Scholarly article on topic 'Thermal, structural and spectroscopic properties of Pr3+-doped lead zinc borate glasses modified by alkali metal ions'

Thermal, structural and spectroscopic properties of Pr3+-doped lead zinc borate glasses modified by alkali metal ions Academic research paper on "Chemical sciences"

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{"Rare earth doped glasses" / "Judd–Ofelt theory" / "Emission properties" / Absorption / FTIR}

Abstract of research paper on Chemical sciences, author of scientific article — M.V. Sasi kumar, B. Rajeswara Reddy, S. Babu, A. Balakrishna, Y.C. Ratnakaram

Abstract This paper offers a study on Pr3+-doped alkali and mixed-alkali borate glasses prepared by the melt quenching technique and characterized by thermal, structural and spectroscopic studies. The amorphous nature of the glassy systems was identified based on X-ray diffraction. The thermal behaviour of glasses was studied using differential thermal analysis (DTA). The functional groups contained in the glasses were identified by Fourier transform infrared spectroscopy (FTIR). Spectral intensities were evaluated from the absorption spectra and used for calculating J–O intensity parameters, Ω λ (λ =2, 4 and 6). Further, these parameters were used for calculating different radiative properties. The best radiative state was identified as the laser transition state among the various states. Emission analysis was performed for this state by calculating the branching ratios and stimulated emission cross sections (σ p) for all the prepared glasses. These studies suggest that borate glasses are useful for visible fluorescence.

Academic research paper on topic "Thermal, structural and spectroscopic properties of Pr3+-doped lead zinc borate glasses modified by alkali metal ions"

Accepted Manuscript

Title: Thermal, structural and spectroscopic properties of Pr3+ ion doped in lead-zinc borate glasses modified by alkali metal ions

Author: M.V. Sasi kumar B Rajeswara Reddy S. Babu A. Balakrishna Y.C. Ratnakaram

PII: DOI:

Reference:

S1658-3655(16)30018-8

http://dx.doi.org/doi:10.1016/j.jtusci.2016.04.004 JTUSCI 299

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Received date: Revised date: Accepted date:

21-1-2016

25-4-2016

26-4-2016

Please cite this article as: M.V.S. kumar, B.R. Reddy, S. Babu, A. Balakrishna, Y.C. Ratnakaram, Thermal, structural and spectroscopic properties of Pr3+ ion doped in lead-zinc borate glasses modified by alkali metal ions, Journal ofTaibah University for Science (2016), http://dx.doi.org/10.1016/j.jtusci.2016.04.004

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Thermal, structural and spectroscopic properties of Pr3+ ion doped in lead-zinc borate glasses modified by alkali metal ions

M. V. Sasi kumar1*, B Rajeswara Reddy2, S. Babu3 , A. Balakrishna3, Y. C. Ratnakaram3* 1. Sree Vidyanikethan Engineering College, A. Rangampet, Tirupati. 2. Governament Degree college, Gudur, Nellore. 3. Department of physics, Sri Venkateswara University, Tirupati - 517 502.

Abstract

This paper offers a study on the Pr3+ doped alkali and mixed alkali borate glasses prepared by melt quenching technique and characterized by thermal, structural and spectroscopic studies. The amorphous nature of the glasses system identified based on X-ray diffraction spectra. Using differential scanning calorimetry (DSC), the thermal behavior of borate glass carried out. The presence of function groups which are contain in the glasses identified by Fourier Transform Infra Red spectra. From the absorption spectra, spectral intensities were evaluated and used for calculating J-O intensity parameters, Q^ (A,=2, 4 and 6). Further, these parameters used for calculating different radiative properties. Among various radiative states, one identified as best laser transition state. For this state, emission analysis was performed by calculating branching ratios and stimulated emission cross sections (oP) for all the prepared glasses. From all these studies, suggest that the borate glasses are useful for visible fluorescence.

Key words: Rare earth doped glasses; Judd-Ofelt theory; Emission properties; Absorption; FTIR.

