Chapter 3

The de-clustering influence of aluminium

ions on the emission features of Nd3+

ions

in PbO-SiO2 glasses

Nd3+

doped lead alumino silicate glasses with varying concentration of Al2O3

(from 5 to 10 mol%) have been synthesized. The IR and Raman spectral studies of

these glasses have indicated that there is a gradual increase in the depolymerization

of the glass network with increase in the concentration of Al2O3.The optical

absorption, luminescence spectra and fluorescence decay curves of these glasses were

recorded at room temperature. From these studies, the radiative parameters viz.,

spontaneous emission probability A, the total emission probability, the radiative

lifetime τ, the fluorescent branching ratio β of different transitions originated from

4F3/2 level of Nd

3+ ions have been evaluated. A clear increase in the quantum efficiency

and luminescence emission of the two prominent NIR bands of Nd3+

ions viz.,

4F3/2→

4I11/2, 13/2, is observed with increase in the concentration of Al2O3. The increase

has been attributed to the possible admixing of wavefunctions of opposite parities and

due to the declusterization of Nd3+

ions by Al3+

ions in the glass network.

The de-clustering influence of aluminium ions on the

emission features of Nd3+

ions in PbO-SiO2 glasses

3.1 Introduction

Aluminium containing silicate glasses are being widely used as low cost

optical connectors, dielectric and sealant materials for solid oxide fuel cells, in

actinide immobilization, as laser ion hosts, in optical lenses, seals and as in vivo

radiation delivery vehicles because of their high mechanical strength, high

electrical resistance and chemical durability [1-6]. Apart from these, it has been

widely accepted that aluminum ions produce greater enhancement on

fluorescence of all rare-earth ions embedded in the host material [7]; the

enhancement is ascribed to the association of RE ions with Al3+ ions in different

ways. The traditional interpretation, is that rare-earth ions will be preferably

partitioned by Al3+, forming Al‒O‒RE bonds, rather than sitting together to

form RE‒O‒RE bonds. Subsequently, the spacing among RE ions becomes

larger in the alumina-doped silica host rather than in the non–alumina-

containing host and facilitates for the enhancement of fluorescence [8].

Among various rare earth ions, Nd3+ ions are known due to their

significant emission transitions in NIR region. Out of three prominent emission

transitions of Nd3+ ions, the emission at 1.3 µm (4F3/2→4I13/2) is more important

because of its significant impact on telecommunications. In general, an efficient

73

host material for laser operation should exhibit large emission cross section to

provide high gain, long fluorescence lifetime to minimize pump losses incurred

from spontaneous emission, large absorption cross section at the pump

wavelength, good photothermal properties, absence of scattering centers and the

possibility to incorporate high concentration of rare earth ions. Aluminium

silicate glasses meet all the above requirements and can therefore be considered

as the most suitable glass for hosting Nd3+ ion with zero dispersion and low loss

at 1.3 µm wavelength. Majority of the studies on 1.3 µm emission are devoted

to Pr3+ doped glasses, whereas most of the studies on Nd3+ emission are

concentrated upon 1.06 µm emission [9-11].

Normally neodymium oxide is immiscible in silica glass and forms

aggregates or clusters in which cross relaxation can give rise to non-radiative

de-excitation of neodymium, resulting in very short lifetimes [12]. Different

research groups working in the field of luminescence have shown that the non-

radiative de-excitation of neodymium ions can be minimized and enhancement

of fluorescence is possible when Al is used as a co-dopant in the glasses

because aluminium ions disperse RE clusters, reduce inter-ionic interactions

and fluorescence quenching [7].

In this study we have synthesized PbO−Al2O3−SiO2: Nd2O3 glasses with

different contents of Al2O3 and investigated the influence of variation of Al2O3

74

concentration on 4F3/2→4I11/2,13/2 fluorescence transitions and fluorescence time

decays of Nd3+ ions in the titled glasses and evaluated the mechanism

responsible for the variation in the intensity of above transition in terms of

interactions between Al3+ ions and Nd3+ ions. The IR and Raman spectral

studies have also been included so as to asses some pre-structural information

on the glass network.

