Spectroscopic and glass transition investigations on Nd3+-doped NaF–Na2O–B2O3 glasses

Materials Research Bulletin 39 (2004) 1507–1515

Spectroscopic and glass transition investigations on Nd3þ-doped NaF–Na2O–B2O3 glasses B. Karthikeyana, S. Mohanb,* a

Raman School of Physics, Pondicherry University, Pondicherry, India, 605014 Department of Material Science, Faculty of Applied Sciences, Asian Institute of Medicine, Science and Technology, 2 Persiaran Cempaka, Amanjaya, 08000 Sungai Petani, Kedah Darul Aman, Malaysia

b

Received 1 March 2003; received in revised form 22 March 2004; accepted 15 April 2004

Abstract New developments in photonic technology need new materials for various applications. In the present report, Nd3þ-doped NaF–Na2O–B2O3 glasses were prepared and the spectroscopic and glass transition properties were analysed. The Fourier transform infrared spectral studies reveal that the glass contains BO3 and BO4 units as the local structures and the Naþ ions as the network modifiers. The absorption studies were carried out by using Judd–Ofelt theory, the experimental and theoretical oscillator strengths were also calculated. The emission spectral study was done for the 1 mol% Nd-doped glass and the spontaneous emission probability and stimulated emission cross-sections for the 4 F3=2 ! 4 I9=2 , 4 I11=2 transitions were calculated using the J–O parameters. # 2004 Elsevier Ltd. All rights reserved. Keywords: A. Optical materials; C. Infrared spectroscopy; D. Luminescence

1. Introduction Fluoride glasses are the most important materials for the optical fiber technology [1–4] due to their improved infrared transmission characteristics. But these glasses are marginally stable and the glass forming composition space is quite narrow. The alkali haloborate glasses [5–7] are also becoming most important due to their fast ion conducting nature. The alkalifluoroborate glasses are well known for their applications in phosphors, solar energy converters and in a number of electronic devices. Shelby and Ortolano [8] prepared of NaF–Na2O–B2O3 glasses and studied the refractive index, density and glass transition temperatures of all glasses. In the present work, owing to the importance of rare-earth-doped fluoride glasses in optical communications [9–15], we have prepared and investigated the structural, glass transition and optical properties of Nd3þ-doped NaF–Na2O–B2O3 glasses of different compositions. *

Corresponding author. E-mail address: [email protected] (S. Mohan).

0025-5408/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2004.04.025

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Table 1 Composition of Nd3þ-doped NaF–Na2O–B2O3 glasses Serial number

Sample code

Composition

1 2 3

OFA0 OFA1 OFA2

25NaF–25Na2CO3–50B2O3 25NaF–25Na2CO3–49B2O3–1Nd2O3 25NaF–25Na2CO3–48B2O3–2Nd2O3

2. Experimental details Nd3þ-doped NaF–Na2O–B2O3 glasses were prepared by using high purity analar grade H3BO3, NaF, Na2CO3 and Nd2O3 in the composition range 25NaF/25Na2CO3/(50  x)B2O3/xNd2O3, where x ¼ 0, 1 and 2. The sample codes and the corresponding sample compositions are presented in Table 1. The appropriate quantities of chemicals are weighed accurately and ground in a mortar to produce 20 g each of glass mixture. The stoichiometric compositions was taken in an open silica crucible and kept in a electric muffle furnace for heat treatment. Initially, the samples were heated slowly and maintained at 430 8C for 2.5 h for the decarbonisation of sodiumcarbonate and decomposition of boric acid. Then the temperature is raised up to 1000 8C and maintained for half an hour. The crucibles were shaken frequently for the homogeneous mixing of all the constituents. Then the melt was quenched at room temperature in air by pouring between two stainless steel plates. The quenched glasses were lilac in colour. The glasses were cut into proper shape and polished for further characterisation. The amorphous nature of the samples were confirmed by XRD studies in Rigaku Miniflux table top ˚ at the scanning rate of 28 min1 and 2y varies spectrometer with Cu-Ka line of wavelength l ¼ 1:5418 A from 10 to 808. The refractive index of all the prepared glasses were measured by Abbe’s refractometer by using sodium vapour lamp. The density measurements of all glasses were carried out by using the Archimedes’s principle. The measurements were taken with Dhona single pan balance and xylene as an inert immersion liquid. The density is obtained from the relation d (gm cm3 Þ ¼ ða=ða  bÞÞx (density of the xylene), where ‘a’ is the weight of the glass sample in air, ‘b’ is the weight of the glass sample when immersed in xylene and the density of the xylene is 0.86 gm cm3. The thermal studies were carried out using Mettler Toledo differential scanning calorimeter (DSC) in the temperature range of 50–500 8C with Table 2 Physical properties of 1 mol% Nd3þ-doped OFA1 glass Serial number