Corresponding author. Tel: +91 9440465523 Email address: drsasi.mv@gmail.com

1. Introduction

The tripositive lanthanide ions have unique spectroscopic properties [1]. Since the 4f shell is efficiently shielded by the close 5s and 5p shells, the ligand environment has only a weak influence on the electronic cloud of the lanthanide ion. Although weak, this perturbation is responsible for the spectral fine structure. In the absorption spectrum the peak positions of the spectral lines reveal the electronic structure of the 4f configuration. The ligand field splitting gives information about the symmetry of the lanthanide site and the shape of the coordination polyhedron. The intensities of spectral transitions reflect the interaction between the lanthanide ion and its environment. The origin of these properties has been strongly related to the local order around boron atoms. The coordination geometry of boron atoms is decided by the glass composition and the nature of the glassy network modifiers. In order to dope high level of rare earth concentration in the host glass matrix, glass modifiers are added which breaks the bridging anion bonds thus increasing the nonbridging bonds.

Among rare earth (RE) ions, the electronic configuration of the Pr3+ ion is (closed shells)+4f2. The total number of 4f2 quantum states is 91. The electron-electron

3+ 3 3 1113 1

interaction creates the LS terms for Pr ion are H, F, G, D, I, P and S. The spin orbit interaction splits the LS terms into J sublevels, but mixed states with the same J and different L and S. Because of this term mixing, there are departures from the Lande interval rule. Pr3+ is an attractive optical activator, since its energy level spectrum contains several metastable multiples 1G4, 1D2, 3P0, 1; 2, which offers the possibility of simultaneous blue, green and red emission for laser action as well as infrared emission for optical amplification [2]. Studies on Pr3+ ion are ideal for the comparison of different relaxation and cross relaxation due to numerous energy levels [3]. A 1.3 p,m Pr3+: ZBLAN optical fiber amplifier has been demonstrated [4]. Furthermore, the broad band interconfigurational 4f2 ^ 4f5d transition of Pr3+ which are used in scintillator detectors of ionizing radiation [5] offer the potential for tunable laser action at near ultraviolet wavelengths. A renewed interest in studying Pr3+ doped materials for laser purposes, both continuous wave and pulsed wave in crystals such as YAlO3:Pr3+ [6] and LaCl3: Pr3+ [7] as well as in optical fibers [8].

The Judd-Ofelt theory has a remarkable success in the characterization of radiative transitions in rare earth doped solids [9, 10]. The application of Judd-Ofelt theory to Pr3+ ion exhibits some problem to the small energy difference between the ground configuration 4f2 and the first excited state configuration 4f15d1 [11]. This demonstration itself both as a large deviation between the measured and calculated spectral intensities and as such the difficulty experienced to fit the 3H4^3P2 hypersensitive transition [12]. In this point of view, modified Judd- Ofelt theory is applied which is introduced by Kornienko et al. [13].

Recently Kubliha et al [14] studied the structural peculiarities and electrical and optical properties of 70TeO2.30PbCl2 glasses doped with Pr3+, prepared in Pt or Au crucibles. They shown that six PL peaks, both in pure and doped glasses. They can be attributed to 3P0 ^ 3F2; 3P1 ^ 3F2; 3P0 ^ 3F3 and 3P1 ^ 3F3 transitions in Pr3+ ions. Compositional dependence of the 1.3 p,m emission and energy transfer mechanism in Ge-Ga-S glasses doped with Pr3+ was reported by Park et al [15] and reported the energy transfer through the cross relaxation to be increased by the inhomogeneous distribution of Pr3+ ions. Concentration quenching of fluorescence from the 3P0 + 3P1 manifold in heavy doped Pr3+: ZBAN glass was reported by Gibbs et al [16] and applied multipolar energy transfer model to analyze the fluorescence from the (3P0 + 3P1) manifold in Pr3+-doped ZBAN glass at concentrations of up to 12mol%. Orange emission in Pr3+ doped fluorinate glass was investigated by Manzani et al [17] concluded that the praseodymium concentrations lower than 0.5%, the 3P0 level was found to exhibit a lifetime longer than 40 |is, which is compatible with efficient laser emission. Zhang et al [18] reported the comparative investigation on the spectroscopic properties of Pr3+ doped boro-germo-silicate and tellurite glasses. This study explained multiphonon relaxation theory. Pr3+ doped ZBLA fluoride glasses for visible laser emission was studied by Olivier et al [19] and observed emission in red, orange and green wavelengths. Jose et al [20] studied the application of a modified Judd-Ofelt theory to Pr3+ doped phosphate glasses and the evaluation of radiative properties and concluded the superiority of the modified J-O theory over the original theory in predicting the observed oscillator strengths has been confirmed. In particular the oscillator strength corresponding to the hypersensitive transition 3H4 ^ 3P2 of Pr3+ in phosphate glass is obtained with satisfactory accuracy.