A particular composition (40-x) PbO–(5+x) Al2O3–54SiO2: 1.0 Nd2O3 (in

mol%) with three values of x ranging from 0 to 5.0, is chosen for the present study

The detailed compostions are as follows:

AN5: 40 PbO–5Al2O3–54SiO2: 1.0 Nd2O3

AN8: 37 PbO–8Al2O3–54SiO2: 1.0 Nd2O3

AN10: 35 PbO–10Al2O3–54SiO2: 1.0 Nd2O3

3.2 A brief review on the spectroscopic studies of Nd3+

ions in

various glass systems

The studies as such on spectroscopic properties of alumino silicate

glasses are very few. However, a brief review on recent studies on

luminescence properties of different other glass systems including some silicate

glass systems is presented here.

75

Pérez-Rodríguez et al. [13] have studied relevance of radiative transfer

processes on Nd3+ doped phosphate glasses for temperature sensing by means

of the fluorescence intensity ratio technique. Their studies the reabsorption

measurements in the samples with a higher Nd3+ content revealed that the

reliability of the sensing performance is compromised with the doping

concentration and the experimental conditions of the measurement. Pal et al.

[14] have reported fluorescence and radiative properties of Nd3+ ions doped

zinc bismuth silicate glasses. Their results have pointed out this glass system as

good candidate to be used in the development of photonics devices operating in

the near infrared spectral range. Sobczyk [15] has investigated temperature-

dependent luminescence and temperature-stimulated NIR-to-VIS up-conversion

in Nd3+-doped La2O3–Na2O–ZnO–TeO2 glasses. The results of this study

indicated that the investigated glasses are potentially applicable as a 1063 nm

laser host as well as an optical sensor for temperature measurements. Boetti et

al. [16] reported the results of spectroscopic investigations of Nd3 + single

doped and Eu3 +/Nd3 + co-doped phosphate glass for solar pumped lasers. This

study has indicated that although energy transfer from Eu3 + to Nd3 +occured, a

large Eu3 + concentration dependent quenching of Nd3 + fluorescence. This latter

process reported to decrease the radiative quantum efficiency of Nd3 +:4F3/2

emitting level. Gupta et al. [17] have studied the influence of bismuth on

76

structural, elastic and spectroscopic properties of Nd3+ doped Zinc–Boro-

Bismuthate glasses. These studies indicated that due to Bi2O3 incorporation

reduced host phonon energy and its high optical basicity effect remarkably

improved the Nd3+ luminescence properties such as emission intensity, quantum

yield and emission cross-section. Martin et al. [18] reported nanocrystal

formation using laser irradiation on Nd3+ doped barium titanium silicate glasses.

Evidence of nanocrystal formation induced by laser irradiation was confirmed

by optical spectroscopic, X-ray diffraction, scanning electron and atomic force

microscopy. Lu et al. [19] reported crystallization, structure and fluorescence

emission of Nd3+- and Yb3+-doped NCS transparent glass-ceramics. In this

report it was said that there is strong fluorescence emission in the Nd3+-and

Yb3+-doped NCS glass-ceramics when compared with the singly ion doped

glasses. Sola et al. [20] studied the stress-induced buried waveguides in the

0.8CaSiO3–0.2Ca3(PO4)2 eutectic glass doped with Nd3+ ions. The objective of

this study was to characterize and to evaluate in what extent their optical

properties could be modified by the waveguide fabrication process.

Brahmachary et al. [21] have investigated spectroscopic properties of Nd3+

doped zinc-alumino-sodium-phosphate (ZANP) glasses. Mahamuda et al. [22]

have studied the spectroscopic properties and luminescence behavior of Nd3+

doped zinc alumino bismuth borate glasses. From these studies it was

77

concluded that 1 mol% of Nd3+ ion concentration is optimum for zinc alumino

bismuth borate glasses to generate a strong laser emission at 1060 nm.

