3.93  0.3158  54.676 
4.2  28.84  25.103  3.737  0.3157  55.217 
4.4  28.76  25.7  3.06  0.3155  56.075 

Crystalline volume Vc for all studied glass samples is calculated using the relation

where Vi is the molar volume of crystalline form of each component [17] (i.e., Vi=28.148, 28.300, 14.5 and 47.557cm3/mol for TeO2, B2O3, ZnO and Eu2O3 by taking crystalline density 5.67, 2.46, 5.05 and 7.40g/cm3 for TeO2, B2O3, ZnO and Eu2O3, respectively). The volume deviation Vo (=VmVc) is listed in Table 1, and we can notice that from this table that the Vm of the glasses is usually much greater than the corresponding values of Vc, which indicate the presence of excess structural volume in these samples; this is characteristic of their glassy nature.

The oxygen packing density (OPD) values were calculated from the equation relation as follows:

where ρ, Mi and C are density, molar mass and number of oxygen ions present in the formula unit. The values of OPD increase with the increase in Eu2O3 content [18].

3.2Thermal analysis

Fig. 1 represents the DTA measurements of all studied glass samples, from which it is observed that the glass transition temperature Tg increases with the addition of Eu2O3. The increase in Tg may be attributed to the increase in compactness as revealed by the decrease in the molar volume Vm and the increase in oxygen packing density (OPD) (Table 2). The Tg value increases by increasing the bond strength; thus, increase in Tg may also be attributed to high bond strength of EuO bond compared to ZnO bond [19,20]. The (Tc) is the crystallization temperature. The glass thermal stability ΔT (TgTc) increases with the increase in Eu2O3 content. The studied glasses have high thermal stability values where ΔT>100οC. Thus, high values of Tg and ΔT suggest that the addition of Eu2O3 to Zinc–boro tellurite glass increases its ability for fiber drawing. The glass forming ability Kg (Hurby factor) was given by the relation:

The Kg value increased with the increase in Eu2O3 for sample 2 (Eu2O3 content=0.5). However, with further addition of Eu2O3, there is a small decrease in the Hurby factor values. This means that there is a little effect on the nucleation and crystallization processes inside the glass, or the ability for glass formation is considerably constant [21].

Fig. 1.
(0.21MB).

DTA of all studied samples. (1) Base, (2) 0.5mol% Eu2O3, (3) 1mol% Eu2O3, (4) 3mol% Eu2O3.

Table 2.

The thermal properties of 24B2O3(25x) ZnO51TeO2, x Eu2O3 glasses.

Glass sample  Tg (οC)  Tc (οC)  Tm (οC)  ΔT (οC)  Kg 
0.0  395  564  625  169  0.734 
0.5  409  629  662  220  0.869 
410  652  755  242  0.701 
431  660  747  239  0.724 
3.3FTIR measurements

Generally the presence of different structural groups in the different glass samples can be identified by using measured infrared spectra of these glasses. In our study, the infrared spectra of base glass (free of Eu ions) were characterized by presence of several peaks at 469, 630, 940, 1100, 1230, 1390 and two small peaks at 1610 and 1640cm−1 as revealed in Fig. 2. We can classify these peaks into 3 regions. The first region lying around 600–700cm−1 can be attributed to the presence of BOB bending vibration bond in borate network, second region from 800 to 1200cm−1 which may be attributed to presence of BO stretching presence in BO4 units, and the third region extended from 1200 to 1600cm−1 due to BO stretching of BO3 units. Besides, the band from 2400 to 3600cm−1 was due to OH vibration of water group. There was a very weak band centered at 469cm−1, which may be attributed to the presence of the stretching vibration of equatorial and axial TeO bonds in the TeO4 trigonal bipyramids units, respectively [22–24].

Fig. 2.
(0.13MB).

FTIR for base glass and glass doped with x mol% Eu2O3.

Due to the absence of the band at about 806cm−1, we can say that the boroxol rings are absent in the network of our studied glass because it is well known that this band is assigned to the boroxol ring in borate glass network. Hence, the glass system contains [BO3] and [BO4] groups.

There are two broad bands, in all the samples, centered at 1360cm−1 and 940cm−1, respectively, and are attributed to BO bonds found in both [BO3] units and [BO4], respectively. The bond at 1360cm−1 is attributed to symmetric stretching vibration of BO bond of orthoborate, pyroborate and metaborate groups in the form of [BO3] units [22,23] and the other band at 940cm−1 is attributed to stretching vibration of BO bond in [BO4] tetrahedral units of di-borate groups [22,24]. It was observed that the intensity of the first band decreased while the second increased progressively when europium oxide content increased by replacing an equal amount of ZnO from the glass system. Also, BO3 band shifted toward lower wave number [1358–1352cm−1], whereas, BO4 band shifted toward the longer wave number (961cm−1) with the rise in europium content. Hence, the continuous increase of europium concentration helped conversion of [BO3] to [BO4] groups in the base glass ZnO TeO2–B2O3, which was already confirmed by the study of density and molar volumes.