Physical properties

1 2 3 4 5 6 7 8 9 10 11


(g) Average molecular weight M Density (g cm3) Number of RE ions (1022 ions cm3) ˚) Polaron radius (rp) (A ˚) Inter-ionic distance (ri) (A Field strength (1016 cm2) Refractive index (nd) Dielectric constant Electronic polarizability (1024 cm3) Glass molar refractivity (cm3) Reflection loss (R) (%)

74.48 2.631 2.127 1.454 3.608 1.418 1.511 2.28 1.310 8.478 4.128

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heating rate of 10 8C min1 in an inert nitrogen gas atmosphere. The local structure of Nd3þ-doped NaF– Na2O–B2O3 glasses were determined by vibrational spectroscopy. The FTIR spectra of all the samples were recorded by using Shimadzu FTIR-8700 spectrometer in the wave number range 400–4000 cm1 using KBr pellet technique. The optical spectra were recorded in Shimadzu UV 1600 spectrophotometer in the wavelength range 400–950 nm. The photoluminescence emission spectrum was also recorded by using Arþ laser as a excitation source and adopted a similar experimental set up reported in the literature [16]. The physical properties [17] of 1 mol% Nd3þ-doped glass are presented in Table 2.

3. Results and discussion 3.1. XRD and thermal analysis

Heat Flow

The XRD pattern of all the glasses show no sharp peaks, indicating the absence of crystalline nature and further confirms the amorphous nature of glasses. The DSC thermogram shows that doping of 1 mol% Ndþ3 in the host does not make significant change in the glass transition temperature (Tg). The DSC thermogram of OFA1, OFA2 are shown in Fig. 1. The glass crystalline onset is above 500 8C. The Tg and the corresponding glass composition are presented in Table 3.

OFA1 OFA2

75

150

225

300

375

450

o

Temperature( C) Fig. 1. DSC thermograms of OFA1 and OFA2 glasses.

Table 3 Glass transition temperatures of Nd3þ-doped NaF–Na2O–B2O3 glasses Serial number

Sample

Tg (8C)

1 2 3

OFA0 OFA1 OFA2

440 445 435

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Fig. 2. FTIR spectra of OFA0, OFA1, OFA2 glasses.

3.2. FTIR spectral analysis Infrared spectra of NaF–Na2O–B2O3 glasses (OFA0, OFA1 and OFA2) are shown in Fig. 2. All spectra show the broad absorption band at 3440 cm1 which is mainly due to the hydroxyl groups present in the glasses and is attributed mainly to the O–H stretching vibrations. The IR spectral vibrations of the borate glasses are divided into three main regions. The first region occurs between 1200 and 1600 cm1 which is due to asymmetric stretching relaxation of the B–O bond of trigonal BO3 units. The second region ranges from 800 to 1200 cm1 which is assigned to B–O bond stretching of the BO4 units. The third region which is around 700 cm1 corresponds to B–O–B linkages in the borate network [18]. The IR spectra exhibits several peaks, which are sharp, medium and broad. The broad bands are due to combination of high degeneracy of vibrational states, thermal broadening and photon scattering from the powdered samples. The broad absorption peak at around 1340 cm1 is ascribed to the B–O stretching vibrations of trigonal (BO)3 units in metaborate, pyroborates and orthoborate groups. This broad peak becomes much broader in the OFA1 than OFA0 and OFA2. The band at 1404 cm1 is attributed to the B–O vibrations of the units attached to the large segments of borate network. The shoulder at 1512 cm1 is due to B–O bonds from isolated pyroborate groups [19]. The broad band around 1000 cm1 is assigned to the vibrations of some boron atoms attached to non bridging oxygen in the form of BO4 units. This absorption peak is observed broader in OFA1. The band between 615 and

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Table 4 Peak Table of FTIR spectra of OFA0, OFA1, OFA2 glasses Serial number