Kawamura et al. [23] have reported the extraction of Nd3+-doped LiYF4

phosphor from sol–gel-derived oxyfluoride glass ceramics by hydrofluoric acid

treatment. The photoluminescent (PL) properties were investigated for the

sample before and after HF treatment. The results indicated that the Nd3+ ions

were predominantly incorporated in LiYF4, and the extraction of LiYF4

crystallites was successfully carried out without changing the PL properties of

Nd3+ ions. da Silva et al. [24] have investigated the frequency upconversion in

Nd3+ doped PbO–GeO2 glasses containing silver nanoparticles. Their results

indicated that the nucleation of silver NPs in Nd3+-doped PGO glasses

contributes to increase the UC efficiency. Fu et al. [25] have reported studies on

single-mode waveguides generated in Nd3+-doped silicate glass by nickel ion

irradiation. Shanmugavelu et al. [26] have studied optical properties of Nd3+

doped bismuth zinc borate glasses.

Surendra Babu et al. [27] have recently reported the results of their

studies on 1.06 µm emission in Nd3+-doped alkali niobium zinc tellurite glasses.

From this study they have concluded that 4F3/2→4I11/2 is the probable lasing

transition excited with low threshold power. Cankaya and Sennaroglu [28] have

reported lasing emission of Nd3+ ions in TeO2–WO3 glasses at 1.37 µm. In this

78

report the authors have claimed to have measured the luminescence efficiency

as much as 78%. Saleem et al. [29] have reported optical absorption and near

infrared emission properties of Nd3+ ions in alkali lead tellurofluoroborate

glasses. In this study the authors have evaluated total radiative transition

probabilities (AT), stimulated emission cross-sections (σE) and gain bandwidth

parameters (σE × ∆λP) and compared with the earlier reports. Zhong et al. [30]

have reported 2.7 µm emissions of Nd3+, Er3+ codoped tellurite glass. In this

study the authors have observed the enhanced mid infrared emission due to the

co-doping. Verma et al. [31] have studied effect of modifiers (BaF2 BaCl2 and

BaCO3) and heat treatment on the fluorescence bands of Nd3+ ions in TeO2

glasses. Their study has indicated that the BaCl2 modified glass exhibits

maximum upconversion intensity among the three modifiers. From the

temperature dependent fluorescence studies of these glasses the authors have

concluded that Nd3+ doped tellurite glass can be used as a temperature sensor.

Courrol et al. [32] have studied spectral properties of lead fluoroborate

glasses doped with Nd3+. Wilhelm et al. [33] have reported the fluorescence life

time enhancement of Nd3+-doped sol–gel glasses by Al–codoping and CO2-

laser processing. Karthikeyan and Mohan [34] have reported the structural,

optical and glass transition studies on Nd3+ doped lead bismuth borate glasses.

Annapurna et al. [35] have investigated the NIR emission and upconversion

79

luminescence spectra of Nd3+:ZnO–SiO2–B2O3 glasses. Saisudha and

Ramakrishna [36] have found large radiative transition propabilities in bismuth

borate glasses doped with Nd3+ ions. Shen et al. [37] have reported the

compositional effects and spectroscopy of rare earths (Er3+, Tm3+, and Nd3+) in

tellurite glasses. Kumar et al. [38] have explored the stimulated emission and

radiative properties of Nd3+ ions in barium fluorophosphate glass containing

sulphate. Chen et al. [39] have studied ion-implanted waveguides in Nd3+ doped

silicate glass and Er3+/Yb3+ co-doped phosphate glass. Kam and Buddhudu [40]

have studied Luminescence enhancement in (Nd3++Ce3+) doped SiO2: Al2O3

sol-gel glasses. Rosa-Cruz et al. [41] have reported the results of their study on

spectroscopic characterization of Nd3+ ions on barium fluoroborophosphate

glasses. Fernandez et al. [42, 43] have evaluated the upconversion losses in Nd-

doped fluoroarsenate glasses. Surana et al. [44] have investigated the Laser

action in neodymium-doped zinc chloride borophosphate glasses. Vijaya

Prakash reported [45] his results of absorption spectral studies of Pr, Nd, Sm,

Dy, Ho and Er ions doped in NASICON type phosphate glass, Na4AlZnP3O12.

Rao et al. [46] have reported luminescence properties of Nd3+: TeO2–B2O3–

P2O5–Li2O glass. Xiang et al. [47] have studied the up-conversion emission in

violet from yellow in Nd3+: SiO2–TiO2–Al2O3 sol-gel glasses. Bouderbala et al.

