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Influence of mixed alkalis on spectroscopic parameters of Sm3+, Dy3+ doped chloroborate glasses
Optical Materials 33 (2011) 799–806
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Influence of mixed alkalis on spectroscopic parameters of Sm3+, Dy3+ doped chloroborate glasses C. Venkateswarlu, M. Seshadri, Y.C. Ratnakaram ⇑ Sri Venkateswara University, Tirupathi 517 502, AP, India
a r t i c l e
i n f o
Article history: Received 29 May 2010 Received in revised form 21 December 2010 Accepted 30 December 2010 Available online 31 January 2011 Keywords: Borate glass Rare earths Judd–Ofelt theory Optical properties
a b s t r a c t This article reports the effect of mixed alkalis on the optical absorption and emission spectra of Sm3+ and Dy3+ doped chloroborate glass matrices of the compositions 69.5B2O3xLiCl(30 x)NaCl0.5R2O3 and 69.5B2O3xLiCl(30 x)KCl0.5R2O3 (where R = Sm and Dy and x = 5, 10, 15, 20 and 25). Using Judd–Ofelt theory, the spectral intensities and Judd–Ofelt intensity parameters (X2, X4 and X6) were obtained from the measured absorption bands of the spectra. Using these intensity parameters, total radiative transition probabilities (AT), radiative lifetimes (sR), branching ratios (b) and peak emission cross-sections (rP) were obtained for the two rare earth ions in these two glass matrices. Variation of these parameters with x in the glass matrix has been discussed. It is found that for Sm3+ ion, the transition, 4G5/2 ? 6H9/2 shows highest emission cross-section and is maximum at x = 10 mol% in lithium–sodium and at x = 20 mol% in lithium–potassium glass matrices. For Dy3+ ion, the transition, 4F9/2 ? 6H13/2 shows highest emission cross-section and is maximum at x = 20 mol% in lithium–sodium and at x = 10 mol% in lithium–potassium glass matrix. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The electronic energy levels of rare earth ions determine the lasing characteristics of rare earth doped materials and are influenced by the presence of other ions in their vicinity [1,2]. The radiative properties of rare earth ions doped in different host matrices are different from that of a free ion due to local perturbations. Additional changes will be observed if there are other ions present in the neighborhood of the first. These studies are extensively investigated in crystals and glasses by several authors to understand the role of rare earth ion on efficiencies of lasers, amplifiers and up-converters [3–6]. Borate glasses are considered to be suitable hosts for optical materials because of their transparency, low melting point and good rare earth ion solubility [7,8]. But the applications are restricted due to their high phonon energy. Borate glasses in particular, have been the subject of numerous infrared studies in order to study their structural peculiarities [9,10]. The vibration modes of borate net work are seen to be active mainly in three different spectral regions (1200–1600 cm1), (800–1200 cm1) and at 700 cm1 [11]. ESR has been used to study intrinsic spin centers in alkali borate, silicate and phase separated borosilicate glasses. The mixed alkali effect is a term describing the nonlinear variation in glass properties such as viscosity, glass transition tem⇑ Corresponding author. E-mail address: [email protected] (Y.C. Ratnakaram). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.12.022
perature and conductivity when one alkali in multicomponent glass is systematically replaced by another species [12–14]. This mixed alkali phenomenon is useful in manufacturing the low loss electrical glass and in understanding the chemical strengthening of glass [12]. Earlier, the authors have studied the effect of mixed alkalies on various spectroscopic properties of Sm3+ and Dy3+ ions [15]. Spectroscopic studies on various Sm3+ and Dy3+ doped glass systems have been reported by many researchers [16–19]. Among various rare earth doped glass materials, samarium containing glasses have received relatively less attention though it offers many interesting features. Samarium has promising characteristics for spectral hole burning studies [20]. The Sm3+ ion is one of the most interesting lanthanide ions to analyze the fluorescence properties of its emitting 4G5/2 level which exhibits high quantum efficiency and also shows different quenching emission channels. Similarly, the optical properties of Dy3+ ions in various glasses have attracted much practical interest because its 1.3 lm emission can be utilized for the optical amplification and its visible up conversion emission can be used as a solid state laser [21]. The emission and the laser action at the wavelengths of 2.9 and 4.4 lm from Dy3+ doped in different crystals and glasses have been reported by several authors [22,23]. In the present work, the authors have studied the spectral properties of Sm3+ and Dy3+ doped lithium–sodium and lithium–potassium chloroborate glasses keeping in mind the effect of mixed alkalis on various spectroscopic parameters and the relationship between the structural modifications and various parameters.
