Accepted Manuscript Composition dependent structural and optical properties of PbF2–TeO2 –B2O3 – Eu2O3 glasses Akshatha Wagh, Y. Raviprakash, Vyasa Upadhyaya, Sudha D. Kamath PII: DOI: Reference:
S1386-1425(15)30064-0 http://dx.doi.org/10.1016/j.saa.2015.07.016 SAA 13909
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
25 November 2014 30 June 2015 1 July 2015
Please cite this article as: A. Wagh, Y. Raviprakash, V. Upadhyaya, S.D. Kamath, Composition dependent structural and optical properties of PbF2–TeO2 –B2O3 – Eu2O3 glasses, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2015), doi: http://dx.doi.org/10.1016/j.saa.2015.07.016
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Composition dependent structural and optical properties of PbF2–TeO2 – B2O3 – Eu2O3 glasses Akshatha Wagh, Raviprakash Y., Vyasa Upadhyaya, Sudha D. Kamath* Department of Physics, Manipal Institute of Technology, Manipal University, Manipal. *Email: [email protected]
Abstract: Boric oxide based quaternary glasses in the system PbF2–TeO2–B2O3–Eu2O3 have been prepared by melt quenching technique. Density, molar volume, FTIR, UV-Vis techniques were used to probe the structural modifications with incorporation of europium ions in the glass network. An increase in glass density & decrease in molar volume (Vm) values proved the structural changes occurring in coordination of boron atom [conversion of BO3 units to BO4]. This resulted in the increase of the compaction of the prepared glasses with increase in Eu2O3 contents. The amorphous natures of the samples were ascertained by XRD and metallization criterion (M) studies. XPS study showed the values of core-level binding energy [O1s, Eu3d, Eu4d, Te3d, Te4d, Pd4f, Pb5d, O1s, and F1s] of (PbF2-TeO2B2O3-Eu2O3) the glass matrix. The frequency and temperature dependence of dielectric properties of present glasses were investigated in the frequency range of 1 Hz-10 MHz and temperature range of 313–773 K. The study of dielectric measurements proved good insulating and thermal stability of the prepared glasses. At room temperature, dielectric loss [tan δ] values were negligibly small for prepared glasses and increased with increase in temperature. FTIR spectroscopy results were in good agreement with optical band energy gap, density, molar volume and hardness values revealing network modifications caused by europium ions in the glass structure.
Keywords: Eu2O3, Melt quench, Dielectric measurement, FTIR, XPS.
Correspondence Author: Dr. Sudha D. Kamath Associate Professor-Senior Scale, Department of Physics Manipal Institute of Technology, Manipal University Manipal - 576104, Karnataka, India Tel: 9964668975, Fax: +91-820-2571919, Email: [email protected]
, [email protected]
1. Introduction The competition of speed, cost and reliability of opto-electronic devices in various applications has led to the research for new materials which can meet the distinct requirement . The study of tellurite glasses is of scientific and technical interest because they have low melting points, high refractive index, high dielectric constant and good infrared transmission [2,3]. But, tellurite oxide (TeO2) is a conditional glass former  and forms glass only with a modifier such as alkali, rare earth, and transitional metal oxides or other glass formers . Recently, an addition of oxides of heavy metals to tellurite glasses is being studied extensively because such additions seem to show remarkable changes in both physical and optical properties of these glasses . Tellurite glasses with good glass forming oxides like boron trioxide (B2O3), are of scientific and technical interest on account of their various unique properties, and have been considered as promising materials for use in optical amplifiers because of their low phonon energy and nonlinear optical devices and their large third-order nonlinear susceptibility [1-3]. The addition of fluoride content to borate glasses decreases the phonon energy and increases the moisture resistance and transparency in the visible region, which in turn contribute to the reduction in the non-radiative losses. Thus, lead fluoride (PbF2) gives some special significance with a good ability to form stable glasses over a wide range of concentrations due to dual role as glass modifier and glass former. PbF2 is a conditional glass former, B2O3 is a glass forming oxide and with these two chemicals in the glass matrix a low rate of crystallization, moisture resistance, stable and transparent glasses were achieved . The rare earth (RE) doped borate glasses are among those materials which have number of optics and photonics applications. An interesting characteristic of the borate glass is that, there are variations in its structural properties when RE cations are introduced. The structure of the borate glasses is not a random distribution of BO3 triangles and BO4 tetrahedral, but a gathering of these units to form well-defined and stable borate groups such as diborate, triborate, tetraborate, etc., that constitute the random three-dimensional network . These things make the borate glasses as one of the best choices for RE doping. Borate glass is a suitable optical material for RE ions with high transparency, low melting point, high thermal stability, good RE ion solubility [5, 6] and show more clear relationship between glass structure and physical properties.
Among the possible REs Europium (Eu3+) is one of the most investigated and is also one of the best optically active elements. The optical properties of trivalent Eu3+ ion are well known to be highly sensitive to environment in which it is surrounded. The selection of Eu3+ ions are a deserving candidate for the study of disordered materials, because of its relatively simple energy level structure with non- degenerate ground 7F0 and emitting 5D0 states. Though there is enough information available on Eu3+ doped borate glasses, it is interesting to find out the variation of properties of Eu3+ ions in fluoroborate glasses by changing the chemical composition. The addition of an extra cations to the glass network exerts an influence on the glass structure leading to the local change of the Bridging Oxygen (BO) and Non Bridging Oxygen (NBO) distribution . Due to technological importance and the advantages of Europium (Eu3+) ion, the present work investigates the dominant role of Eu2O3 on physical, structural, optical and mechanical properties in PbF2-TeO2--B2O3 glass system. The structural and physical properties are studied by using
X - Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy
(FTIR), Dielectric measurements, density and molar volume techniques. The optical properties of glasses are determined by using UV-VIS spectroscopy measurements. 2.