Photoluminescence from Pr3+ doped chalcogenide glasses excited by bandgap light was reported by Harada et al [21] and recommended the bandgap-excited photoluminescence at 1.6 |im in Pr3+-doped chalcogenide glasses is understood to be induced through resonance energy transfer from photo-generated dangling bonds. Spectroscopic properties of Pr3+ doped in tellurite glass were reported by Rai et al [22] and investigated the temperature dependence of the fluorescence intensity and the lifetime of the 3P0 level has been investigated and found that they decrease with the increase of the temperature. Nemec and Frumar [23] reported the synthesis and properties of Pr3+ doped Ge-Ga-Se glasses. Kam and Buddhudu [24] studied the near infrared to red and yellow to blue upconversion emission from Pr3+: ZrF4 - BaF2- LaF3 - YF3 - AlF3 - NaF glasses and concluded the normal emission spectra obtained upon excitation both at visible and UV wavelengths provide sufficient evidence to identify these materials as potential red and blue color emitting luminescent devices.

Spectroscopy of Pr3+ ions in lithium borate and lithium fluroborate glasses were studied by Babu et al. [25] and found that pR for the 1G4 ^ 3H5 (1.3 |im) transition, useful for the fibre amplifiers. Dominiak-Dzik et al. [26] reported the spectral properties and dynamics of luminescent states of Pr3+ and Tm3+ in lead borate glasses modified by PbF2. It was observed that the increase of PbF2 content leads to narrowing of the 1D4^3H4 band from 680 to 590 cm-1 and time constants of luminescence decays increase as a result of increasing the PbF2 content. Optical absorption and emission properties of Pr3+ and Er3+ in lithium cesium mixed alkali borate glasses were studied by Ratnakaram et al. [27] and reported that the radiative lifietimes (tr) of all the excited state of Pr3+ are more at 8mol%. Voda et al [28] examined the optical properties of Pr3+ doped lithium tetra borate glasses and concluded the nonradiative relaxations which reduce the quantum efficiency and consequently the emission of the 1D4 ^ H4 transition.

2. Experimental

In the present work, the author has synthesized Pr3+ doped different alkali and mixed alkali heavy metal borate glass samples using the conventional melt quenching technique. High purity H3BO3, Li2CO3, Na2CO3, K2CO3, ZnO, Pb3O4 and P^On were

used for the batch melting. The powders were weighed in order to obtain a 10 g batch whose molar compositions are given below.

54 B2O3 + 15 PbO + 10 ZnO + 20Li2O + 1Pr6O11 54 B2O3 + 15 PbO + 10 ZnO + 20Na2O + 1Pr6O11 54 B2O3 + 15 PbO + 10 ZnO + 20K2O + 1Pr6O11 54 B2O3 + 15 PbO + 10 ZnO + 10Li2O + 10Na2O +1Pr6O11 54 B2O3 + 15 PbO + 10 ZnO + 10Li2O + 10K2O + 1Pr6On 54 B2O3 + 15 PbO + 10 ZnO + 10Na2O + 10K2O + 1Pr6O11

These powders are thoroughly ground and loaded in a silica crucible. These mixtures were heated upto 950° C for 40 min in an electric furnace. The melt was poured on to a pre-heated brass plate and pressed with another brass plate. For the optical measurements, the samples were polished carefully. Using Abbe refractometer and 1-bromonaphalene as contact liquid, the refractive indices were measured at ^sodium=589.3 nm. The densities of the samples were measured using classical Archimedes method with Xylene as immersion liquid.

The XRD profiles were taken with the XRD 3003 TT Scifert Diffractometer with CuKa radiation. Differential thermal analysis (DTA) thermograms were recorded using SDT Q600 DTA analyzer in the temperature range 150 to 600° C with heating rate of 10°C/min at the Nitrogen gas atmosphere. The Fourier transform infrared spectrum (FTIR) of the host glass matrices were carried out with the Perkin-Elmer Spectra-one FTIR Spectrophotometer using KBr pellet technique. UV-VIS-NIR absorption spectra measurements were recorded for synthesized glass matrices using JASCO - V630 spectrophotometer in the wavelength regions 400 - 700 nm and 900 - 2600 nm at room temperature. Emission spectra of Pr3+ doped alkali and mixed alkali heavy metal borate glass matrices under excitation wavelength 444 nm, recorded using the Spex Fluorolog-2 fluorometer in the wavelength range 450-750 nm.

