Optical absorption and photoluminescence characteristics of Sm3+ ions in

lead silicate glasses mixed with different concentrations of Al2O3 (5 to 10 mol%)

have been investigated. From these studies, the radiative properties viz.,

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

lifetime R, the fluorescent branching ratio of emission transition of 4G5/2 → 6H7/2 along with other transitions for Sm3+ have been evaluated and found to be the

highest for the glass mixed with 8.0 mol% of Al2O3.The IR spectral studies have

indicated that Al3+ ions do participate in the glass network with AlO4 and AlO6

structural units and further revealed that the concentration of octahedral

aluminium ions induce bonding defects in the glass network. Such bonding

defects are assumed to be responsible for low phonon losses in these glasses and

lead to higher values of radiative parameters for the glass mixed with 8.0 mol% of

Al2O3.

Chapter 5

The structural influence of aluminium ions on emission characteristics of Sm3+ ions in lead aluminium silicate glass system

The structural influence of aluminium ions on emissioncharacteristics of Sm3+ ions in lead

aluminium silicate glass system

5.1 Introduction Samarium ion exist in Sm3+ and Sm2+ states but between these two

states, Sm3+ is found to be more stable. The electronic configuration of ion is

4f5 with 6H5/2 ground state. Earlier it was shown that the oscillator strengths of

Sm3+ ions may be arranged in two groups, one referring to transitions upto

10,700 cm-1 and the second to transitions in the range 17,600-32,800 cm-1 and

the Judd-Ofelt parameters can be calculated separately for these two regions

[1]. Such separation was attributed to the splitting of fN configuration being

smaller than the f-d energy gap [1]. In such a case it is incorrect to use the

oscillator strengths of transitions which are about 10,000 cm-1 for calculations

of parameters by means of the Judd-Ofelt theory. The transitions

6H5/2→4F3/2, 4F3/2 of Sm3+ occurring in the absorption spectrum in the near

infrared region is hypersensitive [2, 3]. In the emission spectra of Sm3+ ion, the

transitions, 4G5/2→6F9/2 and 4G5/2→6H9/2 occurring in the near infrared and

visible regions, respectively are also identified as hypersensitive [4].

Sm3+ doped laser materials are of interest in lasers for next generation

nuclear fusion [5]. These materials can be used as a gain media in the microchip

laser at high doping levels [6] since this rare earth ion has a very simple energy

172

level scheme with desirable properties for a laser system. The Sm3+ doped laser

materials have several advantages (compared to other rare-earth ions doped

laser materials) such as weak concentration quenching effect, no excited state

absorption, no upconversion losses, low quantum defect and thermal load since

there is lack of high lying excited states. It was reported that Sm3+ gives out

high luminescence output in NIR region and exhibit lifetime (of the order of

ms) when compared with those of other rare earth ions of the same host

medium. Considerable recent studies are available on NIR luminescence

emission of Sm3+ ion in a variety of glass hosts [7–9].

The objective of this chapter is to synthesize PbO−Al2O3−SiO2: Sm2O3

glasses with different contents of Al2O3, and to investigate the influence of

variation in the concentration of Al2O3 on the fluorescence characteristics of

Sm3+ ions in the glasses, in particularly their fluorescence time decays, which

are important for the optical triggers and laser gain media. The study is further

intended to evaluate the probabilities of principal luminescence transition of

Sm3+ ions, determining their intensities in these glass matrices. The IR spectral

studies have also been carried out to have some pre-assessment over the internal

structural arrangement of the glass network which may play an important role

on luminescence efficiencies of the rare earth ions.