2.1. Sample Preparation Series of Eu3+doped lead fluoroborate glasses (in mol%) 20 PbF2– 20 TeO2 – (60-x) B2O3 – x Eu2O3 (where x = 0, 0.1, 0.5, 1.0, 1.5, 2.0, 2.5 mol%) were prepared by the melt–quenching technique. The chemical powders were procured through Sigma Aldrich with purity 99%. These powders were thoroughly mixed in an agate mortar and pestle with respect to their glass compositions. The thoroughly mixed chemical powders in the porcelain crucible were then kept for melting in an Indfurr electric furnace at 980oC for 1.30 Hrs. After the retirement of melting period the molten mass is quenched rapidly on a stainless steel mold maintained at 200 oC. The glasses thus obtained were found to be clear, bubble free, transparent and yellowish in color. These samples were annealed for 3 Hrs. at 200oC to remove induced residual thermal or mechanical stresses caused due to rapid quenching. Samples were then polished with different grain size emery polishing sheets.
3. Results and Discussion 3.1. Physical Properties 3.1.1. X-Ray Diffraction [XRD] Powder X-Ray Diffraction (XRD) spectra for all the glass samples in the present investigation were recorded at lab temperature using a Rigaku Miniflex 600 X-Ray Diffractometer with Cu Kα radiation (40 KV & 15 mA) and a graphite monochromator with 2Ѳ (Ѳ being Bragg angle) from 10o to 90o. XRD spectra of all glass samples are shown in Figure 1. The pattern of XRD shows no discrete or continuous sharp peaks. The absence of Bragg peaks confirms the amorphous nature of the glass samples. This type of scattering shown in Figure 1 is the characteristic of long range structural disorder and the amorphous nature of the glass samples in the studied composition range [7-10]. 3.1.2. Density and Molar volume These two measurements are simple but powerful tool to examine the changes occurring in the structure of the glasses. They are affected by the structural softening or compactness. Densities of the present glasses were measured using Archimedes principle. The weight of the prepared samples were measured in air using xylene (ρ = 0.865 g/cm3) as an immersion liquid. The experiment was performed using Contech Analytical Balance with accuracy of 0.0001g. The density (ρ) was determined using the relation ρ=
Where is the weight in air, ( = 0.863 g/cm3).
is the weight in xylene and
is the density of xylene
Molar volume Vm also indicates the spatial distribution of the oxygen in the glass network which measures compaction or expansion of the glass structure. It provides better structural information than density. Hence, we calculated molar volume, Vm for all glass samples by using the relation: = Where
is the molar fraction and
molecular weight of the ith component and
density of the sample. We also calculated the crystalline volume (Vc) of the glass samples using the relation, 4
is the crystalline molar volume of the ith component phase  (i.e.,
28.148, 28.300 and 47.557 cm3/mole for α- PbF2, TeO2, B2O3 and Eu2O3 by taking crystalline density 8.44, 5.67, 2.46 and 7.40 g/cm3 for α- PbF2,TeO2, B2O3 and Eu2O3 respectively). The measured values of density and calculated values of molar volume (Vm), crystalline volume (Vc) and the volume deviation V0 (= Vm Vc) are included in Table 1. Figure 2 shows the variation of density (ρ), and molar volume (Vm) with Eu2O3 concentration. From Table 1 it is clearly evident that, the Vm of the glasses is always greater than the corresponding values of Vc, indicating the presence of excess structural volume in these samples; this is characteristic of their glassy nature. From Table 1 and Figure 2, it is clearly evident that the density of PbF2-TeO2-B2O3-Eu2O3 glasses gradually increases while the molar volume decreases with increase in Eu2O3 concentration. An increase in glass density with increasing Eu2O3 contents can be explained on the basis of structural changes occurring in coordination of boron atom in glass network. Due to the increase in larger molar mass content [Eu2O3 =351.93 amu] at the expense of lesser molar mass [B2O3=69.62 amu], a large number of oxygen ions are available in glass structure. These oxygen ions help to convert three coordinate boron units to four coordinated boron units which are confirmed through FTIR study given in section 3.1.6. The [BO4] tetrahedral are considerably denser than the symmetric [BO3] triangle . A decrease in molar volume can be attributed to decrease in bond length or interatomic spacing among the atoms of glass network which causes compaction of structure. The average boron-boron separation is calculated to confirm the modification of glass network due to the presence of Eu2O3. The boron atoms are the central atoms BO3 with negatively charged tetrahedral BO4 units, thus the volume, corresponds to the volume that contains one mole of boron within the given structure has been found as: Vm 2 (1 X B ) Where Vm is the molar volume and XB is the molar fraction of B2O3. VmB
d B B
VB m NA
Where NA is the Avogadro number.