[48] have reported the results of their studies on infrared and visible room

80

temperature fluorescence induced by continuous laser excitation of new Nd3+:

phosphate glasses. Cassanjes et al. [49] have investigated Raman scattering,

differential scanning calorimetry and Nd3+ spectroscopy in alkali niobium

tellurite glasses. Mehta et al. [50, 51] have investigated the spectroscopic

properties including ESR of Nd3+ doped phosphate and borate glasses. Kumar

and Bhatnagar [52] have reported the effect of modifier ions on the covalency

of Nd3+ ions in cadmium borate glasses. Srinivasa Rao et al. [53] have reported

the physical and absorption properties of Nd3+ doped mixed alkali fluoro-

borophosphate optical glasses. Ajit Kumar et al. [54] have reported the

spectroscopic parameters of Nd3+ ion in phosphate glasses. Joshi and Lohani

[55] have investigated the non-radiative energy transfer from Tm3+ to Ho3+ and

Nd3+ in zinc phosphate glass. Dawar et al. [56] have studied the optical and

acousto-optical properties of Nd: phosphate glasses. Ning Lei et al. [57] have

fabricated Ti: sapphire laser pumped Nd: tellurite glass laser. Pozza et al. [58]

have investigated the absorption and luminescence spectroscopy of Nd3+ and

Er3+ in a zinc borate glass. Ratnakaran and Buddhudu [59] have reported the

optical absorption spectra and laser analysis of Nd3+ ions in fluoroborate

glasses. Sen and Stebbins [60] have studied structural role of Nd3+ in SiO2 glass

using NMR studies. Ebendorff et al. [61] have studied the spectroscopic

properties of Nd3+ ions in phosphate glasses.

81

3.3 Results and Discussion

In the presence of modifiers SiO2 participates in the glass network with

meta, pyro and ortho−silicates in the order: [SiO4/2]0 (Q4), [SiO3/2O]− (Q3),

[SiO2/2O2]2−(Q2), [SiO1/2O3]

3−(Q1) and [SiO4]4− (Q0) [62]. Earlier NMR studies

on aluminium silicate glasses have indicated that aluminium ions occupy

mainly tetrahedral (AlO4) and octahedral (AlO6) sites [63].

2Al2O3 → [Al3+] o + 3[AlO4/2]t

The AlO4 tetrahedrons enter the glass network and alternate with SiO4

tetrahedrons, whereas octahedral Al3+ induce bonding defects in the glass

network and participate in the declusterization of Nd3+ ions. Recently, it was

demonstrated by Schmucker and Schneider [64] using NMR studies that the

edge-sharing [AlO4] tetrahedra also participate in inducing bonding defects in

the silicate glass network. The oxygen triclusters of such units (oxygen ion

bonded to two [AlO4] and one [SiO4] or one [AlO4] and two [SiO4]) take part in

inducing bonding defects in the glass containing Al2O3 and SiO2. PbO,

normally in the concentration range used in the present study acts

as a modifier and as a result Si-O-Si and Si-O-Al bonds are cleaved in

succession. Such behaviour also lead to a de-polymerization of the glass

network.

82

From the measured values of the density and average molecular weight

M of the samples, various other physical parameters such as Nd3+ ion

concentration Ni, mean Nd3+ ion separation Ri and molar volume for all the

glass samples were evaluated and presented in Table 3.1. The dependence of

density d on the concentration of Al2O3 for all the titled glasses shows slight

decrement with increasing Al2O3 content. The decrease of density can be

understood due to the replacement of PbO by Al2O3.

Table 3.1 Physical parameters of PbO−Al2O3−SiO2 glasses doped with different concentrations of Nd2O3.

Physical parameter↓ Glass →

Nd3+ series AN5 AN8 AN10

Density d (g/cm3)

4.6218 4.4147 4.3014

Refractive index

1.647 1.646 1.645

Nd3+ion conc. Ni (x1020 ions/cm3)

2.14 2.10 2.09

Interionic distance Ri (Å)

16.72 16.82 16.86

Polaron radius Rp ( Å ) 6.74 6.78 6.79

Field Strength Fi (1014, cm-2)

4.41 4.35 4.33

83

DSC studies on Nd2O3 doped PbO−Al2O3−SiO2 glasses mixed with

different concentrations of Al2O3 have been carried out. All DSC traces exhibit

typical glass transition (Tg) with the inflection point between 850–870 K; the

glass transition temperature decreases slightly with increasing Al2O3 content. At

about 1240 oC, the process of crystallization begins and all the traces exhibited

well defined exothermic peaks at TC.