OFA0 (cm1)

OFA1 (cm1)

OFA2 (cm1)

Assignments

1 2 3 4 5 6

3402 1512 1404 1340 1000 715

3400 1512 – 1350 1002 719

3436 1512 1404 1340 998 717

O–H stretching vibrations B–O bonds from isolated pyroborate groups B–O vibrations B–O stretching vibrations B–O vibrations, attached to BO4 units Presence of pyroborate groups

740 cm1 is assigned to the presence of pyroborate and orthoborate units. The observed wavenumbers of FTIR spectra for all the glasses and the corresponding assignments are shown in Table 4. 3.3. Optical analysis The optical absorption spectrum of 1 mol% Nd3þ-doped NaF–Na2O–B2O3 glass in the wavelength range of 400–900 nm is shown in Fig. 3. In the present study, absorption spectrum of 1 mol% Nd3þ-doped NaF–Na2O–B2O3 glass was analysed. The absorption transitions pertaining to 4 F3=2 , 4 F5=2 þ 2 H9=2 , 4 S3=2 þ 4 F7=2 , 4 F9=2 , 2 H11=2 , 4 G5=2 þ 2 G7=2 , 4 G7=2 , 4 G9=2 , 4 G11=2 þ 2 K15=2 þ 2 G9=2 þ 2 D3=2 , 2 P1=2 þ 2 4 D5=2 I9=2 have been observed. Their energies and the experimental oscillator strengths were calculated from the absorption spectra by using the relation: Z n2 9 eðnÞ dn fmeas ¼ 4:32  10 n1

4

4

4

G9/2

1.50

F5/2+2H9/2

G5/2+2G7/2

where e(n) is the molar extension coefficient at the wavenumber (n).

F7/2+4S3/2

4

G7/2

F9/2

4

2

H11/2

4

F3/2

4

G11/2+2K15/2+2G9/2+2D3/2 4

0.75

P1/2+2D5/2

1.00

2

Absorbance(a.u)

1.25

0.50 400

500

600

700

800

900

Wavelength(nm) Fig. 3. Optical absorption spectrum of OFA1 glass.

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According to Judd–Ofelt theory [20,21] the oscillator strength of a transition between an initial J manifold (S, L)J and a final manifold (S0 , L0 )J0 is given by: " # 2 2 2 8p mc ðn þ 2Þ fcal ðaJ; bJ 0 Þ ¼ Sed þ nSmd 3hð2J þ 1Þ 9n   P where Sed ½ðS; LÞJ : ðS0 ; L0 ÞJ 0  ¼ l¼2;4;6 Ol jhðS; LÞJ U ðlÞ ðS0 ; L0 ÞJ 0 ij2 is the electric dipole line strength and Smd is the magnetic dipole line strength. The bands which are produced by the magnetic dipole mechanism, have very low spectral intensity magnitude compared to those of electric dipole bands. Hence, Smd has not been considered. jhðS; LÞJjjU ðlÞ jjðS0 ; L0 ÞJ 0 ij2 is the reduced matrix elements and ‘n’ is the refractive index of the medium. Hence, for the calculation purpose, we write the equation as: X Tl jhðS; LÞJjjU ðlÞ jjðS0 ; L0 ÞJ 0 ij2 fmeas ðaJ; bJ 0 Þ ¼ l¼2;4;6

where Tl is the related to Ol as: Ol ¼

3h 9n ð2J þ 1ÞTl 8p2 mc ðn2 þ 2Þ2

and the Tl factors can be obtained by least-square fit method. The experimental and theoretical oscillator strengths were calculated by least square method and presented with the J–O parameters in Table 5. The weak bands are not taken in the least square analysis because of uncertainties in the determination of the experimental oscillator strengths. The hypersensitive band shows a good agreement with the calculated oscillator strength. The reduced matrix elements and the assignments of all transition were made on the basis of Carnall et al. [22]. For calculation purposes, the overlapping bands such as 4 F5=2 þ 2 H9=2 , 4 S3=2 þ 4 F7=2 , 4 G5=2 þ 2 G7=2 , 4 G11=2 þ 2 K15=2 þ 2 G9=2 þ 2 D3=2 , 2 P1=2 þ 2 D5=2 , were considered as a single band by adding the corresponding reduced matrix elements. Table 5 Measured and calculated oscillator strengths of 1 mol% Nd3þ ions in NaF–Na2O–B2O3 glass Serial number

1 2 3 4 5 6 7 8

Assignments 4

F3=2 F5=2 þ 2 H9=2 4 S3=2 þ 4 F7=2 4 F9=2 4 G5=2 þ 2 G7=2 4 G7=2 4 G9=2 2 P1=2 þ 2 D5=2 4

O2 (1020 cm2) O4 (1020 cm2) O6 (1020 cm2) Drms ¼ 1:982  106 .