173

5.2 A brief review on the spectroscopic studies of Sm3+ ions in various glass systems

Yaru et al [10] have prepared the Sm3+ doped B2O3Al2O3SiO2 glasses

by high temperature solid-state method. They have concluded that network

units, namely, [SiO4], [BO3], [BO4], [AlO4] existed in Sm3+ doped

B2O3Al2O3SiO2 glasses, and Sm3+ existed as network modifier. Mohan Babu

et al [11] have reported the effect of concentration on photoluminescence

properties of Sm3+ ions doped lead tungstate tellurite glasses by using the

absorption, emission and decay measurements. They have concluded that these

glasses could be useful for photonic devices like visible lasers, fluorescent

display devices and optical amplifiers. Osvet et al [12] have studied the

influence of different metallic reducing agents on the reduction of Sm3+ to Sm2+

in borate glasses as well as the influence of phase separation in borosilicate

glasses on the spectroscopic properties of Sm2+ ions. Zhang et al [13] have

reported the luminescence properties of Sm3+ doped Bi2ZnB2O7 glasses.

Tarafder et al [14] studied the enhanced photoluminescence properties of Sm3+-

doped ZnO–Al2O3–B2O3–SiO2 glass derived willemite glass–ceramic

nanocomposites. Fu et al [15] have reported the Sm3+-doped lithium–yttrium–

aluminum-silicate (LYAS) glasses. These investigations on multi-channel

radiative transition emissions of Sm3+ in LYAS glasses provide a new clue to

develop tunable lasers, compact light sources and optoelectronic devices. Abbas

174

et al [16] have reported the optical absorption of some gamma-irradiated

lithium-boro-silicate glasses doped with some rare-earth metal oxides. Li and

Su [17] have studied the effect of samarium on Mn activated zinc borosilicate

storage glasses. Pal et al [18] have studied the Investigation of spectroscopic

properties, structure and luminescence spectra of Sm3+ doped zinc bismuth

silicate glasses. Tian et al [19] have reported that the X-ray photoelectron

spectroscopy of Sm3+-doped CaO–MgO–Al2O3–SiO2 glasses and glass

ceramics. Barros et al [20] have studied the effect of Er3+ and Sm3+ on phase

separation and crystallization in Na2O/K2O/BaF2/BaO/Al2O3/SiO2 glasses.

Huang et al [21] have reported the spectroscopic properties of Sm3+-ions in

tellurite, fluorophosphate, and fluorine-modified silicate glasses for designing

the visible and UV laser hosts. Nogami et al [22] studied the change in Sm3+

ions doped in silicate glasses consisting of Al2O3 and TiO2 by using

Femtosecond laser pulses. Rao et al [23] studied the optical and structural

investigation of Sm3+–Nd3+ co-doped in magnesium lead borosilicate glasses.

Malchukova et al [24] reported the luminescence of pristine and γ-irradiated

Sm3+ doped borosilicate glasses as a function of Sm content. Gandhi et al [25]

studied the influence of tungsten on the emission features of Nd3+, Sm3+ and

Eu3+ ions in ZnF2–WO3–TeO2 glasses with help of optical absorption,

fluorescence spectra and fluorescence decay. Jimenez et al [26] reported the

175

spectroscopic properties of trivalent samarium ions in a melt-quenched

aluminophosphate glass containing silver and tin. Venkateswarlu et al [27]

studied the effect of mixed alkalis on the optical absorption and emission

spectra of Sm3+ and Dy3+ doped chloroborate glasses. Lakshminarayana et al

[28] reported the photoluminescence properties of Sm3+, Dy3+, and Tm3+-doped

transparent oxyfluoride silicate glass ceramics containing CaF2 nanocrystals.

Sharma et al [29] studied the spectroscopic investigations and luminescence

spectra of Sm3+ doped soda lime silicate glasses. Their studies indicated that

4G5/2→6H7/2 and 4G5/2→6H9/2 transitions are responsible for orange

luminescence and the material might be useful in the development of materials

for LED's and other optical devices in the visible region. Seshadri et al [30]

have reported the spectroscopic and laser properties of Sm3+ doped different

phosphate glasses. Lakshminarayana and Qiu [31] have reported the

photoluminescence of Pr3+, Sm3+ and Dy3+: SiO2–Al2O3–LiF–GdF3 glass

ceramics and Sm3+, Dy3+: GeO2–B2O3–ZnO–LaF3 glasses. Lakshminarayana et

al [32] studied the spectral analysis of RE3+ (RE = Sm, Dy, and Tm): P2O5–

Al2O3–Na2O glasses. Raj and de Araujo [33] have reported the fluorescence

intensity ratio technique for Sm3+ doped calibo glass as a temperature sensor.