The value decreases progressively with increase of Eu2O3 contents (Table 1) Hence, the presence of europium helps to decrease the average boron- boron separation. Thus, the addition of Eu2O3 at the expense of B2O3 leads to compaction of glass network causing an increase in the density and decrease in molar volume. It is also believed that the presence of europium increases the density of glass as the ionic radius of Eu3+ (1.02 A) is almost comparable with ionic radius of Te4+ (0.99 A) and smaller as compared to Pb2+(1.32 A), There is a finite probability that Eu3+ ions may also be situated in the interstitial sites of the glass network . Above mentioned factors (conversion of BO3 to BO4, decrease in bond length and possibility of europium ions to be sited in interstitial sites) are responsible for the increase in density followed by decrease in molar volume. The graph showing the increase in density with decrease in molar volume reveals the change in the structure of glass with increasing Eu2O3 contents. It is evident from the Table 1 that 2.5 mol% Eu2O3 doped borate glass has the most compact structure since it has minimum volume deviation Vo= 2.042 cm3/mol. 3.1.3. Refractive index (n) and other Physical Parameters The refractive index of these glasses was measured at lab temperature using Brewster’s angle method. The block diagram of experimental setup and the experimental details used for the measurement of refractive index are given elsewhere . Like density, refractive index also showed increasing trend with rise in Eu2O3 contents which is given in Table 2. Some other physical parameters useful for the characterization of these selected glasses, doped with rare earth oxides are estimated from the measured value of density, the average molecular weight Mw, and refractive index using the following equations: The molar refraction (Rm) derived by Volf, Lorentz and Lorenz is given by (i)
Where n is the refractive index, Mw is the molecular weight and ρ is the density of the glass samples. The (
) ratio is the molar volume (Vm) .
The rare earth ion [Eu3+] concentration (N) could be obtained from: (ii) Number density (N) (
) = (% mol of RE)
From the obtained ‘N’ values, the polaron radius (rp) and inter – ionic distance (ri), Field strength (F) of the rare earth ions [Eu3+] could be evaluated: (iii)Polaron radius ( ) (
(iv) Inter nuclear distance ( ) (
The field strength around Eu3+ ion is calculated according to the equation: (v) Field Strength (F) (
The observed decrease of rp with increasing Eu2O3 is most likely related to the increased value of (N) for Eu3+ ions. It is worth mentioning that the rare earth ions are situated between the layers and thus the average RE-oxygen distance decreases. As a result of that, the Eu–O bond strength increases, producing stronger field strength around Eu3+ ions. Oxygen packing density (OPD) which is a measure of the tightness of packing of the oxide network was calculated using the following relation, (vi) OPD = (
Where M is the molecular weight and n is the number of oxygen atoms per formula unit. The above calculated values are tabulated in Table 2. It was known that, larger the value of OPD; greater will be the tightness of packing. Plot drawn oxygen packing density (OPD) and field strength versus Eu2O3 composition is shown in Figure 3 (a). The calculated values of OPD and F of modifier cations (Eu3+) increased with increase in Eu2O3 content. As, it was observed by the values of density and molar volume [section 3.1.2.], this study once again showed the compaction of the structure with increase in Eu2O3 content in the glass structure. All of these changes can be understood by the tendency of higher field strength modifier cations (Eu3+) to promote the concentration of negative charges on bridging oxygens in their local coordination environment, systematically converting three- to four-coordinated boron . The molar refraction is related to the structure of the glass and it is proportional to the molar electronic polarizability of the material, αm, through the following Clausins-Mossotti [9,11] relation, (vii)
Where NA is the Avogadro’s number. Dimitrov and Sakka  calculated the average electronic polarizability of oxide ions in numerous single component oxides on the basis of refractive index = [( Where
is the molar cation polarizability and
is the number of oxide ions in the
above relation . For the glass sample 20 PbF2–20 TeO2–59.9 B2O3–0.1 Eu2O3, the value of [0.2
+ 2 (0.599)
+ 2 (0.001)
]. The molar cation polarizability ( )
values of Pb2+, Te4+, B3+ and Eu3+ions are respectively = 0.002
is given by
. The average oxide polarizability of TeO2 and PbF2 is
large and their cation polarizability is also high. So the present glass, as expected have shown higher values electronic polarizability ( The values of molar refraction ( oxide ions (
), molar polarizability (
, electronic polarizability of
are given in Table 2. The increase in the values of
oxygen packing density are observed with the substitution of RE element in the glass matrix. The molar refraction increased with the increase in refractive index which in turn increases oxide ion polarizability and electronic polarizability. The increase in molar refraction and refractive index (n) accompany increase in polarizability . Figure 3 (b) shows the variation of (a) refractive Index (n) and oxide ion polarizability αo2-
concentration (mol %). Hence refractive index of doped glasses not only depends on density values but also on the polarizability values of the glass samples. The degree of acidity and basicity of glass is related to the electron donor power of oxygen atom. Oxides with low electron donating power having high chemical hardness are acids. Those having high electron density and weak chemical hardness are bases. For various oxide ions Duffy proposed the following relationship between the oxide ion polarizability (
and optical basicity : (ix) Ʌ = 1.67 (1 -
The above relationship gives trend of increase in the oxide ion polarizability with increasing optical basicity. The condition for predicting metallic or insulating behavior in the condensed state matter is metallization criterion, (x) M = 1 – (
> 1, then the materials show metallic nature and if
< 1 they exhibit insulating