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Judd–Ofelt intensity parameters (X2, X4 and X6) were obtained from the measured absorption bands of the spectra. From these three intensity parameters, total radiative transition probabilities (AT), radiative lifetimes (sR), branching ratios (b) and peak emission cross-sections (rP) are calculated in these two glass matrices and variation of these parameters with the glass composition has been discussed.
tained under excitation wavelengths 400 nm (for Sm3+) and 348 nm (for Dy3+) using SPEX Fluorolog – 2 fluorometer (Model-II). 3. Results and discussion 3.1. Absorption spectra: Judd–Ofelt analysis
In the present work, two different mixed alkali chloroborate glasses were prepared using standard melt quenching technique. The chemicals used in the present work are H3BO3, LiCl, NaCl, KCl, Sm2O3, and Dy2O3. These chemicals are obtained from Himedia laboratories, Mumbai, India. All these chemicals are analar grade with 99.9% purity. The final glass compositions studied in the present work are 69.5B2O3xLiCl(30 x)NaCl0.5R2O3 (lithium– sodium) and 69.5B2O3xLiCl(30 x)KCl0.5R2O3 (lithium–potassium) (where R = Sm and Dy and x = 5, 10, 15, 20 and 25). About 6–10 g of the batch composition was thoroughly mixed and ground in an agate mortar. This homogeneous mixture was taken into crucible and heated in an electric furnace at a temperature of 900 °C– 1000 °C for 1–2 h. Then the melt was quenched between two well polished preheated brass plates and glass samples in a size of 1 cm diameter and 1 mm thickness were obtained. These samples were polished for optical measurements. Refractive index measurements were performed using an Abbe refractometer at sodium wavelength (589.3 nm) with monobromonaphthalene as contact liquid. The density of the glasses was determined by Archimedes’ method using xylene as immersion liquid. Absorption spectra were recorded on a JASCO-V570 spectrophotometer in the wavelength regions 350–1650 nm (for Sm3+) and 400–1800 nm (for Dy3+) with a spectral resolution of 1 nm. The luminescence spectra were ob-
Figs. 1 and 2 show the absorption spectra of Sm3+ and Dy3+ ions doped lithium–sodium chloroborate glasses for different x values in the glass matrix. Except for small variations in spectral intensities of a few bands, the absorption spectra of these two ions in lithium–potassium glass matrix are similar to the lithium–sodium glass matrix. Hence the spectra are not shown. In the present work, nine absorption transitions are observed for Sm3+ ion and seven absorption transitions are observed for Dy3+ ion in the two glass matrices. These absorption transitions start from 6H5/2 in the case of Sm3+ ion and 6H15/2 in the case of Dy3+ ion. The Judd–Ofelt theory [24,25] has been applied to analyze the spectral characteristics of the two ions in these glass matrices. The best set of three Judd–Ofelt parameters (X2, X4 and X6) is obtained from the experimental spectral intensities and the doubly reduced matrix elements using the procedure explained in Ref. [26]. These values are presented in Table 1 for both Sm3+ and Dy3+ ions in all the glass matrices studied. It is observed that for Sm3+ ion, in lithium–sodium glass matrix, X2 and X6 parameters are minimum at x = 25 mol% (i.e., at lithium rich) indicating lower covalency of Sm–O bond and lower rigidity of glass matrix at x = 25 mol%. These values are maximum at x = 10 mol% in this glass matrix indicating higher covalency of Sm–O bond and higher rigidity of glass matrix. In lithium–potassium glass matrix, X2 and X6 parameters are minimum at x = 15 mol% (i.e. at equal mol% of lithium and potassium) indicating lower covalency and lower rigidity of the glass matrix at this composition.