3. Results and Discussion

3. 1. XRD and SEM with EDS Analysis

Fig. 1 shows the X-ray diffraction profiles of Pr3+ doped sodium and sodium-potassium heavy metal borate glass matrices. The diffractograms of other glass matrices are similar in shape. Hence they have not been shown. From these profiles, it is observed that the studied glass matrices exhibits amorphous nature, because these profiles are not showing any diffraction peaks. Fig. 2 shows the Scanning Electron Microscopy (SEM) images of Pr3+ doped sodium and sodium-potassium heavy metal borate glass matrices, recorded using Carl Zeiss EVO-MA15 Sacanning Electron Microscope. The morphology of these glass matrices are not showing any grains indicating the amorphous nature of the glass matrices. These results coincide with the XRD results. From the above discussion it is clear that the prepared glass matrices are purely amorphous in nature.

Fig. 3 represents the Energy Dispersive X-ray spectroscopy (EDS) diagrams of Pr3+ doped sodium and sodium-potassium glass matrices carried out using Carl Zeiss EVO-MA15 Microscope. The spectra give the elements that are present in the investigated glass samples in terms of percentages.

3. 2. Differential Thermal Analysis

The author has measured the thermal properties of Pr3+ doped alkali and mixed alkali heavy metal borate glass matrices using differential thermal analysis (DTA) and are shown in Fig. 4. The glass transition temperature (Tg) and thermal stability parameter against devitrification, AT(= Tx - Tg), where Tx is the crystallization temperature, are commonly used to estimate the thermal stability of the glasses [29]. The glass transition temperature (Tg) and the crystallization temperature (Tx) vales for Li, Na, K, Li-Na, Li-K and Na-K glass matrices are 331.5, 340.3, 303.3, 309.5, 302.5 and 294.9° C and 396.3, 446.3, 344.3, 385.1, 375.5 and 339.5° C respectively. It was noticed that sodium glass matrix has high transition temperature (Tg) in alkali heavy metal borate glasses, whereas in case of mixed alkali glasses, lithium-sodium glass matrix exhibits high transition temperature (Tg). Among all the six glass matrices, sodium glass matrix exhibits large transition temperature. AT values for the above prepared glasses are 64.8, 106.0, 41, 75.6,

73 and 44.6 ° C. From the above data, If AT > 100° C glasses can be considered as a glass with relatively good thermal stability [30]. From the above data, noticed that sodium glass matrix exhibits large thermal stability among the three alkali glasses. In case of mixed alkali glasses, lithium-sodium glass matrix exhibits larger thermal stability than the other glass matrices.

3. 3. FTIR Spectroscopy

Fig. 5 shows the Fourier Transform Infrared spectra (FTIR) recorded in the 450 -4000 cm-1 region for Pr3+ doped alkali and mixed alkali heavy metal borate glass matrices. In general, the vibration mode of the borate glass network consist of three important band regions. The first group of bands, which occur at 1200 - 1600 cm-1 is due to asymmetric stretching vibration of B-O bonds in BO3 units. Secondly, the groups lie between 800 - 1200 cm-1 and is due to the B-O bond stretching of the tetrahedral BO4 units. Thirdly, a group is observed around 700 cm-1 is due to bending of B-O-B linkage in the borate network. In the present work all the glass matrices possess an absorption band around 484 - 494 cm-1 appeared on the spectra and it is assigned to the loosing of BO4 units [31]. An absorption band around 709 - 719 cm-1 was due to the band vibration of BO linkage in the borate network [32]. An absorption band is observed in potassium glass matrix at ~856 nm and it is due to stretching vibrations of B-O bond in BO4 unit from diborate groups [33]. The peak around ~950 cm-1 is assigned to B-O stretching vibration of BO4 tetrahedra [32]. One shoulder is observed at 1242 cm-1 in only potassium glass matrices and this may arise from B-O stretching vibrations of (BO3)3- unit in metaborate chains and orthoborates [34]. The band around ~1380 cm-1 is assigned to the stretching vibrations of B-O bonds of trigonal (BO3)3- unit in metaborate, pyroborate and orthoborates [35]. An absorption band is observed at 1658 cm-1 and it is due to asymmetric stretching of the B-O bond of BO3 units [36]. The intensity changes of those bands are correlated with the change in the properties of the glass system. The absorption band at 3377 - 3436 cm-1 is attributed to O-H stretching vibration of water molecules [37].