Som and Karmakar [34] have studied the infrared-to-red upconversion

luminescence in samarium-doped antimony glasses. Huang et al [35] reported

176

the spectroscopic properties of Sm3+-doped oxide and fluoride glasses for

efficient visible lasers. Babu et al [36] reported the spectral investigations of

Sm3+ doped lead bismuth magnesium borophosphate glasses. Ravi et al [37]

studied the structural and optical studies of Sm3+ ions doped niobium

borotellurite glasses. Kesavulu and Jayasankar [38] have reported the

spectroscopic properties of Sm3+ ions in lead fluorophosphate glasses. Ha et al

[39] studied the optical properties of samarium-doped strontium orthosilicate

for near ultra-violet excitation.

5.3 Characterization

A particular composition (40-x) PbO–(5+x) Al2O3–54 SiO2: 1.0 Sm2O3

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

study; the samples are labeled as SA5 (x=0), SA8 (x=3), SA10 (x=5).

The detailed compositions are as follows:

SA5: 40 PbO–5Al2O3–54SiO2: 1.0 Sm2O3

SA8: 37 PbO–8Al2O3–54SiO2: 1.0 Sm2O3

SA10: 35 PbO–10Al2O3–54SiO2: 1.0 Sm2O3

The samples were prepared using the methods described in Chapter 2.

177

5.3.1. Physical parameters

From the measured values of density d and calculated average molecular

weight M , various physical parameters such as Samarium ion concentration Ni

and mean samarium ion separation ri of these glasses are evaluated using the

conventional formulae [40] and are presented in Table 5.1.

Table 5.1

Various physical parameters of PbO–Al2O3–SiO2 glasses doped with Sm2O3

Physical parameter ↓ sample → Pure SA5 SA8 SA10

Density (g/cm3) 4.9624 5.2822 5.0557 4.9014 Sm3+ ion conc. Ni (x1020 ions/cm3) --- 2.44 2.40 2.376

Interionic distance Ri (Ao) --- 16.00 16.08 16.14

Polaron radius Rp (Ao) --- 6.45 6.48 6.51

Field strength (x1015 cm-2) --- 0.722 0.714 0.709

Molar volume 25.68 24.67 25.06 25.35

Refractive index 1.735 1.772 1.75 1.741

Optical band gap (eV) --- 3.08 3.02 3.06

178

5.4 Results

5.4.1 Infrared transmission spectra

The summary of the data on the positions of various bands observed in

the IR spectra of PbO–Al2O3–SiO2: Sm2O3 glasses are presented in Table 5.2.

The spectra exhibited the bands due to conventional SiO4, AlO6 and AlO4

structural units as described in the earlier chapters. The spectra further indicated

that the intensity of the bands due to AlO6 units and also the asymmetrical

bands of SiO4 units is the highest for the glass mixed with 8.0 mol% of Al2O3.

Table 5.2 Summary of data on band positions (cm-1) in IR spectra of PbO–Al2O3–SiO2: Sm2O3glasses

Glass Si-O-Si asymmetric

Si-O-Si symmetric/ AlO4 units

Si-O-Si rocking/

AlO6 units

Symmetric bending vibrations

of PbO4 SA5 997 795.06 445 490 SA8 987 804.2 435 498 SA10 990 798.4 440 495

179

5.4.2 Optical absorption spectra

The optical absorption spectra of PbO−Al2O3−SiO2: Sm2O3 glasses (Figs. 5.2(a)

and 5.2(b)) measured at room temperature in the spectral wavelength range

300-1600 nm with spectral resolution of 0.1 nm, have exhibited absorption

bands corresponding to the following transitions [7]:

6H5/2→4I13/2+4I11/2+4M17/2 (UV–visible region),

6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/2, 6F1/2, 6H15/2 (NIR region)

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

same; however, the absorption strength under the given peak is found to vary

with the change in aluminium oxide concentration. It may be noted here that

though the Sm3+ ions are expected to give several bands in the UV- region due

to the higher cut-off edge of the glass host those bands could not be detected.