Energy (cm1)

11415 12422 13369 14641 17094 19011 19493 23256

Oscillator strength fmeas (106)

fcal (106)

3.921 5.363 6.97 3.521 16.271 5.010 3.427 2.124

3.632 7.441 5.80 0.1515 16.264 4.515 1.949 1.106 1.197 8.58 3.914

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Among the three parameters O2, O4 and O6, the magnitude of O2 is an indicative of crystal field symmetry of the rare earth site while the other two parameters are inversely proportional to the rigidity of the host [23,24]. Usually, halide-containing glasses possess smaller value of O2 due to weak local fields and narrow linewidths [25]. In the present investigation, O2 parameter of OFA1 has less value due to weak local fields although the rare earth site has higher asymmetry. But the other two parameters clearly show that the glass has low rigidity. The O2 parameter not only depends on the intensity of the hypersensitive transition but also has a very large value for the reduced matrix elements jjU 2 jj2 . Nearly all the other observed transitions, have small values for jjU 2 jj2 but other two reduced elements, jjU 4 jj2 and jjU 6 jj2 have significant values. 3.4. Radiative properties The emission spectrum of OFA1 glass is shown in Fig. 4. The Arþ laser at 514.5 nm, in resonance with the 4 G7=2 absorption line of Nd3þ, was used to pump the metastable level of 4 F3=2 . The absorption spectral results have been utilised to understand the radiative properties of the Nd3þ ions. The observed two emission lines corresponds to the radiative transitions of Nd3þ ions from 4 F3=2 ! 4 I9=2 and 4 F3=2 ! 4 I11=2 . To obtain the spontaneous emission probability, the following relation was used: " # 64p4 n3 e2 nðn2 þ 2Þ2 Sed A¼ 3hð2J þ 1Þ 9 where Sed is the electric dipole line strength. From this relation, the stimulated emission cross-section was also calculated by using the relation: s¼

l4 A 8pcn2 Dleff

where Dleff is the effective bandwidth which was calculated by integrating the intensity of the emission line shape and dividing it by the intensity at the maximum wavelength [26]. The total transition

ex 4

=514.5 nm

4

4

F3/2 I11/2 =1060nm em

4

Intensity (a.u)

F3/2 I9/2 = 888nm em

800

850

900

950 1000 1050 1100 1150

Wavelength (nm)

Fig. 4. Emission spectrum of OFA1 glass.

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Table 6 Radiative parameters of the OFA 1 glass Serial number

Transition

l (nm)

Dl (nm)

A (s1)

s (1020 cm2)

b (%)

t (ms)

1 2

4

1060 888

57.2 60.35

378 501

0.5 0.3

43 56

1138

4

4

F3=2 ! I11=2 F3=2 ! 4 I9=2

probability P of the emission state was obtained by summing the individual probability values as, AT ¼ A and the radiative lifetime tR ¼

1 AT

By using A and AT, the branching ratio in percentage is given by b ¼ ðA=AT Þ  100. The evaluated values are presented in Table 6. Among the two transitions (4 F3=2 ! 4 I9=2 , 4 F3=2 ! 4 I11=2 ), the 4 F3=2 ! 4 I11=2 has potential application for the laser transition because it has higher stimulated emission cross-section than the 4 F3=2 ! 4 I9=2 transition.

4. Conclusion The glasses were prepared through melt quenching method. The amorphous nature of the samples were confirmed through XRD analysis and the optical absorption spectrum of the glasses also assures the same. The FTIR spectra show the weak band of O–H vibrations in all the glasses and all the glasses has the BO3 and BO4 units in the glassy system. The theoretical and experimental oscillator strength were calculated and the J–O parameters were used for the spontaneous emission probability and stimulated emission cross-section calculations. The evaluated s values imply that the OFA 1 is suitable for the 1060 nm emission.

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