nature. The so called metallization parameter values of the present glasses are found to be less than one and are given in Table 2. Hence the present glasses with their metallization parameter values should exhibit insulating nature [11, 12]. 3.1.4 X-Ray Photoelctron Spectroscopy [XPS] The composition of the samples were studied by X-Ray photoelectron spectroscopy (XPS), performed through ESCA Microprobe with a monochromatic x-ray source of Al Kα radiation to identify the chemical valency of Eu ions. The pressure in the analysis chamber was lower than 10-8 Pa and the source characteristics were of 10 kV and 5mA. A freshly abraded silver paste was used for standardizing the BEs of the spectrometer on the well-known Ag3d3/2 and Ag3d5/2 XPS transitions at 374.3 and 368.3 eV, respectively. As charging effects are unavoidable in the XPS [13,14] study of our insulating glass samples, the BE were calibrated with the contamination of carbon in each sample. For this internal reference, we used the C1s level of unsaturated hydrocarbons at 284.6 eV. The Binding energy (BE) positions of Os1, Eu3d, Eu4d, F1s, Te3d, Te4d, and B1s have been determined for all Eu2O3 doped glass samples. Figure 4 shows a plot of measured BE values of Eu4d, Eu3d, Pb4f, Pb5d, Te4d, Te3d, B1s, F1s and O1s of 2.5 mol% Eu2O3 doped glass sample by XPS with charging corrections. The measured BE values of the elements present in the PbF2-TeO2-B2O3-Eu2O3 glass matrix for 2.5 mol% Eu2O3 doped glass are given in Table 3. The similar peaks were observed in other samples also with slight variations in peak intensity. Although the Eu4d signal is less intense than the Eu3d one, it is interesting to use both peaks, since there is great difference in BEs which is a evidence for depth heterogeneities of the samples. Lanthanides with an incompletely occupied 4f subshell exhibit splitting into two signals separated . This is observed in (Eu3d/Eu4d) energy state into two levels (3/2,5/2). BE values of Eu3d transitions (Eu3d5/2 and Eu3d3/2) have been found at 1130 and 1195 eV respectively, while Eu4d transitions (Eu4d5/2 and Eu4d3/2) were respectively at 142.9 and 148.6 eV. Our measured value of the O1s signal for 2.5 mol% Eu2O3 doped glass matrix were found to be at 9
537.8 eV and with a shoulder peak at lower level BE (527.5 eV). The present XPS study on (PbF2-TeO2-B2O3-Eu2O3) glass samples, showed slight greater BE values for all compounds (Eu, Pb, Te, B, O, F) as compared their BEs with literature referenced values. It may be attributed to water adsorbed (OH adsorbed) during the formation of the sample [13-15] (typical nature of borate glasses). 3.1.5 Dielectric Properties The study of dielectric properties such as dielectric constant ε, loss tan δ, over a range of frequencies and temperatures and dielectric break down strength of the glasses help in assessing their insulating character . The study of dielectric properties of glasses acquired enormous importance due to rapid expansion of glass science. Of all, dielectric constant (ε’), and dielectric loss factor (tan δ) are the most important properties which decides the applications of the glasses [17, 18]. The dielectric constant (ε’) and dielectric loss (tan δ) measurements on the as-quenched (annealed) polished glass plates that were silver painted were done using impedance gain analyzer (HP 4194 A) in the 1 Hz- 10 MHz frequency range with a signal strength of 1.0 Vrms at various temperatures (313 -773 K). The silver leads were bonded to the sample using silver epoxy. Based on these data, the dielectric constant was evaluated taking the dimensions and electrode geometry of the sample into account. Dielectric loss was calculated by taking the ratio of imaginary and real parts of the dielectric constant. The dielectric constant ε’ of pure lead fluoroborate [20PbF2-20TeO2-60 B2O3] glass at 35 0C and at 1 MHz is found to be 18.5 and is almost independent of frequency. With addition of europium ions into these glasses, ε’ values at room temperature are found to decrease with increasing Eu2O3 content. Figure 5 (a) shows variation of dielectric constant, ε’, at 35 oC for 0.1, 1.0 & 2.5 mol % Eu2O3 doped glasses. From Figure 5 (a), it is clearly evident that, the glasses with lesser amount of Eu2O3, showed frequency dispersion in the low frequency region which was not observed with 2.5 mol% doped glass. Decrease in dielectric constant (ε’) with increase in Eu2O3 and decrease in B2O3 content may be due to the increase in porous and gap structure. Relaxation time (τ) was calculated from τ =1/2πfmax with condition ωcτc =1 (where ωc=1/2πfmax) and plotted as log τ vs. 1000/T. The variation of logarithmic relaxation time
with temperature of 2.5 mol% Eu2O3 doped glass sample is shown if Figure 5 (b). The plot was found to be linear and fitted using the Arrhenius relation:
τr = τ0 exp [-Ea/KBT] Where τ0 is the pre-exponential factor and Ea is the activation energy for relaxation. KB is the Boltzmann constant and T is the absolute temperature. The activation energy (Ea) calculated form the slope of the fitted line is found to be 1.623 eV for 2.5 mol% Eu2O3 doped glass system. From Figure 6, it is evident that, the dielectric constant ε’ increases with increase in temperature at lower frequencies and reaches a low frequency plateau, εp (value of the dielectric constant at quasistatic fields), usually associated with the polarization effects of the long range hopping of mobile ions with respect to the immobile glass matrix . This increase in ε’, in lower frequency and higher temperature region, may be due to the application of the field, which assists electron hopping between two different sites in the glasses. At high temperatures, the jump frequency of the charge carriers becomes large and comparable with the frequency of the applied field. Accordingly at low frequency, the charge carriers hop easily out of the sites with high free energy barriers. This leads to a net polarization and gives an increase in the dielectric constant. However, at higher frequency and at lower temperature, the ε’ approaches a constant value, which results from rapid polarization processes occurring in the glass under applied field . The charge carriers at higher frequency will no longer