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Prosiding Pertemuan Ilmiah XXV HFI Jateng & DIY 9

ISSN 0853-0823

Spectroscopic Investigation of The Optical Properties of Rare EarthDoped Phosphate and Tellurite Glasses

M. R. Sahar, M. S. Rohani, R. Ariffin And S. K. GhoshalAdvanced Optical Material Research Group, Department of Physics, Faculty of Science,

Universiti Teknologi Malaysia, 81310 UTM Skudai, JohorEmail: [email protected]

Abstract- Improving the up-conversion efficiency is the key issue in rare earth doped glasses. The quantum efficiency, optical gap, radiative transition rate and lifetimes of excited states are greatly influenced by the optical properties of the hostmaterial, ligand field, multi-phonon relaxation processes, impurities, temperature and concentration of rare earth ions. This presentation gives a panoramic view of the state of art of investigating experimentally the optical properties of rare earthdoped phosphate and tellurite glasses with different compositions and preparation techniques that our group has been carriedout in the recent past. We have prepared these glasses by using two different methods melt quenching and sol-gel, however in

the present paper we only report on the former method. Series of erbium-doped magnesium-phosphate glasses based on(P 2O5 )0.5 –(MgO)0.5-x –(Er 2O3 / Nd 2O3 ) x , ytterbium-doped sodium-tellurite glasses based on (80– x)TeO2 –20Na2O-(x)Yb2O3

(x=0.0-2.0 mol%), samarium-doped glasses having composition of (P 2O5 )50-x –(MgO) 50-(Sm2O3 ) x (0≤ x≤ 4 mol%), and (50- x)P 2O5-50MgO-xSm2O3 (x= 0.0-3.0 mol%) have been studies. In addition, ytterbium-doped sodium-tellurite glasses having

composition of (80– x)TeO2 –20Na2O-(x)Yb2O3 (x=0.0-2.0 mol%) is also presented. The amorphous nature of all these samplesis confirmed using the X-ray diffraction technique. The optical properties of the glass have been measured employing Infrared,

FTIR and UV-Visible spectroscopy. The vibrational frequencies are attributed predominately due to the OH - band, P=O, P-O-

P, and P-O- stretching vibrations respectively. Furthermore, it is found that the absorption cut-off wavelength very much

depending on the MgO contents and the integration mode area of absorption band is strongly affected by the phosphatecontents. Our systematic spectral analysis revealed that the substitution of P 2O5 by a small amount of Sm2O3 has negligibleeffect on the absorption band and hence on the glass structure. It is further observed that MgO does not affect the P-O bondcharacters. The optical energy gap (E g ) and the Urbach energy ( ∆ E) has been estimated from the absorption edge studies. It is

found that E g depend on the concentration of the non bridging oxygen in the glass network. Meanwhile, ∆ E is found to bedepending on the Yb2O3 concentration. Interestingly, samarium doped magnesium-phosphate glasses have been found to

change from colorless to light yellow on increasing the Sm2O3 content. The transmission spectra of ytterbium-doped sodium-tellurite glasses revealed that the predominant absorption peaks are due to the vibration of Te-O-Te and Yb3+ ions. The mainresults on vibrational frequencies, optical gap, Urbach energy and absorption edge has been found to be quantitatively

consistent with other observations. Our detailed systematic spectroscopic studies provide useful information for furtherdevelopment of up-conversion lasers.

Keywords: Phosphate glass, Tellurite glass, Melt-quenching, Sol-gel, Optical absorption, UV-VIS Spectroscopy, X-raydiffraction, IR-spectroscopy, Optical properties, Cut-off wavelength, Urbach energy.

I. INTRODUCTION In the new millennium, there has been a renaissance in

the study of rare-earth doped glass materials for photonicapplications, e.g. phosphors, display monitors, X-rayimaging, scintillators, lasers, up-conversion and amplifiersfor fiber-optic communications [1–10]. There are many

candidates in the family viz., borate, chalcogenide, fluoride,germinate, oxynitride, silicate, phosphate, sulphide,zirconate, and tellurite glasses. Rare-earth ions, especiallyerbium, ytterbium and samarium, have played an importantrole in the development of broadband fiber amplifiers inoptical communication technology during the past fewdecades [5-7]. Phosphate glasses has attracted muchattention in recent years due to their unique high thermalexpansion, low melting temperature, high transmission inthe UV region and radiative properties [1, 2]. The mainadvantage of a phosphate glass over other oxide glasses (e.g.silicate and borate) is its ability to accommodate highconcentration of transition metal ions and remain

amorphous. In addition, phosphate glasses enjoy a range ofcompositional and structural possibilities (ultra, meta, pyro,and ortho) that facilitate tailoring chemical and physical

properties of interest for specific technological applications[3, 4]. It is also an excellent material as host material due totheir good chemical durability, ion exchange ability, highgain coefficient, wide bandwidth capability and low upconversion emission.

