1
Effect of Li
2O on the Optical Properties of Tertiary Phosphate Glass
A. M. Nurul Ain1,a, M. R. Sahar2,b*, E. S. Sazali2,c, and K. Azman1,d
1Faculty of Applied Sciences, Universiti Teknologi MARA Pahang, 26400 Jengka, Pahang, Malaysia
2Advanced Optical Material Research Group, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia
a[email protected], b[email protected], c[email protected],
*Corresponding author: [email protected]
Abstract: Glasses with chemical composition of (60-x)P2O5-25ZnO-(15+x)Li2O with 0.0 ≤ x ≤ 5.0 mol % are prepared by melt quenching technique. Some of the physical properties such as densities and molar volume and XRD are well determined. Meanwhile, the optical properties by mean of their absorption spectra, Urbach energy, direct and indirect energy band gap as well as refractive indices had also been determined. In this work, the glasses obtained are amorphous in nature with densities and molar volume ranging from 2.70 - 2.78 gcm-3 and from 37.48 - 40.75 gcm-3 respectively. Meanwhile, the optical band gap energy for direct and indirect transition are found decreases from 3.074 eV to 2.525 eV and 2.699 eV to 1.670 eV with respect to Li2O content.
Whereas, the Urbach energy is varies with respect to Li2O content. The refractive index of these glasses is ranging from 2.48 to 2.90.
Keywords: phosphate glass, Li2O, energy band gap, Urbach energy
Good reputation of phosphate glasses over some others conventional oxide glasses such as high thermal expansion coefficient, low melting and softening temperature, promising refractive index, high ultra-violet (UV) and far infrared transmission [1-3,9]. Phosphate glass have become a promising UV transmitting material because of its high UV transmittance and short UV cut-off [6].
Phosphate glasses are also used as biomaterials since its chemical composition similar to that of natural bone. Although phosphate possesses excellent reputation in application, however phosphate glasses are lacking chemical durability which may due to its hygroscopicity. Also, the properties of phosphate glass might be influenced by its different structural groups, such as PO4, PO43−
, P-O− and P=O. [12]. In this work, incorporation of alkali metal ions such as Li2O are expected to modify the structural network of phosphate glass and increase the ionic conductivity [4]. An additions of Li2O into phosphate glass result in a breakdown of bridging bond P-O-P and created of non-bridging oxygen (NBO) [5]. Furthermore, an existence of zinc oxide in the phosphate glass capable to modify the physical, structural and chemical durability properties of the host network former because it acts as metal transition [7]. In this paper, the glass preparation, some physical properties as well as the optical properties of phosphate tertiary glasses will be analysed and discussed.
The glasses with the composition of (60-x) P2O5-25ZnO-(15+x) Li2O where 0.0 ≤ x ≤ 5.0 mol
% are prepared by using melt-quenching techniques. Analytical grade powder of P2O5 (98% purity), ZnO (99% purity) and Li2CO3 (98% purity) for about 15g batch of each sample are well mixed in alumina crucibles before placed into furnace at temperature of 900 ºC for about 30 min. The molten is then poured into steel mold and annealed at 300 ºC for 3.5 hours to reduce the residual stress trapped in the glass. The samples are then allowed to cool down to room temperature and then are kept in desiccators to prevent moisture attack.
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Figure 1 shows a typical XRD pattern for phosphate glass. From Figure 1, it clearly been seen a broad peak in range of 20º-30º that verifies the characteristic of an amorphous nature of the glass. Meanwhile, the absence of sharp peaks in XRD pattern reveals that the glass is amorphous nature [10]. Meanwhile, Table 2 shows the results of density, molar volume, energy band gap, Urbach energy and the refractive indices of phosphate glass. Figure 2 shows the results of glass density and molar volume of the phosphate glass. From the result, the glass densities show inversely trend of results to molar volume with respect to Li2O content. The results revealed that an addition of lithium to replace the phosphate interstitial tends to compact the glass structure. Whereas, in addition of lithium which having higher atomic radii (r = 167pm) to phosphorus (r = 98pm) tend to reduce the molar volume. Similar trend of results was also been found elsewhere [11].
10 20 30 40 50 60 70 80
0 10 20 30 40 50 60
2(degree)
Intensity (a.u)
Figure 1. A typical XRD pattern for phosphate glass
Table 2. Density , molar volume ( ), indirect ) and direct band ) gap energy, Urbach energy , and refractive index (n) for (60-x)P2O5-25ZnO-(15+x)Li2O glass.