be able to rotate with enough speed, so their oscillation will begin to lay behind this field resulting in the decrease of dielectric constant ε’. At low temperatures, jump frequency of the charge carriers becomes smaller than the frequency of the applied field. The periodic reversal of the applied field takes place so rapidly that there are no excess charge carrier jumping in the field direction and the polarization due to charge piling up at high free energy barrier sites disappears, which lead to a decrease in the values of ε’ [13,14]. Also, the decrease of the dielectric constant with increasing frequency means that the response of the permanent dipoles decreases as the frequency increases . At elevated temperature (>550 K), the dielectric constant is rather high (>20), and it falls against frequency at first and then becomes more or less stabilized down to above 153 Hz. The low value of dielectric constant at higher frequencies and at lower temperatures is important for extending the material applications towards photonic and optoelectronic fields. 11
The inset of Figure 7 shows the temperature dependence of the dielectric constant ε’ at frequencies 0.1,1,10,100 KHz, for 2.5 mol% Eu2O3 doped glass sample. In the present study, dielectric properties of the glass samples were studied at 313-773 K temperature range. At 313-550 K, the sample of 2.5 mol% Eu2O3 glass has ε’ value in the range of 10-13 and above 550 K, it increased very rapidly above the glass transition at all frequencies under study. This behavior is typical to the polar dielectrics in which the orientation of dipoles is facilitated with rising temperature and thereby the dielectric constant is increased. This ensures thermal stability of these glasses up to 550 K and can be utilized for high temperature applications. From this plot, we can see the pronounced dielectric dispersion at lower frequency (0.1 kHz). The loss factor, tan δ, with logarithmic frequency at different temperatures (313-773 K) of the PbF2–TeO2–B2O3–Eu2O3 (where x = 2.5 mol %) glass sample is shown in Figure 7. Insets (a) & (b) of Figure 7, shows variation of loss factor tan δ with frequency at temperature range, 313-523 K and variation of loss factor, tan δ with temperature at different frequencies, 0.1-100 kHz respectively. The frequency dependence of tan δ at different temperatures (313-773 K) for 2.5 mol% Eu2O3 doped glass is shown in Figure 7. At low frequencies (<150 Hz) and high temperatures (>550 K), the dielectric loss was found to be high values and then decreases with frequency and increases with temperature. Inset of Figure 7 (a), shows variation of loss factor tan δ with frequencies and temperatures, 313 -523 K and it is evident that the dielectric loss tan δ, below 550 K is in the range 0.00086-0.2 which is negligibly small. This may be due to the rapid response of active component than its reactive component. At higher frequencies, the loss tangent decreases with increasing frequency as, the reactive component increases in proportion to the frequency, whereas the active component of the current is practically independent of frequency . Inset (b) Variation of loss factor, tan δ with temperature at different frequencies (0.1-100 kHz). This plot shows shifting of tan δ curve to higher frequency with increasing temperature.
3.1.6 Fourier Transform Infrared Spectra [FTIR] The infrared spectra of the glasses were used to get more information about the presence of different structural groups. Figure 8 shows FTIR spectra of the glass samples recorded at room temperature in the range of 400-4000 cm–1 using Shimadzu FTIR 8400S spectrophotometer [resolution of 0.85 cm-1] by KBr pellet technique. The powdered samples 12
were thoroughly mixed with dry KBr in the ratio of 1:20 by weight and then pellets were formed under a pressure of 9–10 tons. In the present study, FTIR spectra were characterized by three distinguished regions. The first region extended from 1200-1600 cm-1 due to B-O stretching of BO3 units, second region from 800-1200 cm-1 was due to B-O stretching of BO4 units and the third region lying around 660-700 cm-1 due to B-O-B bending vibration in borate network. Besides is, the band from 2400-3600 cm-1 was due to O-H vibration of water group. Apart from these prominent regions, there were two very weak bands centered at 460 cm-1 and 656 cm-1 which correspond to Pb-O bond and the stretching vibration of equatorial and axial Te-O bonds in the TeO4 trigonal bipyramids unit respectively [19-23]. A band centered at 691cm-1, is most likely due to bending of B-O-B linkage of bridging oxygen atoms in BO3 units of the borate network or can be attributed to vibrations of pentaborate groups [20-22]. In general the band at 806 cm-1 is assigned to the boroxol ring in borate glass network [20, 21]. In the present study, this band is found to be missing which indicates the absence of boroxol rings in borate network. Hence the glass system contains [BO3] and [BO4] groups. An absorption band around 1016-1026 cm-1, which was almost present in all compositions, is a signature of the absorption made by penta-borate and di-borate groups in BO3 units. Two broad prominent bands, in all the samples, centered at 1356 cm-1 and 948 cm-1 were respectively due to symmetric stretching vibration of B-O bond in [BO3] units of metaborate, pyroborate and orhtoborate groups [22,23] and stretching vibration of B-O bond in [BO4] tetrahedral units of di-borate groups [20-22]. It was observed that the intensity of these two bands decreased progressively when europium oxide content increased by replacing equal amount of B2O3 from the glass system. Also, BO3 band shifted towards lower wave number [1358-1352 cm-1] whereas BO4 band shifted towards the longer wave number (961-978 cm-1) with rise in europium content. But, the decrease in intensity of BO4 unit with decrease in B2O3 concentration was relatively less as compared with the decrease in intensity of BO3 unit. Hence the continuous increase of europium concentration helped conversion of [BO3] to [BO4] groups in the base glass PbF2TeO2-B2O3, which was already confirmed by the study of density and molar volumes. This reveals europium has played the role of network modifier in the present glass system.