Compared to silicates phosphate sol-gels studies

are very few in the literature. The solution chemistry of phosphate materials is very different from silicate, which,consequently, continues to make the phosphate systemsmuch more complex than the silicates [5]. Nowadays,ytterbium (Yb3+) ion is regarded as the main dopant for theapplication of high-power diode-pumped laser systems.Since there are only two manifolds in the Yb3+ energy levelscheme namely the 2F7/2 ground state and 2F5/2 excitationstate, it is commonly believed that concentration quenchingand multi-phonon relaxation should not affect the excitationwavelength [6]. However, metaphosphate glasses containingrare-earth ions have potentially important applications inoptical communications and laser technologies. Glasses of

the R(PO3)3 formulas (where R represents one of thelanthanide ions) have been reported to exhibit the largestmagnetic contributions to the low temperature specific heats



10 Prosiding Pertemuan Ilmiah XXV HFI Jateng & DIY

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known in oxide glasses. Even though there have beenstudies of phosphate glasses doped with the lanthanide ions,there have been limited studies of phosphate glasses co-doped with the lanthanide ions [4]. They find a place in the phosphor and luminescence materials applications, cathoderay tube phosphors and scintillate phosphors, because oftheir unique spectroscopic properties [1, 2, 4].

Recently, phosphate and tellurite glasses have beenexploited for various applications in optoelectronic devices,fibre lasers, optical amplifiers, sensing and lasertechnologies [1-2]. These glasses also exhibit reasonablemechanical properties although may show some anomalouscharacteristic due to the excessive amount of the modifier[4]. The study on the effect of rare earth doped phosphateglasses on the optical properties has also been done by manyworkers especially in the ternary systems [5-6]. Our study provides essential information on the band structure and theenergy gap in the non-crystalline material. While theabsorption in the lower energy part gives information aboutthe atomic vibrations, the higher energy part carries

information on the electronic state of the atom.Tellurite glasses are considered as one of the besthosts for doping with rare earth elements. It is a goodcandidate for practical laser applications because of lowcrystallization rate, excellent transparency in a wide spectralrange (3–18µm), good mechanical stability and chemicaldurability [7-9]. The optical properties of tellurite glassesdoped with rare earth have been investigated by severalgroups [9-11]. Tellurite glasses doped with Nd3+ and Yb3+ are important for possible applications as luminescent solarconcentrator [11]. TeO2-based glasses were most studied inthe last 10 years, considering the scientific and technologicalinterest due to their high refractive indices, low melting

temperatures, high dielectric constants and good infraredtransmissions [12].Topically important rare earth doped tellurite and

phosphate glasses in general are intensively studied, as theyare promising for widespread potential applications due toexcellent third-order nonlinear optical performance.Recently, light energy up-conversion property of suchnonlinear optical glasses received special attention becauseof their prospective use in biological labeling and solar nearinfrared concentration for photovoltaic exploitation [3-9]. Meanwhile, erbium doped tellurite glass containing quantumdots or metal nanoparticles (NPs) stimulate intense interestin functionalizing tellurite glass by NPs. In that respect, NPs

dispersed up-converting glasses seem to be the idealcandidates in terms of both efficiency and large areacoverage provided the absorption cross-section be enhanced.To achieve enhanced optical characteristics in these glasses,the concentration of rare-earth ions should be low enough toavoid the quenching effect. Use of two or more rare-earthions together and energy transfer between them or dopingmetallic NPs with rare-earth ions etc. has been found asuccessful way to enhance luminescence. Therefore, glassescontaining metallic NPs doped with low concentration ofRE ions are of particular research interest to us. Thecharacterizations of the nonlinear optical and thermo-optical properties of these glasses are very important for the

optimization and the nanophotonic applications. In spite ofsome experiments on these glasses, the fundamentalunderstanding on the unusual nonlinear optical properties is

still lacking [7-15]. Interestingly, the quantum effect due tometal NPs around the luminescent ion that possiblyenhances the nonlinear optical performance requires furthersystematic experimental theoretical investigation. We willfocus on issues those are relevant to the fabrication oftellurite glass-based nanophotonic devices and photovoltaics. So far, there is no systematic theory or model

and not too many experiments exists to explain the influenceof embedded NPs in the erbium doped tellurite glasses.Lanthanide doped crystal and glasses differ in their

