Journal of Non-Crystalline Solids 354 (2008) 3762–3766
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Spectral properties of PbO–P2O5 glasses L.M. Sharaf El-Deen a, M.S. Al Salhi b, M.M. Elkholy a,* a b
Physics Department, Faculty of Science, Menoufia University, Shibin El-Kom, Egypt Physics Department, Faculty of Science, King Saud University, Riyadh, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 20 October 2007 Received in revised form 27 February 2008 Available online 26 April 2008 Keywords: Oxide glasses Optical properties Spectral properties Phosphate glasses Phosphates
a b s t r a c t The optical absorption spectra of xPbO–(100 x) P2O5 glasses where x = 5, 10, 15, 20, 25, and 30 is reported. The spectral absorption of these glasses was measured in the spectral range 300–900 nm at room temperature. Optical absorption spectra show that the absorption edge has a tail extending towards lower energies. The edge shifts nearly linearly towards higher energies with increasing PbO content. The degree of the edge shift was found to depend on the PbO content and is mostly related to the structural rearrangement and the relative concentrations of the glass basic units. The optical energy gap increases, from 2.55 to 3.05 eV by increasing PbO content from 5 to 30 mol%. The width of the localized states is decreased by increasing PbO content. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Phosphate glasses have unique properties which make them useful for a wide range of technical applications. However, these glasses have a relatively poor chemical durability [1] that often limits their usefulness. Several studies have shown that the chemical durability of phosphate glasses can be improved by the addition of various oxides such as PbO and, especially, Fe2O3 [2,3]. As a result, lead and iron phosphate glasses are now of interest for several technological and bio-compatible applications [3,4]. It has been suggested that the addition of one or both of PbO and Fe2O3 results in the formation of P–O–Pb and P–O–Fe bonds which improve the chemical durability [5]. Phosphate glasses are both scientifically and technologically important materials because they generally offer some unique physical properties better than other glasses, such as high thermal expansion coefficients, low melting and softening temperatures, high electrical conductivity, ultraviolet (UV) transmission and optical characteristics [6–10]. These properties make them useful candidates for fast ion conducting materials and other important applications such as laser hosts, glass-to-metal seals and bio-compatible materials [11]. The addition of alkali oxides (A2O) or alkaline earth oxides (MO) to P2O5 glasses results in conversion of the three-dimensional network to linear metaphosphate chains when the molar ratio of alkali or alkaline earth oxide to P2O5, R = A2O/P2O5 = MO/P2O5, increases * Corresponding author. Tel.: +20 482227574; mobile: +20 106062155; fax: +20 482235689. E-mail addresses:
[email protected],
[email protected] (M.M. Elkholy). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.03.032
from 0 to 1. A2O and MO act only as the network modifiers. On the other hand, the structural role of PbO in many oxide glasses is unique since PbO is known to play a dual structural role both as a network modifier [12] and as a network former [13,14]. The role of network forming and network modifying cadmium cations is also found in oxide glasses [15]. The structural role of Pb in many oxide glasses is unique since lead oxide is known to play a dual role as both a network modifier and a network former [16]. In PbO–B2O3 glasses, lead enters (at low concentration) as modifier Pb2+ ions at the rate of two BO4 tetrahedra for each Pb2+ ion [17]. Raman spectroscopy investigations of PbO–B2O3 glasses indicated that, above 50 mol% PbO, four-coordinated boron ions convert into three-coordinated boron ions [18]. In PbO–P2O5 glasses it was found that Pb2+ ions occupy a position between P–O–P layers [19]. Under this interaction, the oxygen atom of the phosphor (PQO) group is included in the lead coordination polyhedron. The probable role of PbO as a network modifier in lead phosphate glasses is depicted in these structures. Several properties of phosphate glasses have been found to result from the incorporation of PbO into the network. For instance, PbO is useful for shielding against high-energy radiations, including nuclear radiation [3] and its addition may result in the formation of P–O–Pb bonds. These bonds lead to an improvement of the chemical durability of phosphate glasses [2]. Therefore, one of the objectives of the present work is to shed more light on the role of lead ions in the PbO–P2O5 glass structure using optical spectroscopy. In addition, it is aimed to correlate the variation of the conductivity and density of the studied glasses with the structural changes. Optical absorption in glass in the visible spectral region results in coloring the glass, leading to applications not only in optical
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2. Experimental
420 (100-X)P O -X(PbO) 415
Transition temperature, oC
lenses but also in optoelectronic materials such as laser hosts fibers for communications and photonic switches. Borate and silicate glasses containing boron oxide have been widely used for optical lenses with high refractive indices and low dispersion characteristics to many decorative uses. Absorption and transmission in the ultraviolet, visible and infrared regions are important in optical instruments, where the absorption in all three regions can be used to study short-range structure of glasses that is the immediate surrounding of the absorbing atom [20].