It is well established that FTIR spectra consists of a number of overlapping peaks, then to get qualitative information about the structural groups in glass, the spectra were deconvoluted.
Figure 9 (a) illustrates the results of the deconvolution for x = 2.5 mol% Eu2O3 doped glass as a representative example. From the relative peaks area of BO4 and BO3 structural group, which separated by Gaussian deconvolution, the ratio N4 [& N3] was calculated using the relation: N4
BO 4 units BO 4 units BO 3 units
which describes the concentration of BO4 units .The dependence of the ratio N4 and N3 on the concentration of Eu2O3 ions is shown in Figure 9 (b). The N4 values gradually increased while N3 values showed reverse pattern with increase in Eu2O3 content which was already observed in different sections of the paper.
3.2. Optical Properties 3.2.1. UV-VIS spectral analysis The study of optical absorption and band edge is a helpful method for getting information about the band structure and energy gap of crystalline and amorphous materials. In the present study, the optical absorption spectra were recorded at lab temperature using a Shimadzu UV-1800 double beam spectrophotometer working at 190–1100 nm. Figure 10 shows the overlaid optical absorption spectra of PbF2–TeO2–B2O3–Eu2O3 glasses and inset in the same figure shows the absorption spectrum of 2.5 mol% Eu2O3 doped lead fluoroborate glass. Presently, it has been observed that due to increase in europium concentration, the optical absorption edge shifts towards the longer wavelength from 377 to 389 nm. The shift in absorption band edge may be due to conversion of the [BO3] groups into [BO4] groups which was already confirmed through FTIR, density and molar volume studies. The plot between sqrt (αhϑ) and energy (hϑ) have been used to calculate the band gap energy. The relationship between the absorption coefficient
and the incident photon energy can
be written as
Where ‘A’ is an energy independent constant, ‘Eg’ is the band gap energy, ‘n’ is a constant equal to ½ for direct band gap semiconductor and 2 for indirect bad gap semiconductor. The value of ‘α’ is obtained from the relation
α = 2.303 A/t Where ‘A’ is the absorbance and ‘t’ is the thickness of the glass sample. The variation of (αhϑ)(1/2) versus the photon energy (hϑ) plot for the glass sample were not a linear one at the absorption edge which confirms that the sample has indirect optical band gap energy. The optical band gap energies were obtained from extrapolating the straight portion of the (αhϑ)(1/2) versus (hϑ) plot on the (hϑ) axis at (αhϑ)(1/2) = 0 The value of (hϑ) at the point where (αhϑ)(1/2) becomes zero as shown in above formula yields a direct measure of the optical band gap energy . A plot of optical absorption spectra of PbF2-TeO2-B2O3–Eu2O3 glasses is shown in Figure 10. Inset and inset of inset in Figure 10 respectively show the absorption spectrum and Tauc’s plot of optical band gap of 2.5 mol% Eu2O3 doped lead fluoroborate glass. The energy gap values of PbF2-TeO2-B2O3–Eu2O3 glasses are tabulated in Table 2. Band gap is showing a continuous decreasing trend with rise in Eu2O3 content. The continuous decrease of band gap with increase in Eu2O3 content may be due to conversion of a few number of trigonal [BO3] units to tetrahedral [BO4] units of borate. This change is already observed in FTIR and density, molar volume studies. The decreasing values of optical band gap energy with increasing Eu2O3 content is understood in terms of structural changes which are taking place in the studied glasses. Due to addition of Eu2O3 excess oxygen ions are available in glass system which is utilized for the conversion of [BO3] to [BO4] groups. The tetrahedral [BO4] groups are strongly bonded in glasses network because the bond strength of B-O (808.7 kcal/mol) > Eu-O (591.2 kcal/mol> Te-O (376.2 kcal/mol) > Pb-O (331 kcal/mol) results in decrease of optical band gap. Absorption studies for the glasses yield a maximum of five transitions which include 7
D0, 5D1, 5D2, 5D3 and 5L6 transitions for Eu3+ doped glasses. The 7F0
is forbidden by the selection rules, it exhibits relatively weak intensity. The magnetic dipole allowed 7F0
D1 hypersensitive transition which is relatively weaker than that of the
electric dipole allowed transition 7F0
D2 for Eu3+ doped glasses. The 7F0
was hardly observed, since it is forbidden by the selection rule (ΔJ = 3). It was seen that the 7
L6 absorption band is found to be relatively intense than other transitions, though it is 15
forbidden by ΔS and ΔL selection rules but allowed by ΔJ selection rule . The fifth level of the 5D multiplet, i.e., the 5D4, expected to lie above the 5D3 level is generally not observed in absorption spectrum because of the poor intensity of the spectrophotometer. The absorptions bands observed in the absorption spectra of lanthanides are due to intraconfigurational f-f transitions. The majority of the transitions are induced electric dipole transitions. However, a few transitions are magnetic dipole in nature and their contribution is not significant. The intensities of the absorption bands are expressed in terms of measured oscillator strengths (fm) and can be evaluated by the area method using the formula fm = 4.32 * 10-9 ʃ ε(υ) dυ Where ε(υ) is the molar absorptivity at energy υ(cm-1) that can be obtained using Beer– Lambert’s law. As we know, the intensity of a UV-Visible absorption band is a function of the oscillator strength as well as the energy of the absorption band. The oscillator strength may be used to calculate Einstein’s A and B coefficients, transition probabilities, dipole moment matrix elements, and is also useful in determining the validity of theoretical models. The transition line with higher oscillator strength can be used as excitation source for the fluorescence spectrum of the doped glass. The spectral oscillator strength (f x 10-6) of the individual transitions have been determined by integrating absorption for each band and the relationship is mc2 () d e 2 N In [I 0 () / I()] () 2.303 E( ) / t t f
Where m is the mass of electron, c is the speed of light, N is the number of rare earth ions, e is the charge of the electron,
is the wavenumber E()
is the absorbance, and t is the
sample thickness. The assignment of the lines, their frequencies and the spectral oscillator strength (f x10-6) are given in Table 4. 3.3.