From the observed absorption edges, we have evaluated the optical band

gaps (Eo) of these glasses by drawing Tauc plot between ( ) 1/2 and

Fig. 5.3 represents the Tauc plots of all these glasses in which a considerable

part of each curve is observed to be linear. From the extrapolation of the linear

portion of these curves, the values of optical band gap (Eo) obtained for

PbO−Al2O3−SiO2: Sm2O3 glasses and are presented in Table 5.1; the optical

band gap is found to be the lowest for the glass SA8. Conventional Judd–Ofelt

(J–O) theory [41, 42] has been used to perform analysis of the experimental

180

absorption spectra. The reduced matrix elements ||U|| of the unit tensor

operators needed for calculations have been re-calculated, using newer

literature data on the Sm3+. Hamiltonian parameters taken from Ref. [43]; the

experimental and calculated oscillator strengths are shown in Table 5.3. The

quality of fitting is determined by the root mean squared deviation (RMS) and

presented in Table 5.3. The value of rms f is in the typical error range of the

J–O fitting and indicates good agreement between the experimental and

calculated results.

181

Fig. 5.1 IR spectra of PbO–Al2O3–SiO2: Sm2O3 glasses.

400500600700800900100011001200

Si–O

–Si a

sym

met

ric u

nits

Si–O

–Si s

ymm

etric

/ AlO

4 u

nits

Si–O

–Si r

ocki

ng

mot

ions

/AlO

6 uni

ts

Sym

met

rical

ben

ding

vi

brat

ions

of P

bO4

Tran

smitt

ance

%

Fig. 1 IR spectra of PbO-Al O -SiO : Sm O glasses. Wavenumber (cm-1)

SA5

SA10

SA8

182

Fig. 5.2(a) Optical absorption spectra of PbO–Al2O3–SiO2: Sm2O3 glasses recorded at room temperature in UV-visible region. All the transitions are from the ground state 6H5/2.

350 400 450 500 550 600 650 700 750 800

420 470 520

Wavelength (nm)

4I13/2+4I11/2+

4M17/2

SA8

SA5

SA10

Abs

orpt

ion

(arb

. uni

ts)

Wavelength (nm)

SA8

SA5

SA10

183

Fig. 5.2(b) Optical absorption spectra of PbO–Al2O3–SiO2: Sm2O3 glasses recorded at room temperature in NIR region. All the transitions are from the ground state 6H5/2.

850 950 1050 1150 1250 1350 1450 1550

6F11/2

6F9/2

6F7/2

6F5/2

6F3/2

6H15/2

6F1/2

SA5

SA10

SA8

Abs

orpt

ion

(arb

. uni

ts)

Wavelength (nm)

184

The Judd-Ofelt parameters 2, 4 and 6 were computed by the least

square fitting analysis of the experimental oscillator strengths using matrix

elements [44] and are presented in Table 5.4 along with the other pertinent data.

Fig. 5.3 Tauc plot of PbO–Al2O3–SiO2: Sm2O3 glasses and inset shows the variation of optical band gap with concentration of Al2O3.

0.0

2.0

4.0

6.0

8.0

10.0

2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3

( h

)1/2 (c

m-1

eV

)1/2

SA10

SA8

SA5

185

Table 5.3 The absorption band energies, oscillator strengths of various transitions of

Sm3+ ion in PbO−Al2O3−SiO2 glasses

The values ofshow the following order for all the three glasses: 2 > 4

> 6. The order of parameters of Sm3+ ion in the studied glasses is well with

the trends available in various other glass matrices [45–49].