Fig. 1. Absorption spectra of Sm3+ doped lithium–sodium chloroborate glasses (x in mol%) (all the absorption transitions start from the ground state, 6H5/2).
Fig. 2. Absorption spectra of Dy3+ doped lithium–sodium chloroborate glasses (x in mol%) (all the absorption transitions start from the ground state, 6H15/2).
2. Experimental
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Parameter
Lithium–sodium x=5
Samarium 1 2 3 Dysprosium 1 2 3
Lithium–potassium
x = 10
x = 15
x = 20
x = 25
x=5
x = 10
x = 15
x = 20
x = 25
X2 X4 X6
9.20 4.73 3.97
13.14 9.68 7.57
11.09 8.38 6.15
8.56 6.13 5.23
1.72 1.94 1.40
7.36 5.97 4.63
3.36 3.79 2.29
3.35 3.77 1.86
7.47 6.24 4.56
6.34 6.05 3.95
X2 X4 X6
18.13 4.79 4.48
13.79 3.80 3.59
6.15 1.82 2.29
26.63 7.75 6.63
11.70 3.22 3.07
9.73 1.69 3.37
15.46 4.75 3.45
5.03 1.34 1.78
9.13 3.09 1.94
12.21 3.45 4.01
For Dy3+ ion, in lithium–sodium glass matrix, X2 and X6 parameters are minimum at x = 15 mol% (at equal mol% of lithium and sodium) indicating lower covalency of Dy–O bond and lower rigidity of the glass matrix. These parameters are maximum at x = 20 mol% indicating higher covalency and higher rigidity of the glass matrix. In lithium–potassium glass matrix also, X2 and X6 parameters are minimum at x = 15 mol% (at equal mol% of lithium and potassium) indicating lower covalency of Dy–O bond and lower rigidity of the glass matrix. Variation of X2, X4 and X6 parameters with x in Sm3+ and Dy3+ doped lithium–sodium and lithium–potassium chloroborate glass matrices is shown in Figs. 3 and 4 respectively. It is observed that for Sm3+ ion, in the case of lithium–sodium glass matrix, X2, X4 and X6 parameters increased at x = 10 mol%
and then decreased for x = 15, 20 and 25 mol% while they are minimum at x = 25 mol% (i.e. at higher lithium content). But in the case of lithium–potassium glass matrix, these parameters decreased for x = 5, 10 and 15 mol% and then increased at x = 20 mol%. For Dy3+ ion, in the case of lithium–sodium glass matrix, X2, X4 and X6 parameters are decreased for x = 5, 10 and 15 mol% and then increased at x = 20 mol%. For lithium–potassium glass matrix these parameters increased at x = 10 mol% and then decreased at x = 15 mol% and further increased for x = 20–25 mol%. Xk parameter can also be written as [27]
Xk ¼ ð2k þ 1Þ
X
jAt;p j2 N2 ðt; kÞð2t þ 1Þ1 ;
k ¼ 2; 4; 6
ð1Þ
t;p
Fig. 3. Variation of Xk (k = 2, 4 and 6) parameter with the variation of x in Sm3+ doped (a) lithium–sodium (LSCB) and (b) lithium–potassium (LPCB) chloroborate glasses.
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Fig. 4. Variation of Xk (k = 2, 4 and 6) parameter with the variation of x in Dy3+ doped (a) lithium–sodium (LSCB) and (b) lithium–potassium (LPCB) chloroborate glasses.
where At,p are the components of the crystal field operator and depend on the symmetry of the crystal field around rare earth ions. N(t, k) is a function of radial integral and depends reciprocally on the energy separations of the 4f level and admixing levels e.g. 5d, 5g. The sum over k includes only the even values 2, 4 and 6 whereas the sum over t includes only the odd values 1, 3 and 5. It has been suggested by Reisfeld [27] that N is related to the nephelauxetic parameter b which indicates the degree of covalency of RE–O bond.