3. 4. Optical Absorption Spectra

Figs. 6a and 6b represents the UV-VIS and NIR absorption spectra of Pr3+ doped alkali and mixed alkali heavy metal borate glass matrices respectively. The absorption bands originating from praseodymium electronic transitions from the fundamental level 3H4 to different excited states, 3P2, ^1+%, 3P0, ^2, 1G4, 3F4, 3F3 and 3F2. Among all the absorption bands, 3F3 band is more intense in all the glass matrices. It is also observed that, a small splitting in lithium-sodium glass matrix for 3F3 band with energy difference 106 cm-1. The transitions were assigned by comparing the band positions with those reported in literature [38-40].

3. 5. Energies of Absorption bands

From the absorption spectra, the energies (experimental) of different absorption bands of Pr3+ doped different alkali and mixed alkali (Li, Na, K, Li-Na, Li-K and Na-K) heavy metal borate glass matrices are obtained. The calculated energies of the observed absorption peaks are obtained using Tayler series expansion and least square fit method. In the present work 3F4 absorption band was excluded from the fitting procedure. The experimental and calculated energies of observed absorption bands with rms deviations were presented in the Table 1. The rms deviations between experimental and calculated energy values are very small indicating the validity of full matrix diagonalization. Using the observed band energies (Ej), zero point energies (Eoj) and partial derivatives (dE/dJ) [41], the correction factors AEk and A^4f have been calculated by the least squares fit method. From these correction factors, Racah (E1, E2 and E3), spin-orbit parameter (Ç4f)

1 3 2 3 3+

and hydrogenic ratios (E1/E3 and E2/E3) for all the Pr3+ doped glass matrices were determined using the procedure explained in Ref [42] and are presented in Table 2. From the table it is observed that the hydrogenic ratios in all the prepared glass matrices are approximately same, indicating unperturbed radial properties.

3. 6. Spectral intensities and Judd-Ofelt Analysis

The experimental spectral intensities (fexp) are obtained from the areas under each absorption peak and the calculated spectral intensities (fcal) of the observed absorption bands of the Pr3+ doped glass matrices are determined using Judd-Ofelt theory. Table 3

represents the spectral intensities (fexp and fcai) and rms deviations between experimental and calculated spectral intensities. From the table it is observed that, the accuracy of the fit is given by the rms deviations between the experimental and calculated spectral intensities, which indicate the validity of the Judd-Ofelt theory and these rms values are relatively small. It is also observed that, 3H4 ^ 3F3 transition exhibits higher spectral intensity and 3H4 ^ !D2 transition exhibits lower spectral intensity in all the glass matrices. In alkali glass matrices spectral intensities decreases with increasing atomic number (Li > Na > K). In case of mixed alkali glasses lithium-potassium glass matrix exhibits higher spectral intensity for the above transition. All these transitions are mainly electric dipole in nature and only the 3H4^-1G4, 3F4,3 transitions have a small magnetic dipole character.

Using the Judd-Ofelt theory [9, 10], the best set of Judd-Ofelt intensity parameters Q^ (X= 2, 4 and 6) were determined for Pr3+ doped alkali and mixed alkali heavy metal borate glass matrices and these values are presented in Table 4. In the present work, seven observed absorption bands are considered for evaluating the intensity parameters for the Li, Na, K and Li-K glass matrices and eight bands are considered for Li-Na and Na-K glass matrices. These Judd-Ofelt intensity parameters are host dependent and are important for the investigation of glass structure and transition rates of rare earth ions. In general Q2 parameter is an indicator of covalency of metal ligand bond and Q4 and Q6 parameters gives the information about rigidity of the host glass matrix. In the present work, it is noticed that lithium glass matrix exhibits higher Q2 parameter value in alkali glass matrices and sodium-potassium glass exhibits higher Q2 parameter value in mixed alkali glasses, which indicates the higher covalency of Pr-O bond. The Q6 parameter is very high for lithium in alkali glasses and lithium-potassium in mixed alkali glasses, indicating the higher rigidity of the glass matrices.