Various types of continuous indentation tests have come into general use for the determination of mechanical properties of materials. The indentation method is preferred because relatively small amounts of testing material are needed and there are no strict requirements for the shape of the samples, moreover the measurements can be performed 16
without the destruction of the samples. For these investigations a wide variety of testing devices were developed with indenters of various forms working in a scale from the nano indentation to macro hardness region. The common feature of these tests is that the applied load is registered as a function of indentation depth during both the loading and unloading period . Micro indentation experiments were carried out on series of glass samples for evaluation of Hardness (H) using Digital Micro Hardness Tester [Matsuzawa MMT-X 7A]. H is obtained through Vickers Micro-Hardness Tester from Hv values obtained from the system during experiment. (Load: 1000 gf, Dwell time: 15s, Objective: 400x, Magnification: 40x and Eyepiece: 10x). Hv value is then converted to Hardness (H) by multiplying H v to a standard constant value, 0.009807; to obtain the hardness value in GPa [25, 26]. Figure 11 clears shows that the microhardness (H) increases with increase in dopant concentration, and this interprets the increase in the glass rigidity. The increase in hardness of the sample shows that these are less brittle to external pressure and moisture which holds good with the agreement studied through FTIR, that is, as the hydroxyl group decreased in the samples the hardness increased. 4. Conclusion Quaternary europium doped PbF2-TeO2-B2O3 glasses were synthesized by melt quenching technique and their composition dependent structural and optical properties were investigated. X - Ray Diffraction showed the amorphous behavior of the prepared samples. The optical band gap, density, molar volume and FTIR spectroscopy results were in good agreement with other properties and these results ensured the structural modifications of the glass network europium ions. Decrease in average boron-boron separation with Eu2O3 incorporation confirmed the presence of [BO4] units and europium ions in the glass network which resulted in compaction of glass structure. These factors were responsible for increase in density values. Molar volume decrement also indicated a decrease in interatomic spacing among the atoms of glass network. Other physical parameters useful for the characterization of the europium doped glass like refraction loss (αm), electronic polarizability of the oxide ion (αo2-), optical basicity (Ʌ), field strength (F) showed good results. The absorption spectra showed a maximum of five transitions which include 7F0
D0, 5D1, 5D2, 5D3 and 5L6
transitions which gave a good agreement with the reported values. FTIR spectra showed structural modifications occurred due to the addition of europium ions in the glass network. 17
FTIR results were also in good agreement with density and band gap which confirmed the coexistence of BO3 and BO4 units. The characteristics (signal shape, intensity, BE) of corelevel photoelectron peaks O1s, Eu3d, Eu4d, Te3d, Te4d, Pd4f, Pb5d, O1s, and F1s, had been reported and exploited. Dielectric study showed good thermal stability of these glasses up to 550 K which can be utilized for high temperature applications. At room temperature, dielectric losses [tan δ] were negligibly small for all prepared glasses and increased with increase in temperature. Dielectric study also showed lesser frequency dispersion at room temperature for 2.5 mol% europium doped glass. 2.5 mol% doped glass samples have highest hardness value with minimum hydroscopic nature which proves the moisture resistant nature of the samples. Decrease in optical band gap energy with an increase of europium concentration showed the semiconducting behavior of the glasses. Hence, these glasses can be used as good semiconductor materials for solid state applications such as electrical memory switching materials, cathode materials for making solid state devices, solid state lasers, luminescent solar energy concentrators, optical fibers and ultrasonic devices.
Acknowledgements This work was supported by a grant-in-aid for a scientific research from the Department of Atomic Energy (DAE) - Board of Research in Nuclear Science [S.No. 2012/34/17/BRNS] of the Government of India. Authors thank UGC - DAE Consortium for Scientific Research Centre for providing Dielectric Spectroscopy and XPS facility. Heartfelt thanks to Prof. (Dr.) V. Upadhyaya for permitting to use the home made Refractive Index system built by him.
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2.0 mol% 1.5 mol% 1.0 mol% 0.5 mol% 0.1 mol%
Figure 1. XRD patterns of the PbF2–TeO2–B2O3–Eu2O3 glass samples.