3.2. Hypersensitive transitions The band positions and spectral intensities of certain transitions of rare earth ions are found to be very sensitive to the environment of rare earth ion. These transitions follow the selection rules, |DS| = 0, |DL| P 0 and |DJ| P 0 and are called hypersensitive transitions. 6H5/2 ? 6F1/2 is the hypersensitive transition for Sm3+ ion and 6 H15/2 ? 6F11/2, 6H9/2 is the hypersensitive transition for Dy3+ ion.
Table 2 Radiative lifetimes (sR) (ls) of certain excited states of Sm3+ and Dy3+ doped chloroborate glasses. S. No.
Samarium 1 2 3 4 5 6 7 Dysprosium 1 2 3 4 5
Excited level
4
G5/2 F11/2 F9/2 6 F7/2 6 F5/2 6 F3/2 6 F1/2 6 6
4
I15/2 F9/2 F3/2 6 F5/2 6 F11/2 4 6
Lithium–sodium
Lithium–potassium
x=5
x = 10
x = 15
x = 20
x = 25
x=5
x = 10
x = 15
x = 20
x = 25
1773 668 723 839 980 1039 1021
981 355 386 459 562 621 631
1159 430 466 548 666 731 747
1516 532 582 699 864 961 980
5543 1914 2131 2589 3267 3825 4103
1642 591 647 772 953 1071 1096
2966 1130 1221 1446 1786 2041 2144
3059 1259 1327 1540 1846 2065 2135
1601 588 636 756 928 1041 1064
1753 653 710 840 1031 1157 1206
750 351 366 329 340
958 455 464 413 446
1745 896 790 706 960
517 236 248 217 228
1123 534 536 480 521
1201 601 554 502 663
2144 905 415 449 395
2198 1132 986 916 1196
1552 700 756 674 645
952 471 431 388 489
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The shift in peak wavelength of the hypersensitive transition towards shorter wavelength with the increase of x in the glass matrix (because of nephelauxetic effect [27]) indicates decrease in covalency of RE–O bond. For the present work, the peak wavelengths (kpeak) (nm) of the hypersensitive transitions and X2 (1020 cm2) parameters of Sm3+ and Dy3+ ions in lithium–sodium and lithium–potassium glass matrices are given below. Sm3+ x
Lithium– sodium
(in kpeak mol%) x=5 x = 10 x = 15 x = 20 x = 25
Dy3+
X2
Lithium– potassium kpeak
X2
Lithium– sodium kpeak
1523.2 9.20 1522.2 7.36 1255.2 1520.3 13.14 1524.6 3.36 1257.1 1521.6 11.09 1520.2 3.35 1256.8 1520.7 8.56 1520.0 7.47 1258.1 1523.6 1.72 1522.2 6.34 1254.3
Lithium– potassium
X2
kpeak
X2
18.13 13.79 6.15 26.63 11.70
1257.2 9.73 1255.0 15.46 1257.4 5.03 1254.5 9.13 1257.0 12.21
From the above data, it is observed that for Sm3+ ion, in lithium–sodium glass matrix, the peak wavelength of the hypersensitive transition shifts towards shorter wavelength for x = 5–10 mol%. But the X2 parameter increased from 9.20 to 13.14. Similarly for x = 20– 25 mol%, there is a shift in peak wavelength towards longer wavelength but the X2 parameter decreased largely (8.56–1.72). From Eq. (1) these changes may be due to structural changes that might take place at these compositions. The results of earlier studies on borate glasses [28] indicate that in alkali borate glasses, at x = 15– 20 mol%, concentration of tetra borate groups is maximum and boroxyl groups disappear from x = 20 mol% and formation of di-borates takes place. This peculiar and anomalous behavior is referred to as boron anomaly and was first explained in terms of the unique ability of boron to exist in two distinct co-ordination states, the trigonal and tetrahedral. These structural changes might be responsible for the variation of X2 parameter at x = 5–10 mol% and at x = 20–25 mol% in the above glass matrix. Similar type of variations are observed in the case of lithium–potassium glass matrix indicating the structural changes at x = 5–10 mol% and 20–25 mol%.