3. 8. Radiative properties

From the calculated Judd-Ofelt intensity parameters, the radiative properties such as total radiative transition probabilities (AT), radiative lifetimes (tr), branching ratios (P) and integrated absorption cross sections (X) were estimated. Among various states,

radiative transition probability found to be decrease in the order of 3P0> 3P1> 1D2> 3F3 in all glasses. Higher emission probability leads to faster decay of that emission level and hence shortening of the lifetime (tr). The lifetimes of different excited states (3P1; 3P0, 1D2 and 3F3) of Pr3+ doped alkali and mixed alkali heavy metal borate glasses have been calculated. It is noticed that, 3F3 excited state exhibits longer lifetimes in all the six glass matrices and shorter for 3P0 excited state. It is also observed that, sodium glass matrix exhibits longer lifetimes in the alkali borate glasses, whereas in case of mixed alkali glasses lithium-sodium glass matrix exhibits longer lifetimes for all the excited states. Table 5 represents the magnitudes of branching ratios (P) and integrated absorption cross-sections (X) of certain transitions (3P1 —3H5, 3P0—3H6,3H4 and 3F3—3H4) of Pr3+ doped alkali and mixed alkali heavy metal borate glass matrices. From the table, among various

3 3 3 3 1 3 3 3

transitions, the transitions, P1^ H5, P0— H4, D2— H4 and P3— H4 have higher magnitude of integrated absorption cross sections and also predicted branching ratios (from JO theory) in all glasses.

3. 9 Emission Spectra

Under 444 nm wavelength excitation wavelength, Pr3+ ions are quickly excited from ground state 3H4 level to the higher 3P2 excited level. From 3P2 excited level to 3P0 lower excited level, non-radiative emission takes place. Because of the small energy gap between the 3P0 levels to the next lower level, these two levels are thermalized at room temperature. Hence, emission takes place from these two excited levels. It is worth remarking that the multiphonon relaxation processes (MPR) from higher level to next lower 1D2 level is efficient due to small energy gap. Moreover, the gap between the 1D2 level and the next 1G4 level is larger, nearly 7000 cm-1, and thus more phonons are required to bridge the energy gap. Therefore, radiative emission from the both 3P0 and 1D2 levels to lower ground state level is very high. Two bands corresponding with the transitions P0— 3H4 (493 nm) and 3P0 —> H6 (615 nm) are considerably more intensive than the other bands related to P1— 3H5 (533 nm), and 3P0 — 3F2 (649 nm) transitions [14, 15]. It is worthy to note that the intensities of two transitions are much stronger. However, the spectral shape of this peak is the same for different glasses and does not normally depend on their chemical compositions. At lower concentrations (<0.5 mol%)

3+ 1 3 3 3

of Pr ions, the emission D2^ H4 transition is higher in intensity than P0^ H6 transition. If concentration increases, the intensity of emission 1D2^3H4 transition decreases whereas 3P0^3H6 transition intensity increases. In the present work, 1.0 mol % of pr3+ ions are doped [43, 44].

Fig. 7 shows the emission spectra of Pr3+ doped alkali and mixed alkali heavy metal borate glass matrices under excitation wavelength 444 nm in the wavelength range 450-750 nm. These spectra consists of four distinguishable emission peaks 3P0 ^ 3H4,

3 3 3 3 3 3

P1 ^ H5, P0 ^ H6 and P0 ^ F2. From the spectra, it is observed that, the intensity of 3P0 ^ 3H6 transition decreases with increasing atomic number in alkali glasses. In case of mixed alkali glass the emission band, 3P0 ^ 3H6 is having more intensity in Li-Na and Na-K glasses. It is also observed that the intensity of emission band, 3P0 ^ 3F2 decreases in lithium- sodium and sodium-potassium glass matrices. The emission properties such as stimulated emission cross sections (op) and effective linewidths (Aveff) are estimated. Table 6 represents the peak wavelengths (Xp), stimulated emission cross-sections (op) and effective linewidths (Aveff) of Pr3+ doped glasses for all observed emission transitions. From the table it is observed that, the transition, 3P0 ^ 3F2 exhibits higher stimulated emission cross-sections in lithium glass and for remaining glasses 3P1 ^ 3H5 transition exhibits higher stimulated emission cross-sections. It is a well known fact that, the glass matrices which are having higher stimulated emission cross-section are preferable for good optical excitation.