physical properties as well, which influences themanufacturing. Glasses can be produced much moreinexpensively and offer more flexibility in the size andshape. They can be drawn into fibers that microns indiameter and meters in length, or made into bulk rods thatare centimeters in diameter and meters long. Glasses alsohave larger flexibility in their physical properties throughselection of the base material [13]. Analytical method usingX-rays are some of the most powerful techniques formaterials characterization. When X-rays encounter matter, a

variety of processes may take place and each of these processes can be utilized to study particular properties ofmaterial [14]. Infrared absorption spectra of glasses can givevaluable information about atomic configurations in glasseseven though quantitative analysis is rarely possible.Experimental as well as theoretical investigations ofvibrational spectra for vitreous solids have been undertaken[15]. In spite of vast literature, the detailed mechanism behind the origin of linear and nonlinear optical properties isfar from being understood. In view of the topical importanceof these glasses we are tempted to investigate the optical properties of samarium doped magnesium-phosphateglasses, phosphate glass co-doped with the lanthanide ions

by using Infrared and UV-Visible spectroscopy followingtwo different sample preparation roots namely, meltquenching and sol-gel respectively.

II. METHODOLOGY

A. Experimental A.1 Glass Preparation

The glass samples have been prepared from startingmaterial constituents of P2O5, MgO, Er 2O3 and Nd2O3 bymelt-quenching technique with constants phosphate contentat 50 mol% and by changing the MgO content as the Er 2O3 and Nd2O3 change. Four samples co-doped with constant ofthe Er 2O3 contents at 2.5 mol% and 0.25, 0.50, 0.75 and 1

mol% of Nd2O3 has been obtained. In addition, another threesamples co-doped with constant of Nd2O3 contents at 0.50mol% and 1.50, 2.00 and 3.00 mol% respectively has beensynthesized. An appropriate mixture of 20 gm. batch is placein a silica crucible. The mixture was then mixed to becomehomogenous by using milling machine at about 30 minutes.The mixture was heated in an electrical furnace at 1100ºCand then kept inside about 1 hour for the glass to melt. Themelts were quenched into a steel plate mould beforetransferred to another furnace at 450 ºC for 3 hours and thenthe sample was allowed to cool down to room temperature.It is needless to mention that all our samples have been prepared with starting materials from Aldrich products with

99.99% purity.Series of samarium-doped magnesium phosphate glasses,

with composition of [P2O5]50-x-[MgO]50-[Sm2O3]x with




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0≤x≤4 mol% has been prepared from chemically pure rawmaterials by employing the melt quenching technique.Analytical grade of P2O5 MgO and Sm2O3 (purity 99.99%)were used as starting materials. The corresponding weightsof the starting materials were mixed thoroughly in analumina crucible and then placed in air before heated insidethe electric furnace at temperature 900oC-1300oC depending

on composition. Higher temperature was needed for higherSm2O3 content. The melts was then poured on a brass plates before being annealed at 300oC for 3 hours and then allowedto cool down to room temperature.

The raw materials of TeO2-Na2O- Yb2O3 glasswere obtained in the powder form. All samples weresynthesized using the conventional melt-quenchingtechnique. An appropriate amount of TeO2, Na2O and Yb2O3 powder were mixed properly in a silica crucible to thecorresponding composition. 10 gm batches of mixture wereheated at 900 C for about 1 hour. After the mixture wascompletely melted, it was quenched on a mould of stainlesssteel and then allowed to anneal at 300 C for 2 hours.

Finally, the furnace was switched off to allow the sample tocool down to the ambient temperature to remove thermalstrains. All samples obtained are transparent and have goodoptical quality.

The gel with composition of P2O5-Al2O3-Na2O glass was prepared by dissolving aluminum isopropoxide inisopropanol at about 150ºC for 2 hours. Then, a mixture ofHNO3 and NaNO3 in water is added to the alcoholic solutionof aluminum isopropoxide and stirred again at 150ºC until aclear solution is attained before another alcoholic solution ofH3PO4 (obtain by dissolving H3PO4 in isopropanol) isadded. To the resulting solution, YbCl3 solution is addedand was stirred together under reflux at the same

temperature for 3 hours, where a completely clear and stablesolution formed. The gel was obtained after all the solventand water evaporated. The formed gels were then left in afurnace at 150ºC for 72 hours so that the xerogel powderswere formed. The powder as finally melted in air attemperature around 950ºC. The melts was quickly poured ona stainless steel plate to obtain the colorless glass.