Table 3 Branching ratios (b) of certain transitions of Sm3+ and Dy3+ in lithium–sodium and lithium–potassium chloroborate glasses. S. No.
Samarium 1 2 3 4 5 Dysprosium 1 2 3 4 5
Transition
Lithium–sodium
Lithium–potassium
x=5
x = 10
x = 15
x = 20
x = 25
x=5
x = 10
x = 15
x = 20
x = 25
G5/2 ? 6H9/2 F7/2 ? 6H5/2 6 F5/2 ? 6H7/2 6 F3/2 ? 6H5/2 6 F1/2 ? 6H5/2
0.400 0.384 0.431 0.448 0.714
0.356 0.143 0.416 0.440 0.637
0.356 0.407 0.409 0.443 0.634
0.355 0.421 0.426 0.436 0.641
0.312 0.440 0.396 0.433 0.539
0.345 0.421 0.413 0.439 0.616
0.320 0.420 0.378 0.444 0.547
0.327 0.398 0.356 0.454 0.550
0.344 0.416 0.405 0.441 0.610
0.334 0.415 0.390 0.443 0.583
4
0.667 0.706 0.470 0.494 0.922
0.669 0.701 0.474 0.497 0.921
0.704 0.675 0.508 0.540 0.911
0.662 0.703 0.466 0.466 0.919
0.671 0.701 0.474 0.498 0.921
0.728 0.687 0.528 0.567 0.914
0.642 0.709 0.446 0.462 0.924
0.705 0.680 0.507 0.543 0.913
0.628 0.710 0.430 0.445 0.925
0.694 0.685 0.497 0.526 0.915
4 6
I15/2 ? 6H15/2 F9/2 ? 6H13/2 6 F3/2 ? 6H13/2 6 F5/2 ? 6H15/2 6 F11/2 ? 6H15/2 4
Fig. 5. Emission spectra of Sm3+ and Dy3+ doped lithium–sodium chloroborate glasses (x in mol%).
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Table 4 Emission cross-sections (rP 1021) (cm2) of the observed emission transitions of Sm3+ and Dy3+ doped chloroborate glasses. S. No.
Samarium 1 2 3 4 Dysprosium 1 2
Transition
Lithium–sodium
Lithium–potassium
x=5
x = 10
x = 15
x = 20
x = 25
x=5
x = 10
x = 15
x = 20
x = 25
G5/2 ? 6H5/2 G5/2 ? 6H7/2 4 G5/2 ? 6H9/2 4 G5/2 ? 6H11/2
0.511 0.697 1.315 0.140
1.074 1.601 2.034 0.442
0.939 1.313 1.772 0.355
0.728 1.022 1.471 0.287
0.220 0.312 0.331 0.084
0.665 0.907 1.239 0.395
0.387 0.498 0.598 0.222
0.371 0.496 0.579 0.167
0.717 1.017 1.307 0.320
0.688 0.928 1.188 0.295
4
0.667 6.296
0.563 5.074
0.345 2.435
1.059 9.788
0.481 4.328
0.518 4.282
0.569 5.652
0.272 2.057
0.319 3.413
0.687 5.381
4 4
F9/2 ? 6H15/2 F9/2 ? 6H13/2
4
Fig. 6a. Variation of emission cross-sections (rP) with the variation of x in Sm3+ doped lithium–sodium (LSCB) and lithium–potassium (LPCB) chloroborate glasses.