Table 5.4 J-O parameters of Sm3+ ion doped PbO−Al2O3−SiO2 glasses

Transition From 6H5/2

SA5 SA8 SA10 Energy

(cm-1) fexp

(×10-6) fcal

(×10-6) Energy (cm-1)

fexp (×10-6)

fcal (×10-6)

Energy (cm-1)

fexp (×10-6)

fcal (×10-6)

4I13/2+4I11/2+4

M17/2 21109 1.902 1.259 21123 1.857 1.253 21119 1.962 1.2328

6F11/2 10590 0.490 0.743 10594 0.518 0.739 10584 0.476 0.7257

6F9/2 9248 4.101 4.442 9254 4.077 4.421 9257 3.983 4.3507

6F7/2 8106 6.034 5.834 8106 5.999 5.798 8096 5.906 5.7119

6F5/2+6F3/2 7276 2.755 3.559 7284 2.585 3.497 7287 2.579 3.5326

6H15/2+6F1/2 6683 1.381 0.584 6690 1.451 0.548 6692 1.529 0.5834

r.m.s. deviation 0.6389 0.6805 0.7219

Glass (cm-2)

(cm-2)

(cm-2)

SA5 4.495 3.286 1.405

SA8 4.482 3.314 1.333

SA10 4.539 3.336 1.438

186

5.4.3 Photoluminescence spectra

The emission spectra (Fig. 5.4) recorded at room temperature of

PbO−Al2O3−SiO2:Sm2O3 glasses (excited at 470 nm) have exhibited the

following bands:

4G5/2 → 6H5/2, 6H7/2, 6H9/2

The spectral intensities of these bands are found to vary with the content of

Al2O3 in the samples and are observed to be the highest for the glass containing

8.0 mol% of Al2O3.

The energy level diagram containing the observed transitions of Sm3+

ion in the glass SA8 is shown in Fig. 5.5. The values of the radiative properties

viz., spontaneous emission probability A, the total emission probability AT

involving all the intermediate terms of fluorescent transitions, the radiative

lifetime R, the fluorescent branching ratio of various emission transitions for

Sm3+ have been evaluated in all the three glass matrices and are presented in

Table 5.5.

It was well established that an emission level with value nearly equal

to 50% is a potential laser emission [50]. Among various transitions, the

transition 4G5/2→6H7/2 found to have the highest values of for all the three

glasses; this transition may therefore be considered as a possible laser

transitions. Further, the comparison of values of these transitions for the

187

glasses mixed with different concentrations of Al2O3 shows the largest value for

SA8 glass.

Fig. 5.4 Photoluminescence spectra of PbO−Al2O3−SiO2:Sm2O3 glasses recorded at room temperature with exc= 470 nm.

540 590 640

4G5/2 → 6H5/2

4G5/2 → 6H7/2

4G5/2 → 6H9/2

Inte

nsity

(arb

.uun

its)

Wavelength (nm)

SA8

SA5

SA10

188

Table 5.5

Various radiative properties of Sm3+ doped PbO−Al2O3−SiO2 glasses.

Transition Aed (s-1) %

SA5 4G5/2 → 6H9/2 7.1 2.8 4G5/2 → 6H7/2 163.7 64.0 4G5/2 → 6H5/2 85.1 33.2 R 3.91 ms AT = 255.9 s-1

SA8 4G5/2 → 6H9/2 6.5 2.6 4G5/2 → 6H7/2 165.0 65.5 4G5/2 → 6H5/2 80.2 31.9 R 3.97 ms AT = 251.7 s-1

SA10 4G5/2 → 6H9/2 7.2 2.8 4G5/2 → 6H7/2 165.0 63.8 4G5/2 → 6H5/2 86.4 33.4 R 3.87 ms AT = 258.6 s-1

189

Fig. 5.5 Absorption and emission transitions of Sm3+ ions in lead silicate glasses mixed with 8.0 mol% of Al2O3.