Figure 2.Variation of the density (ρ) and molar volume (Vm) with Eu2O3 content.
Figure 3. Variation of (a) field strength (F) and oxygen packing density (OPD) and (b) refractive Index (n) and oxide ion polarizability αo2- (n) with Eu2O3 concentration (mol%).
Figure 4. XPS BE spectra of the Eu4d, Eu3d, Pb4f, Pb5d, Te4d, Te3d, B1s, F1s and O1s energy levels of the 2.5 mol% Eu2O3 doped glass.
(b) (b) 0.1 mol%
Figure 5. Variation of (a) dielectric constant ε’ with temperature at room temperature [35 oC] for 0.1, 1.0 & 2.5 mol % Eu2O3 doped glasses. (b) relaxation time with temperature of 2.5 mol% Eu2O3 doped glass sample.
Figure 6. Variation of dielectric constant ε’(ω) with logarithmic frequency at different temperatures of 2.5 mol % Eu2O3 doped glass. Inset represents the variation of dielectric constant with temperature of the same glass at different frequencies.
Figure 7. The loss factor, tan δ, with logarithmic frequency at different temperatures (313 -773 K) of 2.5 mol % Eu2O3 doped glass. Inset (a) Variation of loss factor tan δ with frequency at temperatures ((313 -523 K). Inset (b) Variation of loss factor, tan δ with temperature at different frequencies (0.1-100 kHz).
Intensity (a.u.) 0 mol% 0.1 mol% 0.5 mol% 1.0 mol% 1.5 mol% 2.0 mol% 2.5 mol%
-1 Wavenumber (cm )
Figure 8. The overlaid FTIR absorption spectra of PbF2– TeO2 –B2O3 – Eu2O3 glasses. Inset: The FTIR absorption spectrum of 2.5 mol% Eu2O3 doped lead fluoroborate glass.
Figure 9: (a) FTIR deconvoluted spectrum of 2.5 mol% Eu2O3 doped PbF2–TeO2–B2O3 glass. (b) The relation between N4 and N3 with respect to Eu2O3 concentration.
Intensity (a.u.) 0 mol% 0.1 mol% 0.5 mol% 1.0 mol% 1.5 mol% 2.0 mol% 2.5 mol%
Wavelength (nm) Figure 10. The overlaid optical absorption spectra of PbF2–TeO2–B2O3–Eu2O3 glasses. Inset: The absorption spectrum of 2.5 mol% and inset of inset: Tauc’s plot of optical band gap of 2.5 mol% Eu2O3 doped lead fluoroborate glass.
Figure 11. Variation in the Hardness (H) parameter as a function of Eu2O3 concentration (mol %).
Table 1. Density, molar volumes [Vm,Vc,V0], average boron-boron separation of the Eu2O3 doped PbF2-TeO2-B2O3 glass samples. Mol % of Eu2O3 0 0.1 0.5 1.0 1.5 2.0 2.5
Density (ρ) 3.8501 3.8638 3.902 3.9989 4.0878 4.1467 4.1944
Vm 31.8773 31.8374 31.8151 31.3971 31.0596 30.9588 30.9433
Molar volume Vc 28.42 28.439 28.516 28.613 28.709 28.805 28.901
V0 3.457 3.398 3.299 2.785 2.351 2.154 2.042
nm 0.407 0.4068 0.4054 0.402 0.3989 0.3969 0.3953
Table 2. Various physical and optical properties of PbF2–TeO2 –B2O3 –Eu2O3 glasses. Physical Parameter Mol % Eu2O3 Refractive Index (n) (±0.001) Oxygen Packing Density (gm-atm/l) Molar refractivity (Rm) (cm3/mol) Metallization criterion (M) Reflection loss (%) Number density (N) (*1020 ions/cm3) Inter nuclear distance (ri) (nm) Polaron radius (rp) (nm) Field15strength (F) (*10 cm-2) Energy Band gap (Eg) Oxide Ion Polarizability ( αo2- (n)) Optical Basicity (Ʌ)
Table 3. Binding energy (BE) values of element’s photoelectron peak in Eu2O3 doped sample [x = 2.5 mol%] (BE values are corrected from contamination carbon at 284.6 eV). Element Oxygen (O1s) Boron (B1s) Lead (Pb4f) Lead (Pb5d) Fluorine (F1s) Tellurium (Te3d) Tellurium (Te4d) Europium (Eu3d) Europium (Eu4d)
BE (eV) 537.8 197.2 130.5, 135.6 12.1,14.8 686.3 581.9, 592.4 36.09 1130, 1195 142.9, 148.6
Table 4. The assignment of the lines, their frequencies and the spectral oscillator strength (f x10-6). Oscillator Strength (*10-6) Concentration (mol%) 7 5 F0 L6 7 5 F0 D3 7 5 F0 D2 7 5 F0 D1 7 5 F0 D0
34.77 24.30 ----------------
13.34 9.49 12.83 -----4.01
13.40 13.60 29.46 39.57 20.39
17.34 22.50 17.02 20.60 17.02
10.61 8.64 16.45 21.38 9.54
10.19 9.89 18.05 31.49 13.04
XPS study confirmed the chemical analysis of the samples.
FTIR, density, molar volume and hardness values proved the compact [Bridging Oxygens] network in the samples.
Dielectric study showed good thermal stability of the samples till 550 K.
Decrease in optical band gap energy values indicated the semiconducting behavior of the glasses.