For Dy3+ ion, in lithium–sodium and lithium–potassium glass matrices, the variation in shift in peak wavelength of the HST and X2 parameter at x = 5–10 mol% is similar to the Sm3+ ion indicating structural changes, but at x = 20–25 mol%, the structural changes are not influencing the covalency of Dy–O bond in these two glass matrices. In addition to the above some more variations are observed at x = 10–15 mol% and 15–20 mol% in the case of lithium–potassium glass matrix. 3.3. Radiative lifetimes Using the three Judd–Ofelt intensity parameters (Xk), radiative transition probabilities (A) of different transitions of Sm3+ and Dy3+ ions, radiative lifetimes (sR) of the excited states 4G5/2, 6F11/2, 6F9/2, 6 F7/2, 6F5/2, 6F3/2 and 6F1/2 of Sm3+ and the excited states 4I15/2, 4F9/2, 6 F3/2, 6F5/2 and 6F11/2(6H9/2) of Dy3+ and branching ratios (bR) are calculated using the formulae given in our earlier article [26]. The estimated radiative lifetimes of the above excited states are presented in Table 2 for both the ions in the two glass matrices. It is observed that among various excited states of Sm3+ ion, 6F11/ 4 2 state has lowest lifetime and G5/2 state has highest lifetime values in the two glass matrices. It is also observed that in lithium–sodium glass matrix, the lifetime values decreased at x = 5 mol% to 10 mol% and increased with further increase of lithium content in the glass matrix. In lithium–potassium glass matrix, the lifetime
values increased with the increase of lithium content up to 15 mol% but decreased for further increase of x to 20 mol%. In the case of Dy3+ ion, among the five excited states, 4I15/2 and 6F5/2 shows highest and lowest radiative lifetimes for all the x values (except at x = 10 and x = 20 mol% in lithium–potassium glass matrix) in the two glass matrices. It is also observed that the lifetime values increased with the increase of lithium content up to 15 mol% but decreased at 20 mol% in lithium–sodium glass. In lithium–potassium glass matrix, there is no specific order. Table 3 gives branching ratios (bR) of certain transitions (which have higher in magnitudes among various transitions) of Sm3+ and Dy3+ ions for different x values in both lithium–sodium and lithium–potassium glass matrices. It is observed that the branching ratio values are higher for 6F1/2 ? 6H5/2 transition of Sm3+ at x = 5 mol% in both the glass matrices. In the case of Dy3+ ion, 4F11/2 ? 6H15/2 transition shows higher branching ratio at x = 5 mol% in lithium–sodium and at x = 20 mol% in lithium–potassium glass matrices. Hence these transitions may be suggested for laser excitation from the magnitude of branching ratio parameters. 3.4. Emission spectra Fig. 5 shows the emission spectra of Sm3+ and Dy3+ ions in lithium–sodium glass matrix for different x values in the glass matrix. Due to similarity of the emission spectra, the spectra of Sm3+ and
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805
Fig. 6b. Variation of emission cross-sections (rP) with the variation of x in Dy3+ doped lithium–sodium (LSCB) and lithium–potassium (LPCB) chloroborate glasses.
Dy3+ ions in lithium–potassium glass matrix have not been shown. In the emission spectra, four emission peaks are observed for Sm3+ ion and are designated as 4G5/2 ? 6H5/2, 4G5/2 ? 6H7/2, 4G5/2 ? 6H9/2 and 4G5/2 ? 6H11/2. In the case of Dy3+ ion, two emission peaks are observed and are designated as 4F9/2 ? 6H15/2 and 4F9/2 ? 6H13/2. Peak stimulated emission cross-sections (rP) are calculated for all the observed transitions in all the glass matrices using the formulae given in Ref. [26] and are presented in Table 4. It is observed that among the four transitions of Sm3+ ion, 4G5/2 ? 6H9/2 transition shows highest emission cross-sections for all the x values in the two glass matrices. Among the five x values in the glass matrix, this transition shows highest cross-section at x = 10 mol% in lithium–sodium glass matrix and at x = 20 mol% in lithium–potassium glass matrix. In the case of Dy3+ ion, 4F9/2 ? 