Glass
(g cm-3) (cm3 mol-1)
(eV)
(eV)
(eV)
S1 2.6991 40.7450 2.4711 2.9567 1.3604 2.5563 S2 2.7010 40.3066 2.6990 3.0737 0.9594 2.4833 S3 2.7023 39.8723 2.6435 3.0451 1.0538 2.5003 S4 2.7350 38.9859 2.6079 3.0336 1.1107 2.5116 S5 2.7829 37.9123 2.6407 3.0515 1.0477 2.5012 S6 2.7848 37.4839 1.6700 2.5253 3.4454 2.8952
3
15 16 17 18 19 20
2.70 2.72 2.74 2.76 2.78 2.80
Li2O Concentration (% mol)
(g cm-3)
(g cm-3)
Vm (cm-3 mol-1)
37 38 39 40 41 Vm(cm -3 mo
l -1
)
Figure 2. Concentration of Li2O dependent glass density and molar volume
Figure 3 shows the typical plot of indirect and direct optical band gap according to Tauc’s plots which the photon energy dependent variation are ( 1/2 and ( 2 respectively. From the results it is clearly been seen that the indirect optical band gap is slightly lower than the direct one.
Meanwhile, in Figure 4, the variation of value with respect to Li2O revealed the structural changes in glass system. The distribution of lithium ions into phosphate glass network are expected to change the glass structure as it has been shown by a drastic change in optical band gap especially when more than 19 mol%. This similar trend of results was also been observed to Urbach’s energy.
Ascertain amount of Li2O contributes the disorder of glass structural hence, decreasing the optical energy band gap [4]. Therefore, by the decreasing trends in optical band gap it will then cause the increasing number of non-bridging oxygen (NBO) [1]. It also can be assumed that the high- polarized of modifying cation, Zn2+ could affect the electronic shell of ion that may cause a decreasing in Eopt.
2.6 2.8 3.0 3.2 3.4 3.6 0
200 400 600 800 1000
(hv)1/2(eV1/2.cm-1/2)
hv (eV)
3.0 3.2 3.4 3.6
0 1x106 2x106 3x106 4x106
(hv)2(eV21.cm-2)
hv (eV)
(a) (b)
Figure 3. A plot of (a) indirect and (b) direct optical band gap for phosphate glass
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15 16 17 18 19 20
1.6 1.8 2.0 2.2 2.4 2.6 2.8
EIopt (eV)
EDopt (eV)
Li2O Concentration (mol%)
EI opt(eV)
2.5 2.6 2.7 2.8 2.9 3.0 3.1
ED opt(eV)
15 16 17 18 19 20
1.0 1.5 2.0 2.5 3.0 3.5
E(eV)
E(eV) n
Li2O Concentration (mol%)
2.5 2.6 2.7 2.8 2.9
n
(a) (b)
Figure 4. A plot of (a) indirect and direct optical band gap energy and (b) Urbach energy and refractive index of phosphate glass against Li2O concentration
The results of Urbach energy and the refractive index are shown in Figure 4 and Table 2. As in Figure 4, the width of the band tail or Urbach energy, can be obtained by plotting the logarithm of absorption coefficient, versus photon energy, . The Urbach energy seem stable as the Li2O content varies from 15 mol% to 19 mol%. However, a drastic change of Urbach energy is observed as the Li2O content is more than 19 mol%. The large value of Urbach energy revealed a tendency of glass structures to convert weak bond into defects that caused the lack of long-range order [4,10]. Similar trend of results is also been observed as the refractive index increases with respect to Li2O content. The incorporation of Li2O tends to alter the glass network to be more compact. Index of refraction has strong relation to the matter’s compactness [1].
In conclusion, glasses of chemical composition (60-x)P2O5-25ZnO-(15+x)Li2O with 0.0 ≤ x ≤ 5.0 mol % has successfully been prepared by melt quenching techniques. All glasses are amorphous in nature as confirmed by XRD. The glass densities had successfully been determined by Archimedes’ method was found ranging from 2.699-2.785g cm−3. Whereas, the optical band gap by mean of their direct and indirect energy was found ranging from 3.074-2.525 eV and from 2.699- 1.670 eV respectively. Meanwhile, the Urbach energy was found varies from 0.959-3.445 eV with respect to Li2O content.
Acknowledgement
The authors gratefully acknowledged the financial support from Universiti Teknologi MARA and Universiti Teknologi Malaysia.
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