A.2. X-Ray DiffractionThe X-ray diffraction studies of all the samples (in

powder form) has been carried out using an automaticPhillips made X ray powder diffractometer to verify theamorphosity of the samples. The running voltage of 30 kV

and current 20 mA has been employed. A radiation sourceof Cu-K α (λ =1.5418Å) is used with a step scan of 0.52. Thedegree of crystallinity of the samples was also determined by using a Siemens made diffractometer D5000 model,equipped with diffraction software analysis. Diffraction patterns were collected in the 2θ range from 10 to 80o, insteps of 0.04o and 4s counting time per step. Meanwhile, theactual glass composition is determined using EnergyDispersive X-ray microanalysis (EDX).

A.3. Infrared Spectral StudiesThe infrared absorption spectra of the glass samples were

recorded using a Perkin-Elmer double beam

spectrophotometer in conjunction with the KBr disctechnique, over the spectral range of 4000–450 cm−1 at roomtemperature. Glass powdered samples of 4mg were

thoroughly mixed and grounded with 200 mg KBr, afterwhich the mixtures were pressed at 10 tonnes/cm2 for 5 mininto a pellet with a surface area of 1 cm2. The IR spectrawere recorded in the spectral range 4000-400 cm-1 usingPerkin Elmer Fourier Transform Infra Red (FTIR)spectrophotometer. The UV-Vis spectroscopy is recorded todetermine the absorption characteristic at 200–700 nm.

A.4. Optical AbsorptionThe optical absorption spectra are recorded at room

temperature. These curves are traced for highly polishedglass samples of ~3mm thickness using a Perkin-Elmerspectrophotometer in the wavelength range of 200 - 800nm.The optical spectra of the sample are recorded using theUV-Vis spectroscopy in the region of 300 – 700 nm atnormal incidence. The transmission cut-off spectra in thevisible and ultraviolet region were recorded at roomtemperature. These curves have been traced for highly polished glass samples of ~3mm thickness using a Perkin-Elmer spectrophotometer in the wavelength range of 200-

800 nm. B. Theoretical

The optical absorption coefficient α ( ω ) for each photonenergy was calculated by using the relation

( )d

A=ω α (1)

where A is the absorbance and d is the thickness of thesample.

In amorphous materials the absorption due to theelectronic transition within the band in relation with theoptical band gap is described by Mott and Davis [16]equation given by,

( ) ( )n g E −= ω ω ω α hh Constant (2)

where α ( ω ) is the absorption coefficient, ħω the photonenergy ( E g ) the optical band gap and n is an index whichcan have any values between ½ and 3 depending on thenature of interband electronic transition. It has been foundout that for most amorphous material n is equal to 2 (indirect band gap) will gives reasonable fit to Equation (2) [17]. Thismeans that at these allowed indirect transition, theinteractions of photons with lattice vibrations will take place. The value of Eg can be obtained by extrapolating thelinear part of the (α ( ω ) ħω )½

versus photon energy, ħω graph to the x-axis. The absorption coefficient, α ( ω ) in the

optical region near the absorption edge, at certaintemperature, always obey the Urbach equation [18] can bewritten as,

( ) ( )( ) E ∆

= ω ω α

hexpConstant (3)

where ∆E is called Urbach energy that can be interpretedas the width of the localized state in the normally forbiddengap. This energy can be obtained by the inverse of the slopelnα(ω) against photon energy (ħω).

C. Result and Discussion The compositions and the XRD patterns of the Er 3+/ Nd3+

co-doped magnesium phosphate glasses obtained are

depicted in Table 1 and Figure 1 respectively. Theamorphous nature of all the glass samples is confirmed by a




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broad halo as that is a characteristic of amorphous structure[19].

TABLE 1. THE COMPOSITIONS (MOL %) OF THEMAGNESIUM

Sample

No.

P2O5

(mol%)

MgO

(mol%)

Er2O3

(mol%)

Nd2O3

(mol%)

S1 50.00 47.25 2.50 0.25

S2 50.00 47.00 2.50 0.50

S3 50.00 46.75 2.50 0.75

S4 50.00 46.50 2.50 1.00

S5 50.00 48.00 1.50 0.50

S6 50.00 47.50 2.50 0.50

S7 50.00 46.50 3.00 0.50

Figure 1. X-ray diffraction pattern of the glass samples. phosphateglasses.

The absorption spectra of all 7 samples are shown inFigure 2. It is clear that the absorption bands are around3420 cm-1-3560 cm-1 and that can be assigned to thevibration peaks of OH- band. The strong absorption reflectsthe higher degree of hygroscopicity of glass samples and can be minimized using high purity materials.