4. Conclusions

XRD and SEM studies reveal that, these Pr3+ doped glasses are amorphous in nature. The sodium glass matrix exhibits higher glass transition temperature among all the glasses. The sodium and lithium-sodium glass matrices exhibit larger thermal stabilities. From the absorption spectra the spectroscopic parameters such as Racah (E1, E2 and E3), spin-orbit parameters (^4f) and hydrogenic ratios (E1/E3 and E2/E3) are determined for all the glass matrices. These parameters are not much varied with the glass matrix. The hydrogenic ratios are nearly constant for all the glass matrices indicate unperturbed radial properties. The spectral intensities are higher for 3F3 band and lower

for 1D2 band in all the glass matrices. The Q2 parameter is higher in sodium glass matrix indicating higher covalency. In case of mixed alkali glasses sodium-potassium glass exhibits higher covalency. From the variation of shift in peak wavelength of hypersensitive transition and Q2 parameter, it is noticed that structural changes are not much influencing the covalency of Pr-O bond. Radiative lifetimes (tr) are estimated for

3 3 1 3 3+

the excited states. 3P1 3P0 1D2 and 3F3 of Pr3+ in all the glass matrices. Among various transitions, 3F3— 3H4 transition consists higher branching ratio value in all the glass matrices. From the emission spectra, the experimental branching ratios (Pexp), emission

3 3 3 3 3 3

cross-sections are obtained for the four emission transitions, P0— H4, P1— H5, P0— H6 and 3P0— 3F2. 3P0— 3H4 transition exhibits higher stimulated emission transition and it is higher for lithium glass matrix and it may be useful for laser excitation.

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Figure captions

1. Fig. 1. X- ray diffraction patterns of Pr3+ doped lithium and sodium-potassium heavy metal borate glass matrices.

2. Fig. 2. SEM images of Pr3+ doped sodium and sodium-potassium heavy metal borate

glass matrices.

3. Fig. 3. EDS profiles of Pr3+ doped (a) sodium (b) sodium-potassium heavy metal borate glass matrices.

4. Fig. 4. DTA thermograms of Pr3+ doped different alkali and mixed alkali heavy metal borate glass matrices.

5. Fig. 5. FTIR spectra of Pr3+ doped different alkali and mixed alkali heavy metal borate glass matrices.

6. Fig. 6a & 6b.Visible and NIR absorption spectra of Pr3+ doped different alkali and mixed alkali heavy metal borate glass matrices.

7. Fig. 7. Emission spectra of Pr3+ doped different alkali and mixed alkali heavy metal borate glass matrices.

10 20 30 40 50 60 70

29 (degree)

Fig. 2

loosing of BO .units

bending of B-O-B

BO units

B03units

OH group

Wavenumber (cm-1) Fig. 5

400 440 480 520 560 600 Wavelength (nm)

Fig. 6a

T-1-1-1-1-r

1200 1500 1800 2100 2400 2700

Wavelength (nm) Fig. 6b

1.2x10 -

9 8.0xl05

500 550 600 650 700 750

Wavelength (nm)

Table.1.

Experimental and calculated energies (Eexp and Ecal )(in cm-1) of certain excited states of Pr3+ in different alkali and mixed alkali heavy metal borate glass matrices.

Li Na K Li-Na Li-K Na-K

Energy Level

E ^exp Ecal Eexp exp Ecal Eexp exp Ecal Eexp exp Ecal Eexp exp Ecal Eexp exp Ecal

3f2 5203 5163 5208 5165 5165 5142 5167 5144 5166 5140 5193 5160

3F3 6522 6481 6521 6488 6527 6473 6542 6474 6540 6492 6549 6491

1d2 16894 16897 16902 16906 16912 16914 16877 16878 16858 16861 16892 16896

3Po 20663 20573 20644 20559 20633 20547 20621 20519 20615 20532 20632 20529

3P1 + 1I6 21198 21309 21191 21301 21186 21275 21154 21254 21182 21272 21161 21274

3P2 ^ 22467 22464 22471 22461 22431 22445 22399 22421 22446 22455 22432 22442

rms deviation ± 109 ± 105 ± 98 ± 114 ± 94 ± 119

Table 2.

Various spectroscopic parameters of Pr3+ doped alkali and mixed alkali heavy metal borate glasses (E1, E2, E3 and £,4f are in cm-1)

S.N o. Parameter Li Na K Li-Na Li-K Na-K

1 E1 2540.6 2500.6 2708.8 2605.6 2688.6 2488.1

2 E2 38.438 38.710 37.327 37.892 37.165 38.725

3 E3 467.91 467.46 467.16 466.49 466.47 466.59

4 u 708.50 710.14 713.92 712.54 722.26 713.06

5 E1/ E3 5.430 5.349 5.798 5.585 5.764 5.332

6 e2/e3 0.082 0.083 0.080 0.081 0.080 0.083

Table. 3.

Experimental and calculated spectral intensities (fexp x 10-5 and fcal x 10-5) of certain excited states of Pr3+ in different alkali and mixed alkali heavy metal borate glass matrices.