410 405 400 395 390 385 380 y = -0.0126x + 1.7298x + 369.23 R = 0.9978
375
3. Results The glass transition temperature, Tg, which have been obtained, from DTA charts for the prepared glasses are tabulated in Table 1. It was noted that, Tg is increased with increasing PbO content. The glass transition temperature is plotted in Fig. 1 as a function of PbO content. From this figure we found that Tg varied with a polynomial second order dependence with PbO content according the equation: y ¼ 0:0126x2 þ 1:7298x þ 369:23. The changes in Tg with PbO addition are due to the closeness of the network structure of these glasses that confirmed by the increase in density in the same direction (see Fig. 2). Also, the increase of Tg and density with incorporation of PbO content may be due to the fact that, incorporation of the PbO content serve to increase the compactness of the glass network due to the smaller size of the ionic radius of Pb relative to the ionic Table 1 Density, transition temperature, number of bonds per unit volume, cut-off wavelength, optical energy gap, width of localized states, for PbO–P2O5 glasses PbO content (mol%)
Density (g/cm3)
Tg (°C)
No. of bonds per unit volume nb 1022
Cut-off wavelength (nm)
Eopt (eV)
DE (eV)
5 10 15 20 25 30
6.08 6.63 7.26 7.89 8.20 8.48
378 385 390 397.7 405.6 414.5
9.00 9.63 10.35 11.04 11.27 11.45
350 340 325 314 310 305
2.65 2.75 2.90 2.95 3.02 3.10
0.78 0.69 0.60 0.52 0.51 0.48
370 5
10
15
20
25
30
35
40
PbO Content (mol.%) Fig. 1. Glass transition temperature as a function of PbO content for PbO–P2O5 glass system.
No. of bonds /unit volume
9.50 9.00 8.50 8.00
13.0
(100-X)P2O5-X(PbO)
12.0 11.0 10.0 9.0 5
15 25 35 PbO Content (mol%) Molar Volume (cm /mol.)
7.50
3
Density (gm/cm3)
PbO–P2O5. glasses were prepared from Analar grades of PbO and P2O5 oxides. The appropriate proportions of these reagents were thoroughly mixed and heated in an electric furnace, open to the atmosphere, using alumina crucibles. In order to prevent the excess boiling and consequent spillage and to reduce hydration, the mixtures were placed in an electric furnace preheated at 250 °C for 1 h and then transferred to a second melting furnace maintained at temperature ranging from 800 to 850 °C depending on composition. The melt was left for 45 min under the atmospheric condition in the furnace during which it was occasionally stirred. The homogenized melt was cast in the required form, annealed (350 °C) and cooled to room temperature. All glasses were examined by X-ray diffraction and no diffraction lines were observed confirming the glass formation. The densities of glasses were determined by a simple Archimedes method, using toluene (density = 0.864 gm/cm3 at 20 °C) as an immersion liquid. The density results were reproducible within +0.01%. The characteristic glass transition temperature (Tg) of the prepared glasses obtained using Shimadzu Differential Thermal Analyzer. The optical absorption spectra in the visible and near ultraviolet region were recorded at room temperature. These curves were traced for highly polished glass samples of 2–3 mm thickness using a Perkin-Elmer 402 double beam spectrophotometer in the wavelength range of 190–900 nm.
7.00 6.50 6.00
26 25 24 23 22 21 20 19 18
5
15
25
35
PbO Content (mol.%)
5.50 5
10
15
20
25
30
35
40
PbO Content (mol.%) Fig. 2. Density, molar volume, and number of bonds per unit volume as a function of PbO content for PbO–P2O5 glass system.