Figure 2. FTIR spectra for the glass samples.

This problem can also be avoided by preparing thesamples in high vacuum or in the environment of nitrogen

gas. The occurrence of strong absorption bands around 1312cm-1-1319 cm-1

can be assigned to the stretching vibration ofP=O [20, 21] that appear in the branching group of Q3 tetrahedral site exist in most phosphate glass system.However, these bands shifted towards a lower wave numberas the amount of Er 3+/ Nd3+ content increase indicating thatthere are some structural changes occur in the glass network.

The bands around 1060 cm-1-1083 cm-1 is assigned to theasymmetrical stretching vibration of P-O-P bonds. The bands for (P-O-P)as-s shifts to lower wave number withincreasing of Er 2O3 and Nd2O3 content, presumably, due tothe changes in the phosphate chain length and the chain P-O-P band angle. Another absorption bands are locatedaround 933 cm-1-938 cm-1 are due to the stretching vibrationof P-O- [22]. A shift towards lower bands is believed to bedue to the existence of the bonds between Er 3+/ Nd3+ ions and

the non-bridging oxygen. A small absorption bands occuraround 764 cm-1-769 cm-1 are due to the symmetricstretching modes of the P-O-P linkages and (P-O-P) s. Theother absorption bands around 471 cm-1 –474 cm-1 can beassigned to the P-O-P bending vibration are actually thesignature of the phosphate glass IR transmission spectra[23].

Figure 3 shows the UV-Vis spectra of the sample 1 withthe thickness ~2.90 mm. Using Figure 3 the cut-offwavelengths is obtained and is summarized in Table 2.

Figure 3: Optical transmittance spectra for Sample 1

TABLE 2. CUT-OFF WAVELENGTH VERSUS MGOCONTENTS.

Sample No. MgO (mol%) cut-off (nm)

S1 47.25 322

S2 47.00 321

S3 46.75 319

S4 46.50 318

S5 48.00 321

S6 47.50 320

S7 46.50 318

A plot for the cut-off wavelength versus sample numbergenerated from Table 2 is depicted in Figure 4.

Figure 4. Cut-off wavelength versus sample number obtained fromTable 2.




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It is clear that the cut-off wavelength shows gradualdecrement with decreasing MgO contents that is due to thechange in glass network structures.

A series of samples of samarium doped magnesium phosphate glass having a composition of [P2O5]50-x-[MgO]

50-[Sm2O3] x, with 0≤x≤4 mol% is represented in Table 3

and the corresponding X-ray diffraction pattern Is shown inFigure 5.

TABLE 3. NOMINAL COMPOSITION OF [P2O5]50-x-[MgO]50[Sm2O3] x WITH 0≤X≤4 MOL% GLASSE SAMPLES.

Sample

No.

Nominal composition (mol%)

P2O5 MgO Sm2O3

S1 50 50 -

S2 49 50 1

S3 48 50 2

S4 47 50 3

S5 46 50 4

Figure 5. X-ray diffraction pattern of samples

The Table 3 shows the possibility of wide glass-formingregion is phosphate glasses. The absence of sharp peaks in

the measured X-ray spectra confirms the amorphous natureof the sample.

Figure 6. IR absorption spectra of the [P2O5]50-x -[MgO]50-[Sm2O3] x glass system with x=0, 1, 2, 3 and 4 mol%.

The room temperature infrared spectra for all five samples isshown in Figure 6.The spectra reveals broad, strong andweak absorption bands over the investigated range of wavenumbers namely, 4000 cm-1-400 cm-1. The comparisonshows the sample modifications with the increment ofSm2O3 contents in the sample. The frequencies of predominant absorption peaks are characterized and presented in Table 4. Figure 6 clearly shows the existence of

a low frequency envelope around 462-474 cm-1consist ofone absorption band. This band is assigned as a bendingvibration of O-P-O units, δ(PO2) modes of (PO-

2) chain

groups [24, 25], while a fundamental band at ~500 cm-1 can be ascribed as a fundamental frequency of (PO4

3-) [26] or asharmonics of P=O bending vibration [26]. The absorption band around 753-769 cm-1is assigned to the symmetricstretch of P-O-P bridges and vs(P-O-P) those are generallythe characteristics of cyclic meta-phosphates. Furthermore,Shikerkar et al [27] reported the occurrence of P-O-Psymmetric stretch absorption band around 731-778 cm-1.