To observe the mass change effects during heating process, we have also

recorded thermal gravimetric traces for all the samples; TG traces for two of the

samples along with corresponding DSC traces are shown. The thermal

gravimetric analysis for all the samples indicated that upto 1200 K virtually no

change in the mass of the samples. In Fig. 3.1 DSC and TG traces for the glass

AN5 are presented. In the inset of Fig. 3.1 we have presented the variation of

variation of Tc-Tg (a parameter that gives the information on thermal stability

of the glass) is found to be decreased slightly with the concentration of Al2O3. It

may be noted here that DSC traces of all the three series of glasses viz.,

PbO−Al2O3−SiO2:Nd3+/Dy3+/Yb3+ exhibited a similar trend. In view of this, the

DSC patterns PbO−Al2O3−SiO2:Dy3+/Yb3+ were not presented in the subsequent

chapters.

84

Fig. 3.1 DSC and TG Traces for the glass AN5. Inset shows the variation of Tc-Tg with the concentration of Al2O3.

The optical absorption spectra of PbO−Al2O3−SiO2: Nd2O3 glasses

recorded at room temperature in the wavelength region 300-2000 nm exhibited

conventional absorption bands corresponding to the following electronic

transitions [65]

4I9/2 → 2P1/2, 2D3/2+

4G11/2, 4G9/2+

4G7/2, 4G5/2,

2H11/2, 4F9/2,

4F7/2, 4F5/2 and

4F3/2

The pattern of absorption spectra for all the three glasses remains the same;

however, considerable variations in the peak positions and the intensity of these

bands have been observed (Fig. 3.2).

85

Fig. 3.2 Optical absorption spectra of PbO-Al2O3 -SiO2: Nd2O3 glasses in the visible region (all transitions are from 4I9/2).

2.0

3.0

4.0

5.0

6.0

7.0

8.0

400 450 500 550 600 650 700 750 800 850 900

2P1/2

2D3/2

4G9/2

4G7/2

4G5/2

2H11/24F9/2

4F7/2

4F5/2

4F3/2

AN5

AN10

AN8

Wavelength(nm)

Abs

orbt

ion

coef

fici

ent (

cm-1

)

86

The experimental oscillator strengths (OS) of the absorption transitions

were estimated from the spectra for all the three rare earth ion doped glasses in

terms of the area under absorption peaks. The procedure of fitting of the

calculated OS to those evaluated from the experimental spectra is described in

Ref.[66]. A set of matrix equations (which includes the U2, U4, and U6 matrices,

the matrices of the experimental OS and the energies of the corresponding

transitions) have been solved to minimize the difference between the calculated

fcal and observed fobs OS. The quality of fitting is determined by the root mean

squared deviation (RMS) defined as [67], and presented in Table 3.2 for all the

three glasses. The deviations indicate reasonably good fitting between theory

and experiment, demonstrating the applicability of JO theory. The values of

Ωλ are found to be in the following order for the three glasses Ω2 > Ω6 > Ω4 and

found to be consistent with the some recently reported values [68, 69]. For

evaluating the JO parameters from the absorption spectra the baseline was

drawn in such a way to remove the background in all absorption spectra, we

mean to leave only those features corresponding to the 4f-4f transitions, which

are clearly seen on the top of the background. Then such a baseline was

subtracted from the spectrum, and the spectrum modified in this way was used

for calculations of the experimental OS.