radius of phosphorus. Therefore, as a result of PbO addition to the present glass system, the chains become shorter and the cross-link density will increase. Both stiffer chains and the increase in strong ionic cross-link serve to increase the transition temperature and density of glass samples with PbO content. The glass transition temperature can be taken as a measure of the onset of diffusion motion and thus it corresponds to the fixed value of viscosity (or of reciprocal fluidity). In such a case, the Tg can be connected with: (i) the average number of bonds per atom (N) which must be broken to obtain fluidity, and (ii) the bond strength that can be related to the optical gap. Based on this idea, attempts are made to calculate the number of bonds per unit volume (nb) for each glass sample using the following expression [21]: nb ¼ nf N A q=M g ;
ð1Þ
where nf is the coordination number, NA is the Avogadro’s number, and Mg is the molecular mass of the glass sample, which is given by: M g ¼ xM 1 þ ð1 xÞM 2 ;
ð2Þ
where M1 and M2 are the atomic masses and x is the ratio of the used oxides. The calculated number of bonds per unit volume using the above equations along with the density is presented in Table 1. From the onset of Fig. 2 it is found that, the number of bonds per unit volume increases with increasing PbO content and thus the
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average cross-link density also increases leading to compactness of this binary glass system confirming the increase in density and Tg for the same glass system. The optical absorption spectra for binary PbO–P2O5 glasses in the visible and near UV range are shown in Fig. 3. This figure shows that, there is no sharp absorption edge and this is the characteristic of the glassy state. The position of the fundamental absorption edge shifts to higher energy with increasing of PbO content in binary PbO–P2O5 glasses. Kordes and Nieder [9] showed that the absorption edge shifts back and forth with increasing alkaline earths content in phosphate glasses. In the present work, the addition of PbO to P2O5 is meant to produce a breakdown of the weak P@O bond in the glass network and to be replaced by a strong Pb@O bond which is reflected in the absorption spectra by a significant shifting of the absorption edge to shorter wavelengths (higher energy). The shifts of the absorption edge are most likely related to the structural rearrangement of the glass and the relative concentrations of the various fundamental units. The absorption coefficient a(x) was determined at different photon energies, near the absorption edge for binary PbO–P2O5. For direct forbidden transitions, the quantity ða hxÞ1=2 plotted against photon energy ð hxÞ according to the Davis and Mott formula [17]: B ð hx Eopt Þn : x h
ð3Þ
10 x=30 x=25 x=20 x=15 x=10 x=05
(100-x)P2 O 5 -xPbO
9 8
Absorbance
7 6 5 4 3 2 1 0 200
300
400
500
600
700
800
900
Wavelength (nm) Fig. 3. Optical absorption spectra for PbO–P2O5 glass system.
9.00 8.00 7.00 6.00
(αhν)1/2
aðxÞ ¼
Fig. 4 shows a linear dependence of ða hxÞ1=2 on photon energy ð hxÞ for the binary PbO–P2O5 glass systems in the high photon energy range, and then tend to deviate from linearity at low values of photon energy. The values of optical energy gap E, are obtained by extrapolation of the linear region of the plots to a hxÞ1=2 ¼ 0 and these values for PbO–P2O5 glasses are given in Table 1. Stevels [22] suggested that, the intrinsic absorption edge of oxide glass corresponds to the transition of a valence electron of an oxygen ion in the glass network to an excited state. He also suggested that the movement of the absorption band to lower energy corresponds to the transition from the non-bridging oxygen which binds an electron more loosely than a bridging one. Therefore, we believed that, the effect of the entry of Pb atom into the glass was to replace the P@O bonds by strong Pb–O, Pb–O–Pb and/or Pb–O–P bridging bonds. Then the number of non-bridging oxygen atom, decreases and therefore, the energy required to excite an electron from bridging oxygen must be higher than that of exciting an electron from non-bridging oxygen. Thus, the absorption edge shifts to higher energy (lower wavelength) with increasing of PbO content. As a result of decreasing the non-bridging oxygen, the compactness of the glass is increased and the therefore, the optical energy gap increases with increasing PbO content. The absorption coefficient a(x) of the optical absorption near the band edge show an exponential dependence on photon energy h x and obey the empirical relation due to Urbach [23]
5.00
(100-x)P2O5 -xPbO
x=30 x=25 x=20 x=15 x=10 x=05
4.00 3.00 2.00 1.00 0.00 2.00
2.20
2.40
2.60
2.80
3.00
3.20 3.40 hν (eV)
3.60
3.80
4.00
Fig. 4. Dependence of (ahm)1/2 on photon energy (hm) for PbO–P2O5 glass system.
4.20
4.40
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h x ; aðxÞ ¼ a0 exp DE
In the present study the variations of E with PbO content in binary glass system (Fig. 6) suggested that the non-bridging oxygen ion content decreases with increasing PbO content, shifting the band edge to higher energies and leading to an increase in the value of E. The estimated values of E for binary glass system are very close to the reported values in many works [25,27–29]. Hogarth and Hosseini [28] studied the optical absorption spectra near the fundamental absorption edge in some binary V2O5– P2O5 and ternary V2O5–P2O5–TeO2 glasses. They found that, for binary system, the width of the band tails, DE, varies between 0.39 and 0.61 eV, and the optical energy gap, E, varies between 2.02 and 0.43 eV; while for ternary system, DE varies between 0.31 and 0.41 eV whereas E changes from 2.10 to 2.33 eV depending on the glass composition. For molybdenum phosphate glasses, the value of DE was greater than 0.16 eV [31]. For ternary phosphate glasses of the type CdO– ZnO–60 mol% P2O5, the reported value of DE vary between 0.40 and 0.58 eV, and E changes from 3.65 to 4.95 eV depending on the glass composition [30] .On the other hand, DE was found to be 0.12– 0.4 eV for WO3–CaO–TeO2 glasses [32]. For telluride and tungsten telluride glasses, the reported value of DE vary between 0.07 and 0.14 eV, while the value of E changes from 3.32 to 3.79 eV depending on the glass composition [29]. Hassan et al. [33] studied the optical properties of glass and evaporated amorphous thin films of BaO–TeO2. They concluded that, the values of E for glasses are greater than those of thin evaporated thin films. Thin films were expected to be more disordered than the blown glass films.