4000.0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400.0

0.0

5

10

15

20

25

30

35

40

45

50.0

cm-1

%T

S5

S4

S3

S2

S1




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TABLE 4. THE IR PEAKS POSITIONS OF [P2O5]50-x-[MgO]50-[Sm2O3]x GLASSES WITH 0≤X≤4 MOL%

The frequency of vibration of P-O-P bonds enhanceswith increasing Sm2O3 content as clearly depicted in Figure7(a). However, at about 3.0 mol% of Sm2O3, the rate ofincrement in frequency starts decreasing. This phenomenonindicates that at this point Sm2O3 may act as a modifier.

The report of Moustafa [28] on IR absorption in the

region 850-1200 cm-1

showed the sensitiveness for thedifferent meta-phosphate groups in the form of chain-, ring-,and terminal groups [29, 30]. The spectral analysis of thisregion shows the existence of seven bands: 927, 931, 1068,1074, 1076, 1082 and 1086 cm-1. The asymmetric stretch ofP-O-P bridges, vas (P-O-P) occur around 931-928cm-1, whilethe absorption band around 1050-1071cm-1can be assignedas symmetric stretch of PO2. It is also found that PO2 absorption band occur at 1068-1086 cm-1 and increases withincreasing of Sm2O3. The bands around 1307-1341cm-1 areassigned to P=O asymmetric stretching vibration modes, inwhich the bands become sharper by the increment of Sm2O3.This attribution is based on the assumption that increasing

Sm2O3 content leads to a breakdown of some terminal bonds(P=O) in the glass network. Such alteration in the structuredecreases the bond strength and consequently, the bandcentre is shifted towards lower wave number as observed[28]. The Sm2O3 concentration dependence of P=O vibrationfrequency mode is shown in Figure 7(b). It can be seen thatas the Sm2O3 content increases the frequency of vibrationdecreases and the behavior is similar to the P-O-P modeexplained in Figure 7(a).

The absorption around 3400 cm-1 is due to OH- ion thatdemonstrates the presence of small amount of water in thesample. The report of Bridge and Patel [31] indicated thatthe intake of water occurred mostly during the preparation

of the pellets. However, the width of the bands, which become progressively weaker with increasing Sm2O3 content, shows that the OH- groups are an integral part ofthe glass network and causes considerable absorption.Although, the presence of small amount of OH- does notdramatically change the structure of phosphate glasses.However, the calculation on the relative integrated area forthese bands show that it is reduced as the Sm2O3 content isincreased which is shown in Figure 8. Relatively broadabsorption band of OH- bonds reflects the amount of thesegroups present in a sample.

(a)

(b)

Figure 7. Variation of frequency of vibration of P-O-P mode (a)and P=O mode (b) as a function of Sm2O3 concentration.

Figure 8. Relative area of OH- absorption band (cm2) of [P2O5]50-

x[MgO] 50-[Sm2O3] x, with 0≤x≤4 mol% glass system.

It is clear from the plot that as the concentration ofSm2O3 is increased, the glass becomes more stable.Moreover, there is an indication that if the dopantconcentration is higher than some threshold, the glass becomes less stable. This is expected, because the Sm3+ ions

Sample

no.

IR absorption Peaks Positions (cm-1)

S1 472.05 752.99 931.82 1086.31 1341.07 3425.98

S2 462.99 757.00 927.77 1076.92 1333.67 3413.79

S3 467.03 762.94 931.59 1068.53 1323.49 3413.87

S4 474.95 764.80 931.36 1074.12 1315.05 3414.16

S5 474.81 768.84 927.80 1082.51 1307.34 3414.20




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either enter in the lattice sites by replacement of OH- ions orreside between the lattices. In case of low Sm3+ content, theSm3+ ions would replace the OH- from the lattices, and thus,the glass becomes more stable. On the other hand, for higherSm3+ content (>3 mol%) the rate of OH- absorption isreduced but the PO4 structure may experience changes intheir structural network that may reduce the stability of the

glass. In this case, the dopant may be featured as an oxidemodifier confirming our earlier explanation. Now we turn our attention to the absorption edge

studies for the estimation of the optical energy gap (Eopt)and the Urbach energy (∆E) and their dependence on thenature of samples with varying Sm2O3 concentration.

Table 5 shows the composition of our preparedsamples. All glasses are found to be very stable except theone with more than 3 mol% of samarium oxide where theglass is easily devitrified. Figure 9 shows the plot of(α(ω)ħω)½ versus photon energy (ħω) for the sample S2. Byextrapolating the linear part of the graph to the x-axis, theoptical energy gaps are obtained and are listed in Table 5.

TABLE 5. NOMINAL COMPOSITION OF THE P2O5-MgO-Sm2O3 GLASSES AND THE VALUES OF Eg AND∆E EXTRACTED FROM THE PLOT.