3.4Optical properties

The transmittance T (λ) and the reflectance R (λ) spectra in the wavelength range of 200–2500nm for ZnO boro-tellurite glass doped with Eu2O3 are illustrated in Fig. 3. It is noticed that the studied samples are in the absorbing region where R+T<1.

Fig. 3.
(0.14MB).

The transmittance T (λ) and the reflectance R (λ) spectra for ZnO boro-tellurite glasses doped with x Eu2O3.

For each of the studied samples, the reflectance values show a gradual increase from 430nm up to 450nm followed by constant values up to 1800nm. A sharp increase in transmittance values is observed at 430nm followed by a gradual increase up to 830nm; thereafter, it became constant. It is noticed that T values decrease with the increase in Eu content, and an absorption region between 500 and 830nm was observed for Eu-containing glass. Such absorption was attributed to the optical transitions from 7F0 to 5D4, 5G4, 5L6, 5D3, 5D2, 5D1 and 5D0 of Eu3+ ions [25].

The absolute values of T (λ) and R (λ) are used to calculate the optical constants, the optical absorption coefficient (α), the absorption index (k) and the refractive index (n) using the following relations [26]:

Increase in (α) values was observed with the increase in Eu content as indicated in Fig. 4. The absorption peaks observed from 850 to 430nm for Eu-containing samples are attributed to the presence of color centers induced by Eu ions.

Fig. 4.
(0.09MB).

The absorption coefficient (α) at different photon energies as function of Eu2O3.

The relationship between the optical absorption coefficient (α) and the photon energy was described by Tauc [27].

where B is constant, is the incident photon energy, Eg is the optical band gap of the glass sample, and r depends upon the type of transition. The value of r is ½ for the allowed direct transition, whereas it is 2 for the indirect transition. The indirect band gap (Eg) was estimated by extrapolating the straight line part of the relation between (αhν)1/2 and photon energy at (αhν)1/2=0, as shown in Fig. 5. It is clear that there is a decrease in the Eg values with the increase in Eu2O3 content as shown in Table 2. This behavior is attributed to the structural changes in the glass network with the addition of Eu2O3. The addition of Eu2O3 enhances the number of non-bridging oxygen (NBO) atoms in the glass network. NBO's ions shift the edge of valence band toward the conduction band [28]. This leads to decreases in the optical band gap values.

Fig. 5.
(0.1MB).

The relation between (αhν)1/2 and photon energy as function of Eu2O3.

The refractive index (n) values (determined from Eq. (9)) at different wavelengths are plotted in Fig. 6. It is noticed from Fig. 6 that the refractive index values decrease with increasing Eu2O3 content. Fig. 6 represents a peak with nmax value at ≅840nm, which shifts to longer wavelength with increasing Eu2O3 content. The values of n and nmax decreased with the addition of Eu2O3. At λ>1100nm, a normal dispersion is observed. Such a dispersion is analyzed by Wemple and DiDomenico (WDD) single oscillator model [29,30]. The (WDD) model suggests that the relation between n and is given in the following equation:

where Eo is the effective oscillator energy and Ed is the dispersion energy, which measures the strength of the inter-band optical transitions. (n21)−1 is plotted versus ()2 as shown in Fig. 7. Eo and Ed values are determined from the slope and intercept of the extrapolated straight line and listed in Table 3. The effective oscillator energy, Eo, is related to the bond energy of the chemical bonds existing in the glass matrix [31]. Thus, the formation of non-bridged oxygen ions decreases the fraction of covalent and increases the fraction of ionic bonds, which have lower bond energy than the covalent bonds. This behavior could explain the decrease of Eo value with increase in Eu2O3 content.

Fig. 6.
(0.12MB).

The relation between refractive index (n) and wavelength (λ) as function of Eu2O3.

Fig. 7.
(0.11MB).

The relation between (n21)2 and ()2 as function of Eu2O3.

Table 3.

The optical properties and nonlinear refractive index n2 and nonlinear optical susceptibility χ(3) for 24B2O3(25x) ZnO51TeO2, x Eu2O3 glasses.