190

5.4.4 Fluorescence decay curves

The fluorescence decay curves of the emission lines corresponding to

4G5/2 level of Sm3+ ion in PbO−SiO2 glasses mixed with different concentrations

of Al2O3 are recorded at room temperature and the log of intensity dependence

with the decay time are shown in Fig. 5.6. From these curves the radiative

lifetimes for all the three glasses have been evaluated and furnished in the Table

5.6.

Table 5.6 Comparison of radiative lifetimes and quantum efficiencies of 4G5/2 → 6H7/2 transition of sm3+ ions in PbO−Al2O3−SiO2 glasses

Glass Radiative lifetime, ( 0.01) (ms)

Quantum efficiency () Calculated Measured m

SA5 3.91 1.95 49.2

SA8 3.97 2.48 63.3

SA10 3.87 1.56 40.4

191

Fig. 5.6 The fluorescence decay curve of the 4G5/2 level for PbO−Al2O3−SiO2: Sm2O3 glasses.

0.0 2.0 4.0 6.0 8.0 10.0

SA5

SA10

SA8

102

103

104

Inte

nsity

192

5.5 Discussion

As has been discussed in the earlier chapters in the IR spectra of glass

SA5, the band due to AlO4 structural units merged with band due to tetrahedral

silicate groups and exhibited remarkable intensity. As the concentration of

Al2O3 is raised from 5.0 to 8.0 mol%, the intensity of this band is observed to

decrease. The band characteristic for the vibrations of Al–O bonds (aluminium

in octahedral coordination) is found to be more intense in the spectrum of the

glass mixed with 8.0 mol% of Al2O3. The intensity of the bands due to Si–O–Si

asymmetric vibrations, Si-O-Si rocking motion is also observed to be the

highest while that of the band due to Si-O-Si symmetric stretching vibrations

and band due to PbO4 structural groups is the lowest for the same glass. Thus

the analysis of IR spectra points out that as the concentration of Al2O3 is raised

in the glass matrix from 5.0 to 8.0 mol%, there is an increasing degree of

disorder in the glass network due to enhancement in the octahedral occupancy

of aluminium ions as is also observed in the other two series of the glasses.

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

determines the symmetry and distortion related to the structural change in the

vicinity of Sm3+ 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

193

average Sm–O distance to increase. Such increase in the bond lengths produces

weaker field around Sm3+ ions leading to a low value of 2 for the glass mixed

with 8.0 mol% of Al2O3 as observed. Additionally the variations in the

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

and also changes of the higher order electrostatic ligand fields as discussed

above also play an important role in the variation of value of 2.

The luminescence spectra of Sm3+ are similar to those reported for a

number of other glass systems [6–9]. The high intensity or high quantum yield

of the luminescence bands of Sm3+ ion in the glasses mixed with 8.0 mol% of

Al2O3 indicates that there is a minor cross relaxation i.e., the transfer of energy

from the excited state of Sm–ion by electric multipole interaction (more

precisely dipole-dipole or dipole–quadrapole interactions) to neighboring Sm–

ion lying in the ground state is low for this particular glass when compared with

other two glasses. The comparison of branching ratio values of yellow

emission (viz., 4G5/2 → 6H7/2), for the three glasses suggests the highest value (~

65%) for the glass mixed with 8.0 mol% of Al2O3. Hence it can be concluded

that around 8.0 mol % of Al2O3 is optimal concentration in the lead silicate

glass matrix to produce high luminescence efficiency. Further, it may be worth

mentioning here to note that the upper occupied quasi-valence states are

prevailingly originated from the 2p O which effectively interacts with the s- and

194

d-localized states; such charge transfer plays principal role on the local

electrostatic field of f-delocalized states of Sm3+ and thereby influence the

luminescence efficiency.

The fluorescence decay curve of the 4G5/2 level for all the three glasses

observed to be single exponential (Fig. 5.6). The fluorescence lifetime τ,

evaluated from these graphs is apparently shorter than calculated lifetimes from

the J–O theory (Table 5.6) suggesting multi-phonon relaxations. Additionally

electron-phonon anharmonicities may also play an important role for such

difference of life times.