Figure 9. Typical plot of (α(ω) ħω)½

versus photon energy (ħω)for S2.

The existence of the linear dependence of the graph on photon energy suggested that the transition is of indirectnature. The dependence of Eg on Sm2O3 content is shown inFigure 10. It can clearly be seen that the optical band gapincreases gradually as the Sm2O3 content is increased.However, as the amount of Sm2O3 is about 1 mol%, the gapis decreased. This indicates that the addition of Sm2O3 intothe glass network in the presence of MgO that acts as amodifier impinged the network into a more compactstructure through the formation of P-O-Mg cross-links. As a

result, the hardness as well as the Young’s modulus alsoincreased with the MgO content [32]. However, if theamount of Sm2O3 is further increased, the effect is lessening

which indicates the increasing amount of non-bridgingoxygen in the glass network. In this sense, the MgO seemsuccessfully opens up the chain by breaking up the oxygen bond which finally forming the bridge between the cornersof the PO4 tetrahedra.

Figure 10. Plot for the variation of Eg versus Sm2O3 mol%.

Figure 11 shows the plot of lnα(ω) against photon energy(ħω) of S2. The Urbach energy, were found to be in the

range of 0.28-0.37 eV and is depicted in Table 5.

Figure 11. Graph lnα(ω) versus photon energy (ħω) of S2.

The variation of Urbach energy (∆E) as a function ofSm2O3 contents is presented in Figure 12. It is observed thatthe Urbach energy decreases as the Sm2O3 concentrationincreases indicating the fact that the indirect nature of the band gap becomes weaker. However, at 1.0 mol% ofSm2O3, the effect gets weaker and thereby the energy isincreased. This result shows that the width of the localizedstate in the forbidden gap become smaller and indicates thatthe density of electrons with higher energy becomes smallerthus reducing the probability of electronic transition fromthe valence band to the conduction band. As a result, weobserve a decrease in the optical band gap as has been

shown in Figure 10. However, with an increase of 1.0 mol%of Sm2O3 the probability of electronic transitions isincreased. At this point, the absorption coefficient is slightlyincreased as has also been observed by Sahar et al [32] inthe samarium doped Mn phosphate glasses.

Now we focus our attention to the tellurite glass. In thisstudy, Ytterbium doped sodium-tellurite glasses havingcomposition of (80–x)TeO2 –20Na2O-(x)Yb2O3 (x = 0.0-2.0mol%) are prepared and X-ray diffraction technique and thetransmission spectroscopy is employed for characterization.The absorption vibrational spectra is analyzed is detail.Table 6 shows the nominal composition of the preparedsamples. The X-ray diffraction patterns for the samples with

Yb2O3 concentration from 0.5 to 2.0 mol% are shown inFigure 13.

Sample

number

Nominal composition

(mol%)

Optical band

gap (Eg) in eV

Urbach energy

( E) in eV

P2O5 MgO Sm2O

3

S1 50 50 - 4.35 0.32

S2 49 50 1 4.45 0.28

S3 48 50 2 4.30 0.29

S4 47 50 3 4.30 0.37




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Figure 12. Variation of the Urbach energy (∆E) as a function ofSm2O3

contents at room temperature.

The X-ray diffraction pattern shows expected broad halocharacteristic of amorphous samples. The broad peaksaround 30º show the characteristic of most tellurite glasses.

TABLE 6. THE NOMINAL COMPOSITION OF THEPREPARED SAMPLES.

Sample

number

Nominal Composition

(mol%)

TeO2 Na2O Yb2O3

S1 70.0 30 -

S2 69.5 30 0.5

S3 69.0 30 1.0

S4 68.5 30 1.5

S5 64.0 30 2.0

Figure 13. X-ray diffraction patterns for the glass system. (80 – x)TeO2 – 20Na2O - (x)Yb2O3 glass system.

The positions of the absorption bands of these glasses arelisted in Table 7 with the specification of theircorresponding attributed vibrational modes. For all glasses presented in this work, the spectra occurs at range 3405 cm-

1-3423 cm-1, 1632 cm-1-1643cm-1, 1377 cm-1-1382 cm-1, 721cm-1-732 cm-1 and 589 cm-1-606 cm-1. The predominant peaks around700 cm-1 is attributed to symmetrical vibration

oscillations of the Te–O bonds. The position of all peaks inTeO2 spectrum showed a close agreement with those previously reported [33]. Rare earths connected to thechains of TeO4 groups are identified based on thesimultaneous presence of the bands at ~721 cm-1-732 cm−1.For rare earth-doped glasses, new bands were detectedaround 592, 599, 600, 580 and 606 cm−1 respectively and

those are attributed to Yb–O stretching vibrations.