S.No Li Na K Li-Na Li-K Na-K

Energy Level f xexp fcal fexp exp fcal fexp exp fcal fexp exp fcal fexp exp fcal fexp exp fcal

1 3F2 7.22 7.29 3.13 4.39 3.02 4.64 2.41 2.42 4.05 4.13 2.74 2.76

2 3F3 12.66 11.23 11.27 9.68 11.23 9.56 5.18 4.97 11.64 10.00 5.68 5.32

3 3F4 4.48 6.52 2.60 5.21 2.19 5.04 2.46 2.77 3.29 5.61 2.35 2.89

3 1G4 - - - - - - 0.26 0.25 - - 0.17 0.26

4 1D2 1.44 2.00 1.54 1.70 1.53 1.66 1.39 0.88 1.36 1.77 1.50 0.93

5 3P0 1.80 3.98 1.98 5.01 1.98 5.26 1.77 2.20 1.84 4.38 1.98 2.55

6 3P1 + 1I6 2.94 6.43 3.18 7.37 3.16 7.68 2.89 3.33 2.68 6.62 2.92 3.82

7 3P2 6.26 6.33 6.71 5.44 7.07 5.28 5.97 2.81 5.93 5.75 6.24 2.96

8 rms deviation ±2.42 ± 3.13 ± 3.46 ± 1.47 ± 2.75 ± 1.59

Table. 4.

Judd-Ofelt intensity parameters (Q^ x 10-19) (cm2) of Pr3+ in different alkali and mixed alkali heavy metal borate glasses.

S. No Glass matrix Q2 Q4 Q6 Q4 /Q6 Order

1 Li 8.06 6.13 9.79 0.63 Q6> Q2> Q4

2 Na 1.21 7.74 7.78 0.99 Q6> Q4> Q2

3 K 1.60 8.15 7.41 1.10 q4> q6> q2

4 Li-Na 1.42 3.40 4.18 0.81 Q6> Q4> Q2

5 Li-K 1.33 6.81 8.62 0.79 Q6> Q4> Q2

6 Na-K 1.64 3.95 4.31 0.92 Q6> Q4> Q2

Table. 5.

Branching ratios (P) and integrated absorption cross-sections (S x 10-18)(cm-1) of certain transitions of Pr3+ in different alkali and mixed alkali heavy metal borate glass matrices.

S. No Glass matrix 3Pl -3H5 ^ 3P0 ^3H4 1D2 ^3H4 3F3 ^3H4

P S P S P S P S

1 Li 0.27 241.6 0.40 316.4 0.26 31.8 0.78 127.7

2 Na 0.38 267.0 0.64 398.7 0.40 27.0 0.85 110.1

3 K 0.38 273.9 0.64 418.4 0.37 26.4 0.84 108.7

4 Li-Na 0.35 123.2 0.56 174.8 0.35 13.9 0.83 56.5

5 Li-K 0.38 248.2 0.61 348.5 0.42 28.2 0.86 113.7

6 Na-K 0.35 138.6 0.57 230.1 0.34 14.8 0.82 60.5

Table. 6.

Emission band positions (XP, nm), effective bandwidths (Aueff cm-1) and peak stimulated emission cross-sections (o p X 10-20 cm2) of certain observed emission transitions of Pr3+ in different alkali and mixed alkali heavy metal borate glass matrices.

Transition Parameter Li Na K Li-Na Li-K Na-K

3Po ^ 3H4 Xp 492.7 492.0 497.6 491.4 490.8 493.0

AUeff 795.4 819.5 908.0 868.0 819.2 837.4

o p 413.7 503.5 487.4 207.4 437.0 251.7

3P1 ^ 3H5 Xp 533.5 533.1 531.0 531.4 531.6 531.2

AUeff 464.3 558.0 490.0 437.4 778.3 309.3

o p 543.0 498.2 577.1 290.2 329.8 462.0

3Po ^ 3H6 Xp AUeff 615.2 846.5 616.2 751.3 616.0 584.2 607.2 932.6 616.2 704.4 608.5 861.3

o p 203.1 181.4 221.1 76.0 212.7 85.3

3Po ^ 3F2 Xp AUeff 649.2 243.8 650.2 248.1 651.0 258.2 650.0 104.2 650.7 259.8 652.4 81.0

o p 2203.5 325.0 414.4 909.8 339.9 1356.2