(3.1) ( )

31

2

exp

−

−=∑

=

N

ff

RMS

N

i

i

calc

i

87

Table 3.2 The absorption band energies, the oscillator strength for the transitions of Nd3+ ion PbO-Al2O3–SiO2 glasses. All transitions are from the ground state 4I9/2

Transition

AN5 AN8 AN10

Barycenter (cm-1)

fexp (×10-6)

fcal

(×10-6) Barycenter (cm-1)

fexp (×10-6)

fcal

(×10-6) Barycenter (cm-1)

fexp (×10-6)

fcal

(×10-6)

4I9/2 → 4F3/2 11358 3.2433 2.5644 11363 4.4655 2.9729 11358 1.4220 1.1288 4F5/2 12381 13.6470 11.7574 12336 15.5183 13.2551 12402 7.9474 7.0369 4F7/2 13403 11.3087 12.3779 13398 12.6297 13.9120 13399 7.4073 7.9067 4F9/2 14622 0.8325 1.6259 14620 1.1444 1.8298 14642 0.6452 1.0328

2H11/2 15735 0.55 0.3541 15686 0.4583 0.3979 15954 0.1115 0.2243 4G5/2 17113 34.7772 34.6050 17121 40.3518 40.1105 17120 24.8989 24.8292 4G7/2 18911 3.7539 5.9892 18912 3.7869 6.9099 18899 2.6207 3.5077 4G9/2 19575 1.1836 2.3194 19624 0.9381 2.6592 19552 0.6464 1.2909

2D3/2, 4G11/2

,

4G9/2 21091 1.6681 1.4947 21210 2.3500 1.7193 20991 0.5374 0.7828

2P1/2, 2D5/2 23141 1.0995 0.5657 23210 0.7121 0.6700 23213 0.4066 0.1776

rms deviation ±1.3349 ±1.7984 ±0.6150

JO parameters Ω2 Ω4 Ω6 Ω2 Ω4 Ω6 Ω2 Ω4 Ω6

( x 1020, cm2) 13.94 3.37 13.58 12.16 2.81 12.08 9.36 0.56 8.42

88

According to the Judd–Ofelt theory, crystal field parameter that determines the

symmetry and distortion related to the structural change in the vicinity of Nd3+ ions. In

the present context, this may be understood as follows: the larger the degree of disorder

or depolymerization in the glass network, the larger is the average distance between Si–

O–Si, Si–O–Pb chains causing the average Nd–O distance to increase. Such increase in

the bond lengths produces weaker field around Nd3+ ions leading to a low value of Ω2 for

the glass mixed with 10.0 mol% of Al2O3 [68]. Additionally the variations in the

concentration of silicate groups with different number of non-bridging oxygens, as

discussed before, also play an important role in the variation of value of Ω2.

The luminescence spectra of Nd3+ doped glasses excited at 582 nm recorded at

room temperature in the spectral range 0.95 to 1.6 µm region are shown in Fig. 3.3; the

spectra exhibited bands due to 4F3/2→4I11/2 (at 1.06 µm) and 4F3/2→

4I13/2 (at 1.3 µm)

transitions. Further a gradual growth of these bands is clearly observed with increase in

the concentration of Al2O3. From these spectra, various radiative parameters viz.,

spontaneous emission probability, A, the total emission probability, AT, the radiative

lifetime τ, the fluorescent branching ratio β of different transitions originated from 4F3/2

level of Nd3+ ions have been evaluated and presented in Table 3.3.

The fluorescence decay curve of the 4F3/2 excited level of Nd3+ doped glasses is

shown in Fig. 3.4; the curves viewed to be double exponential with a smaller (~ 20%) and

a larger (~ 80%) fast decay component (τfd). The evaluated lifetimes with possible errors

89

are presented in Table 3.4. The lifetimes, both experimental and calculated, were found to

be in the increasing order with increase in the concentration of Al2O3.

Fig. 3.3 Photoluminescence spectra of PbO-Al2O3 -SiO2: Nd2O3 glasses in the NIR region excited at 582 nm.