TABLE 7. INFRARED ABSORPTION PEAKS FOR ALLSAMPLES.

Sample Number IR peaks(cm-1)

1 3423 1643 1382 732 606

2 3423 1640 1381 727 600

3 3417 1637 1380 727 599

4 3412 1632 1377 724 592

5 3405 1635 1377 721 589

Figure 14. IR absorption spectra for TeO2–RE glass system.

The main absorption frequencies around 730 cm-1 –600cm−1 as shown in Figure 14 are assigned to the stretchingvibrations of the Te–O bonds. The spectra of the all studiedglasses showed the typical broadening of the observed

bands. In most cases, they are very similar to the spectra ofthe crystalline phases. This is the direct proof for thesimilarities of the structural units and of the short-rangeorder believed to be crystalline. Due to this reason, thespectra of tellurite glasses are interpreted based on theircrystalline phases. The vibrations of a specific group ofatoms in a lattice are regarded as relatively independentfrom motions of the rest of the atoms, as mentioned earlier[34, 35], according to an assumption in vibrationalspectroscopy of the solid-state [36]. The concept of separatevibrations for glasses was discussed in detail for the firsttime in the work of Tarte [35-40]. A similar approximationis used in the present investigation. This empirical analysis

can give some useful information concerning thearrangement of atoms in glasses.

4000 3000 2000 1500 1000 450

545045

40

35

30

25

20

15

105

T%




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III. CONCLUSION

In conclusion, the phosphate glass sample hassuccessfully been made and their amorphous nature has been confirmed by X-ray diffraction techniques. The IRspectra show that there are six broad absorption peakscorresponding to the OH- band, P=O, P-O-P, P-O- stretchingvibrations. The absorption cut-off wavelength in the range

318 nm to 322 nm depends strongly on the concentration ofMgO in the sample.In a separate study the [P2O5]50-x-[MgO]50-[Sm2O3]x

glass with 0≤x≤4 mol% has been prepared again by meltquenching technique. XRD shows all samples are glass innature. While, IR shows that the glass network is dominated by the existence of asymmetric and symmetric of P-O-P bridges around 931 cm-1-928 cm-1 and 753 cm-1-769 cm-1 respectively. While, P=O asymmetric stretching vibrationmodes exists around 1307 cm-1-1341cm-1. For low Sm3+ content, the glass becomes more stable but only up to 3mol% when the glass start to be precisely influence by theexistence of broad OH peak. In this case, the Sm2O3 may act

as a modifier.To estimate the relevant optical parameter a series ofsamarium doped magnesium phosphate glasses have beenmade and the optical absorption characteristics showed thatthe transition is of indirect type with n=2. The doping ofSm2O3 into the glass network with the present of MgO asmodifying oxides does not alter very much the opticalnature of band gap as well as the Urbach energy tail. Onlyafter the further addition of 1 mol% Sm2O3 the optical bandgap is affected because the formation range of the glass become very small beyond a certain threshold concentrationof the samarium ion.

We further investigate the optical behavior of a series of

ytterbium doped sodium tellurite glasses. Glasses with morethan 2 mol% of Yb2O3 are found to be chemically unstable.The X-ray diffraction pattern confirms the amorphousnature of the samples. The infrared absorption spectra for allthe glasses showed bands around 3500, and 600 cm−1. Rareearths connected to the chains of TeO4 groups identified based on the simultaneous presence of the bands at 589 to606 cm−1. Our spectroscopic investigations on phosphateand tellurite samples prepared by sol-gel techniques will bereported in future communications.

Temperature and concentration dependence of themulti-phonon relaxation rates, radiative decay and up-conversion luminescence is worth to look at. It is hoped that

our detail systematic experimental study may provide usefulinformation for exploiting rare earth doped tellurite and phosphate glasses in fabricating up-conversion lasers. Inaddition, there is a need to extend our studies to examine thetemporal behavior of up-conversion emission and Stokesemission for red and green transition. A completemicroscopic picture on optical characteristics, however,require rigorous theoretical model and simulation on thelocal bonding environment and the local vibrational densityof states of the rare earth ion within the matrix becausesome of these vibrational modes may or may not be coupledto electronic excited states. The effect of embedded metallicnanoparticles on the optical properties of these glasses will

be reported elsewhere.

ACKNOWLEDGEMENTS

S.K. Ghoshal especially thanks to the Physics Department,Universiti Teknologi Malaysia for providing researchfacilities to complete this work.

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