The higher value of lifetime for the glasses SA8 advocates a low phonon

loses or higher concentration of dangling Si–O–Si bonds with more non–

bridging oxygens. The quantum efficiency () is defined as the radiative

portion of the total relaxation rate of a given energy level:

radnrrad

rad

WAA

exp

(5.1)

where Arad is the total radiative relaxation rate, Wnr is the rate of total

non–radiative transition exp –experimental lifetime and rad –radiative lifetime.

The value of for the 4G5/2 level) determined for the three glasses is presented

in Table 6. For the glass SA8, the value of is found to be 63.3%, whereas for

SA5 and SA10 glasses it is found to be 49.2% and 40.4%, respectively. In

195

general, for any common glass host, the quantum efficiency for Sm3+ ions

emission in the visible region is expected to be higher when compared with that

of other rare earth ions. This is because, the energy gap between 4G5/2 and the

first underlying level 6F11/2 is ~7000 cm-1; the maximum phonon energy in the

silicate glasses is ~1100 cm-1. The comparison of the two energies shows the

non-radiative decay probability is low and hence higher values of are

expected as is also observed for these glasses. The comparison of the value of

for the three studied glass systems here indicated the higher value for SA8 glass;

as has been discussed earlier, the degree of disorder (ruffling of Si–O–Si

linkages) in SA8 glasses is higher when compared with other two glasses. Such

higher concentration of ruffled Si–O–Si linkages causes the presence of low

energy phonons in these glasses, where as comparatively high energy phonons

are expected in the other two glasses since the structural disorder is

comparatively less in these two glasses. Hence, there is an electron–phonon

coupling of the Sm3+ ion with the high–energy phonons in the glasses mixed

with 5 and 10 mol% of Al2O3 lead to high non–radiative losses; such coupling

in between electron and low–energy phonons and hence minimal non-radiative

losses are expected in the glass SA8 leading to higher value of

196

5.6 Conclusions

PbO−Al2O3−SiO2: Sm2O3 glasses mixed with different contents of Al2O3

are synthesized. The IR spectral studies on these glasses have indicated that

Al3+ ions do participate in the glass network with AlO4 and AlO6 structural units

and further revealed that the concentration of octahedral aluminium ions induce

bonding defects in the glass network. From the evaluated optical band gaps

using Tauc’s plots it is concluded that the concentration of such bonding defects

is more in the glass SA8. The optical absorption of these glasses exhibited

several absorption transitions of Sm3+ ions in the visible and IR regions

originating from 6H5/2 ground state. Photoluminescence spectra (excited at 470

nm) of Sm3+ ions have exhibited the three intense bands (4G5/2 → 6H5/2, 6H7/2,

6H9/2) in the visible region. The spectral intensities of these bands are found to

vary with the content of Al2O3 in the samples and are observed to be the highest

for the glass containing 8.0 mol% of Al2O3. From these spectra, the emission

probabilities and also fluorescence lifetime of the principal transition viz., 4G5/2

→ 6H7/2 of Sm3+ ions have been evaluated. The analysis of results of these

studies has indicated that there is a less radiative trapping and enhanced

fluorescence lifetime and high quantum efficiency in the glasses mixed with 8.0

mol% of Al2O3. The reasons for such changes have been analyzed

quantitatively in the light of variations of structural units of aluminium ions in

the vicinity of Sm3+ ions glass network. Finally, it is felt that around 8.0 mol%

of Al2O3 is optimal concentration for getting high luminescence efficiency of

Sm3+ ions in lead silicate glasses.

197

References

[1] R.Reisfeld, Structure and Bonding 22 (Springer- verlag, New York, 1975) 123.

[2] D.E. Henrie, R.L. Fellows and G.R. Choppin, Coord. Chem. Rev. 18 (1976) 199.

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