950 1050 1150 1250 1350 1450 1550 1650

4F3/2→ 4I11/2

4F3/2→ 4I13/2

Wavelength(nm)

PL I

nten

sity

(ar

b. u

nits

)

AN5 AN10AN8

90

Table 3.3 Various radiative properties of transitions of Nd3+ ions in PbO–Al2O3–SiO2 glasses

Transition AN5 AN8 AN10

A (s-1) β (%) A (s-1) β (%) A (s-1) β (%)

4F3/2 → 4I9/2 1796 29.38 1634 27.29 1657 28.01

→4I11/2 3499 57.26 3538 59.09 3731 63.08

→4I13/2 774 12.67 768 12.83 500 8.45

→4I15/2 42 0.69 47 0.78 27 0.46

τ = 164 µs τ = 167 µs τ =169 µs

Table 3.4 Summary of the data on life measurements of 4F3/2 excited level

Glasses Life time Nd doped samples; λexcitation = 582 nm and λemission = 1064nm

Quantum efficiency

η%

measured (µs) Calculated (µs) τcal

χ2 (possible error in τexpt ) τfd τsd

AN5 67 (21%) 171 (79%) 164 1.6 72.5

AN8 69 (21%) 187 (79%) 167 1.5 76.6

AN10 73 (19%) 191 (81%) 169 1.6 78.1

The Infrared spectra of PbO Al2O3 SiO2: Nd2O3 glasses (Fig. 3.5) exhibited conventional

bands at about 1025 cm-1 due to Si-O-Si asymmetric vibrations, 760 cm-1 due to Si-O-Si

symmetric vibrations and at about 450 cm-1 due to Si-O-Si rocking motion [62, 70].

Incidentally, the vibrational frequency of Al O stretching in AlO4 structural units is

found to exhibit band at about 750 cm-1 and band due to AlO6 structural units lie

91

Fig.3.4 Decay curves of Nd3+ (4F3/2→

4I11/2) doped PbO−Al2O3−SiO2 glasses recorded at room temperature excited at 582 nm.

at about 450 cm-1 [71]. Hence, it is quite likely that tetrahedral Al ion to cross link

with the neighbouring Si-O-Si structural units and form Al-O-Si linkages. Hence the

band observed at 760 cm-1 may be considered as the band due to Si-O-Al linkages. The

band due to the vibrations of PbO4 structural units about 470 cm-1 is also present in these

glasses. With increase in the concentration of Al2O3, the intensity of the bands due to Si-

O-Si asymmetric vibrations and Si-O-Si rocking motion increases at the expense of band

due to Si-O-Si/ Si-O-Al symmetric vibrations. A visible increase in the intensity of the

band due to AlO6 structural units is also observed with the increase in the concentration

of Al2O3. Such features of the spectra indicate an increasing concentration of Al3+ ions

that act as modifiers with increasing content of Al2O3 in the glass matrix.

AN8

AN10

AN5

Inte

nsity

(cy

cles

/s)

103

104

Time (x106, µs)0.32 0.72 1.12 1.52

92

Fig. 3.5 IR spectra of PbO-Al2O3 -SiO2: Nd2O3 glasses.

400500600700800900100011001200

Si–O

–Si a

sym

met

ric

units

Si–O

–Si s

ymm

etri

c/ A

lO 4

uni

ts

Si–O

–Si r

ocki

ng

mot

ions

/AlO

6 un

its

Sym

met

rica

l ben

ding

vi

brat

ions

of

PbO 4

Tra

nsm

ittan

ce %

Wavenumber (cm-1)

AN5

AN10

AN8

93

The Raman spectra of PbO–Al2O3–SiO2:Nd2O3 glasses (Fig.3.6) exhibited bands

at 1050 cm-1 and at 815 cm-1 due to Si–O–Si asymmetric modes and symmetric vibrations

of SiO4 tetrahedra respectively. In the region of Si-O-Si symmetric modes, Al–O

stretching vibrations of Al tetrahedral coordination are also possible. The spectra also

exhibited a band at 550 cm-1 attributed to the bridging oxygen breathing mode in (Si, Al)

three-membered rings and another band at 475 cm-1 due to Si-O-Si rocking vibrations

[72]. The spectra also exhibited a weak envelope at about 330 cm-1 due to the vibrations

of PbO4 units [73]. As the concentration of Al2O3 increases a noticeable enhancement in

the intensity of the bands due to Si–O–Si asymmetric modes and Si-O-Si rocking

vibrations of SiO4 tetrahedra is observed at the expense of the band due to symmetric

vibrations of SiO4. Such changes clearly indicate an increasing disorder in the glass

network with increasing content of Al2O3. Thus both IR and Raman spectra of the studied

glasses suggests that there is an increase in the concentration of Al3+ ions that act as

modifiers (which may de-cluster Nd3+ ions) with increase in the concentration of Al2O3 in

the glass network.