X  Eg (eV)  Eo (eV)  Ed (eV)  ??  β  ??L  χ(3) (esu)  n2 (esu) 
2.695  6.120  19.758  4.230  0.277  4.330  4.737×10−11  8.694×10−10 
0.5  2.624  5.976  17.743  4.161  0.276  4.308  3.390×10−11  6.421×10−10 
2.594  9.746  29.827  4.061  0.275  4.113  3.827×10−11  7.156×10−10 
2.541  5.613  17.216  3.881  0.265  3.962  4.475×10−11  8.243×10−10 

The dielectric constant at infinite frequency ?? is determined from extrapolation of the straight line with the ordinate axis at ()2=0. Petkov et al. [32] proposed the relations between the dispersion energy Ed and other physical parameters as:

where Nc is the coordination number of the cation, which is the nearest neighbor to the anion, Za is the formula chemical valence of the anion, Ne is the effective number of valence electrons per anion and β is a parameter between 0.26±0.04eV for ionic materials and 0.37±0.05eV for covalent materials. Using the calculated β values of Eu2O3-doped and -undoped Zn–boro-tellurite glass (Table 2) and the values of Nc=4, Za=2 and Ne=8 [26,29], the obtained β values are around 0.261. These results confirmed that Eu2O3-doped glasses have more ionic character than the undoped glass, which could be attributed to the increase in the number of non-bridging oxygen (NBO) atoms in the glass network.

The refractive index, n, is related to the lattice dielectric constant (??L) by the following equation [33]:

where e is the electronic charge, ??o is the permittivity of free space, c is the speed of light and N/m* is the ratio of the carrier concentration to the effective mass. Fig. 8 indicates a linear relation between n2 and λ2, for Eu2O3-doped and -undoped glasses, which is consistent with Eq. (13). The ??L values are determined from the extrapolation of the linear relation to λ2=0. It is noticed that ??<??L, which was attributed to free charge carrier contribution [34].

Fig. 8.
(0.1MB).

The relation between (n2) and (λ)2 as function of Eu2O3.

3.5Nonlinear optical properties

Several theoretical models have been proposed for calculation of the third-order susceptibility χ(3) and the non-linear index n2[35–37]. In this work, the model suggested by Tichy et al. [38] is applied. They combine the Miller's generalized rule [39] with the static refractive index, which is evaluated from the WDD single oscillator model [29]. According to their proposed model, χ(3) and n2 are given by the following relations:

where A=1.7×10−11 and no is the static refractive index. From Eq. (11), as 0, then n=no, i.e.,
The third-order susceptibility χ(3) and the non-linear index n2 values for the studied glasses are listed in Table 3. It is observed from Table 3 that χ(3) and n2 values decreased with the addition of 0.5mol Eu2O3. The observed decrease in Eu2O3-doped glass could be attributed to the replacement of a higher polarizable ZnO by a lower polarizable Eu2O3.

However, the χ(3) values indicate a gradual increase with further increasing Eu2O3 concentration. This behavior could be explained by the enhancement of excitation electron with the increasing Eu2O3 concentration [40].

ZnO borate–tellurite glass doped with Eu2O3 showed higher values of nonlinear refractive index n2 and nonlinear optical susceptibility χ(3) than that reported for heavy elements, such as borate and silicate glasses [41,42]. These results suggested that Eu2O3-doped and -undoped ZnO boro-tellurite glasses have promising applications in nonlinear optical switching devices [40,43].

4Conclusion

Glasses of composition 24B2O3(25x) ZnO51TeO2, x Eu2O3, mol% (where x=0.0, 0.5, 1, and 3%) were prepared.

  • 1.

    Density of studied glasses was measured and found to decrease with increasing the Eu2O3 content, whereas the molar volume of the same studied glasses were found to decrease.

  • 2.

    Crystalline volume Vc for all studied glass samples were calculated, and the volume deviation Vo were estimated and found to decrease with increasing the Eu2O3 content.

  • 3.

    FTIR spectra were characterized by the presence of several peaks at 469, 630, 940, 1100, 1230, 1390 and two small peaks at 1610 and 1640cm−1.

  • 4.

    By addition of Eu2O3, there was a change in intensity and position of the peaks related to change from [BO3] to [BO4] groups.

  • 5.

    The glass thermal stability ΔT (TgTc) increases with the increase in Eu2O3 content.

  • 6.

    The glass forming ability Kg (Hurby factor) increased with the increase in Eu2O3.

  • 7.

    The Eg values were with increasing in Eu2O3 content, which attributed to the Enhancement of the number of non-bridging oxygen (NBO) atoms in the glass network.

  • 8.

    The studied glasses have high values of nonlinear refractive index n2 and nonlinear Optical susceptibility χ(3).

The results obtained indicated that ZnO boro-tellurite glass doped with Eu2O3 has high nonlinear optical property values with good thermal stability, which make these glasses suitable candidates for their application in fiber optics and nonlinear devises and broadband optical amplifiers.

Conflicts of interest

The authors declare no conflicts of interest.

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