ð4Þ
where ao is constant and DE is the width of the band tails of localized states. The origin of the exponential dependence of absorption coefficient on photon energy, hx in Urbach equation is not clearly known. It was suggested that it arises from electron transitions between localized states where the density of localized states is exponentially dependent on energy [24]. Davis and Mott [25] reported that this explanation is not valid for all disordered materials since the slope of the observed exponential behavior remains unchanged for many crystalline and non-crystalline materials. Dow and Redfield [26] suggested that it may be arise from random fluctuations of the internal fields associated with structural disorder in many amorphous solids. One possible reason suggested by them is that the slopes of the observed exponential edges obtained from Urbach equation are very close to each other in many semiconductors. Fig. 5 shows the variation of ln a with photon energy hx for some PbO–P2O5 glasses. The values of DE calculated from the slope of the straight lines of these curves are listed in Table 1. The values of DE for a range of amorphous semiconductors lie between 0.045 and 0.67 eV [27]. For the glasses investigated in the present study, the exponential behavior is observed and the value of DE varies between 0.48 and 0.78 eV for binary system, depending on PbO content in the glass composition. The estimated values of DE and E are consistent with the reported values.
3.00 (100-x)P2O5-xPbO
x=30 x=25
2.50
x=20 x=15
ln α
2.00
x=10 x=05
1.50 1.00 0.50 0.00 1.50
2.00
2.50
3.00
3.50
4.00
4.50
hν (eV) Fig. 5. Variation of ln a with photon energy hm for PbO–P2O5 glass system.
3.8 0.9
3.6
y2= 0.0005x- 0.0281x + 0.9128 2 R = 0.9932
ΔE (eV )
0.8
(100-x)P2O5 -xPbO Indirect transition
0.7 0.6 0.5
3.2
0.4
3.0
0
5
10
15
20
25
30
35
PbO content Cut-off wavelength (nm)
Optical energy gap (eV)
3.4
2.8 2.6
y 2 = -0.0005x + 0.0332x + 2.4872 R2 = 0.9913
2.4 2.2
y = 0.0475x - 3.5111x + 367.65 R = 0.9925
350 330 310 290
0
5
10
15 20 25 PbO content
30
35
2.0 0
5
10
15
20 PbO content
25
30
Fig. 6. The variations of E, cutoff wavelength and DE PbO contents for PbO–P2O5 binary glass.
35
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4. Discussion Optical spectroscopy has allowed us to follow the evolution of the glass structure of lead phosphate glasses. The analyses of the optical spectra revealed that the addition of PbO to P2O5 glasses causes a change in the short-range order structure of the phosphate matrix. For PbO < 35 mol%, PbO enters the structure as a network modifier forming non-bridging oxygen ions, while for PbO > 35 mol%, PbO plays the role of a network former in addition to its initial role as a glass modifier. The formation of PbO4 units increases the density, Tg, number of bonds per unit volume and optical energy gap with increasing PbO content. The change in the role of lead ions from the modifier to the former causes an inflection of the optical parameters. In many oxide glasses PbO is known to play a dual structural role both as a network former [34,16,35–38] and as a network modifier [39–41] depending upon its concentration in the glass. When PbO enters the network, it is expected to form PbO4 units in which lead is coordinated to four oxygens in a covalently bonded configuration [34,16]. Several properties of phosphate glasses have been found to result from the incorporation of PbO into the network: for instance PbO is useful for shielding against high-energy radiations, including nuclear radiation [3] and its addition may result in the formation of P–O–Pb bonds, which lead to improvement in the chemical durability of phosphate glasses [42,2]. The observed higher Eopt values and the shift in the absorption edge to higher energies with increasing PbO content indicate the suitability of these glasses for optical device applications and makes them potential candidate for radiation protection devices.
5. Conclusion The optical absorption spectra of PbO–P2O5 glasses were measured in the visible and near UV range. The resulted absorption edge has a tail extending towards lower energies. Increasing PbO content was found to shift the absorption edge nearly linear towards higher energies. The measured values of energy gap vary between 2.55 and 3.05 eV and the width of localized states varies between 0.78 and 0.48 eV with increasing PbO content. References [1] A. Masingu, G. Piccaluga, G. Pinna, J. Non-Cryst. Solids 122 (1990) 52. [2] H.S. Liu, P.Y. Shih, T.S. Chin, Phys. Chem. Glasses 37 (1996) 227.
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