94

Fig. 3.6 Raman spectra of PbO-Al2O3 -SiO2: Nd2O3 glasses.

Neodymium oxide normally exhibits immiscible tendency in silica glass and as a

result it forms aggregates or clusters in which cross relaxation can give rise to non-

radiative de-excitation of neodymium, resulting in very short lifetimes. Comparatively

weak fluorescence intensity and short emission lifetime observed for glass AN5 indicates

possible formation of more concentration of Nd3+ clusters that are responsible for

luminescence quenching in this glass matrix [9]. The observed increase of PL peak

intensity with increase in the Al2O3 concentration clearly suggests that Al increases PL

200 400 600 800 1000 1200Wavenumber (cm-1)

Si-O

-Si r

ocki

ng

grou

ps/A

lO6

units

(Si,

Al)

- T

hree

mem

bere

d ri

ngs

Si-O

-Si b

endi

ng

grou

ps/A

lO 4

units

Ram

an in

tens

ity (

a.u.

)

AN5

AN8

Si-O

-Si

stre

tchi

ng

grou

ps

PbO

4 te

trag

onal

pyr

amid

s

AN10

95

output indicating the possibility of admixing of wavefunctions of opposite parities which

lead to increase the radiative probabilities of all the f-f transitions [74]. Among the two

life time components observed from the decay curves, the value of τfd is found to be

virtually invariant while τsd is found to increase significantly with increase in the

concentration of Al2O3.The fast decay component is obviously due to the emission from

clusterized Nd3+ ions which cross relax (concentration quenching) and give rise to non-

radiative de-excitation of neodymium ions. The slow decay component can be attributed

to the emission from Nd3+ ions that are uniformly dispersed in the silica matrix; the

increasing value of this component with increase in the concentration of Al2O3 indicates

aluminum ions gradually disperse Nd3+ ions uniformly in the glass matrix and reduces

fluorescence quenching due to cross-relaxation. Branching ratio ‘β’ (that defines the

luminescence efficiency of the transition) of the orange emission due to 4F3/2→4I11/2

transition, among various transitions originated from 4F3/2, is found to be the highest

(~63%) for the glass AN10 and it is ∼ 57% for the glass AN5.

The quantum efficiency (η = τexp/τcal), defined as the radiative portion of the total

relaxation rate of a given energy level is found to be increasing (Table 3.3) with increase

in the concentration of Al2O3 (τexp is taken as an average of two components). As has

been discussed earlier, the degree of dispersion of Nd3+ ions is higher in the glass

containing higher concentration of Al2O3 which lead to high non–radiative losses causing

higher value of η.

96

3.4 Conclusions

Lead alumino silicate glasses of the composition (40-x) PbO–(5+x) Al2O3–

54SiO2: 1.0 Nd2O3 (in mol%) with three values of x ranging from 0 to 5.0 were prepared.

IR, Raman, optical absorption, photoluminescence and luminescence decay have been

studied.

(i) The IR and Raman spectral studies have indicated that there is a gradual

depolymerization of the glass network with increase in the concentration of

Al2O3.

(ii) The optical absorption spectra of these glasses exhibited bands due to

4I9/2 → 2P1/2, 2D3/2+

4G11/2, 4G9/2+

4G7/2, 4G5/2,

2H11/2, 4F9/2,

4F7/2, 4F5/2 and

4F3/2

transitions. The spectra have been characterized by JO theory and a reasonable

matching between experimental and theoretical oscillator strengths could be

achieved.

(iii) The luminescence spectra of these glasses excited at 582 nm exhibited two

prominent transitions viz., 4F3/2→4I11/2,13/2 in the spectral region 950 to 1600

nm. An increase in the PL output of these transitions is observed with increase

in the concentration of Al2O3.

(iv) The quantitative analysis of these results has revealed that the Al3+ ions cause

declusterization of Nd3+ ions and thereby decrease the luminescence quenching

due to cross relaxation.

97

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