6H13/2 transition shows highest emission cross-section for all the x values in the two glass matrices. Among the five glass compositions, this transition shows highest cross-sections at x = 20 mol% and at x = 10 mol% in lithium–sodium and lithium–potassium glass matrices respectively. Hence these two transitions are more useful for laser excitation at these compositions. Variation of emission cross-sections (rP) of different transitions with the variation of x in lithium–sodium and lithium–potassium glass matrices is shown in Figs. 6a and 6b for both the ions. 4. Conclusions The magnitudes of X2 and X6 parameters are minimum at x = 25 mol% (i.e. at lithium rich) in Sm3+ doped lithium–sodium chloroborate glass matrix indicating lower covalency of Sm–O bond and lower rigidity of the glass matrix. In lithium–potassium glass matrix, these parameters are minimum at x = 15 mol% (at equal mol% of lithium and potassium) indicating lower covalency of Sm–O bond and lower rigidity of the glass matrix at this composition. For Dy3+ ion, X2 and X6 parameters are minimum at x = 15 mol% in lithium–sodium and lithium–potassium glass matrices (at equal mol% of lithium, sodium and lithium, potassium) indicating lower covalency of Dy–O bond and lower rigidity of the glass matrix. From the variation of shift in peak wavelength of the hypersensitive transition and X2 parameter with x in the glass
matrix, it is concluded that there are structural changes at x = 5–10 mol% and at x = 20–25 mol% in Sm3+ doped lithium–sodium and lithium–potassium chloroborate glass matrices. In the case of Dy3+ ion, in both lithium–sodium and lithium–potassium glass matrices, structural changes are observed at x = 5–10 mol%, but at x = 20–25 mol% the structural changes do not influence the covalency of Dy–O bond. From the magnitude of peak emission cross-sections, the emission transition 4G5/2 ? 6H9/2 of Sm3+ ion at x = 10 mol% in lithium–sodium glass matrix and at x = 20 mol% in lithium–potassium glass matrix is useful for laser excitation. For Dy3+ ion, the emission transition, 4F9/2 ? 6H13/2 at x = 20 mol% and at x = 10 mol% in lithium–sodium and lithium–potassium glass matrices respectively are useful for laser excitation. Acknowledgements One of the authors (YCR) expresses his thanks to University Grants Commission for providing financial assistance in the form of a Major Research Project. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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[25] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [26] Y.C. Ratnakaram, A. Vijaya Kumar, D. Thirupathi Naidu, J.L. Rao, Spectrochim. Acta Part A 61 (2005) 2169. [27] R. Reisfeld, Struct. Bond. 22 (1975) 83. [28] H. Rawson, Inorganic Glass Forming Systems, Academic Press, London, 1967.
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Influence of mixed alkalis on spectroscopic parameters of Sm3+, Dy3+ doped chloroborate glasses C. Venkateswarlu, M. Seshadri, Y.C. Ratnakaram ⇑ Sri Venkateswara University, Tirupathi 517 502, AP, India
a r t i c l e
i n f o
Article history: Received 29 May 2010 Received in revised form 21 December 2010 Accepted 30 December 2010 Available online 31 January 2011 Keywords: Borate glass Rare earths Judd–Ofelt theory Optical properties
a b s t r a c t This article reports the effect of mixed alkalis on the optical absorption and emission spectra of Sm3+ and Dy3+ doped chloroborate glass matrices of the compositions 69.5B2O3xLiCl(30 x)NaCl0.5R2O3 and 69.5B2O3xLiCl(30 x)KCl0.5R2O3 (where R = Sm and Dy and x = 5, 10, 15, 20 and 25). Using Judd–Ofelt theory, the spectral intensities and Judd–Ofelt intensity parameters (X2, X4 and X6) were obtained from the measured absorption bands of the spectra. Using these intensity parameters, total radiative transition probabilities (AT), radiative lifetimes (sR), branching ratios (b) and peak emission cross-sections (rP) were obtained for the two rare earth ions in these two glass matrices. Variation of these parameters with x in the glass matrix has been discussed. It is found that for Sm3+ ion, the transition, 4G5/2 ? 6H9/2 shows highest emission cross-section and is maximum at x = 10 mol% in lithium–sodium and at x = 20 mol% in lithium–potassium glass matrices. For Dy3+ ion, the transition, 4F9/2 ? 6H13/2 shows highest emission cross-section and is maximum at x = 20 mol% in lithium–sodium and at x = 10 mol% in lithium–potassium glass matrix. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The electronic energy levels of rare earth ions determine the lasing characteristics of rare earth doped materials and are influenced by the presence of other ions in their vicinity [1,2]. The radiative properties of rare earth ions doped in different host matrices are different from that of a free ion due to local perturbations. Additional changes will be observed if there are other ions present in the neighborhood of the first. These studies are extensively investigated in crystals and glasses by several authors to understand the role of rare earth ion on efficiencies of lasers, amplifiers and up-converters [3–6]. Borate glasses are considered to be suitable hosts for optical materials because of their transparency, low melting point and good rare earth ion solubility [7,8]. But the applications are restricted due to their high phonon energy. Borate glasses in particular, have been the subject of numerous infrared studies in order to study their structural peculiarities [9,10]. The vibration modes of borate net work are seen to be active mainly in three different spectral regions (1200–1600 cm1), (800–1200 cm1) and at 700 cm1 [11]. ESR has been used to study intrinsic spin centers in alkali borate, silicate and phase separated borosilicate glasses. The mixed alkali effect is a term describing the nonlinear variation in glass properties such as viscosity, glass transition tem⇑ Corresponding author. E-mail address: [email protected] (Y.C. Ratnakaram). 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.12.022
perature and conductivity when one alkali in multicomponent glass is systematically replaced by another species [12–14]. This mixed alkali phenomenon is useful in manufacturing the low loss electrical glass and in understanding the chemical strengthening of glass [12]. Earlier, the authors have studied the effect of mixed alkalies on various spectroscopic properties of Sm3+ and Dy3+ ions [15]. Spectroscopic studies on various Sm3+ and Dy3+ doped glass systems have been reported by many researchers [16–19]. Among various rare earth doped glass materials, samarium containing glasses have received relatively less attention though it offers many interesting features. Samarium has promising characteristics for spectral hole burning studies [20]. The Sm3+ ion is one of the most interesting lanthanide ions to analyze the fluorescence properties of its emitting 4G5/2 level which exhibits high quantum efficiency and also shows different quenching emission channels. Similarly, the optical properties of Dy3+ ions in various glasses have attracted much practical interest because its 1.3 lm emission can be utilized for the optical amplification and its visible up conversion emission can be used as a solid state laser [21]. The emission and the laser action at the wavelengths of 2.9 and 4.4 lm from Dy3+ doped in different crystals and glasses have been reported by several authors [22,23]. In the present work, the authors have studied the spectral properties of Sm3+ and Dy3+ doped lithium–sodium and lithium–potassium chloroborate glasses keeping in mind the effect of mixed alkalis on various spectroscopic parameters and the relationship between the structural modifications and various parameters.
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Judd–Ofelt intensity parameters (X2, X4 and X6) were obtained from the measured absorption bands of the spectra. From these three intensity parameters, total radiative transition probabilities (AT), radiative lifetimes (sR), branching ratios (b) and peak emission cross-sections (rP) are calculated in these two glass matrices and variation of these parameters with the glass composition has been discussed.
tained under excitation wavelengths 400 nm (for Sm3+) and 348 nm (for Dy3+) using SPEX Fluorolog – 2 fluorometer (Model-II). 3. Results and discussion 3.1. Absorption spectra: Judd–Ofelt analysis
In the present work, two different mixed alkali chloroborate glasses were prepared using standard melt quenching technique. The chemicals used in the present work are H3BO3, LiCl, NaCl, KCl, Sm2O3, and Dy2O3. These chemicals are obtained from Himedia laboratories, Mumbai, India. All these chemicals are analar grade with 99.9% purity. The final glass compositions studied in the present work are 69.5B2O3xLiCl(30 x)NaCl0.5R2O3 (lithium– sodium) and 69.5B2O3xLiCl(30 x)KCl0.5R2O3 (lithium–potassium) (where R = Sm and Dy and x = 5, 10, 15, 20 and 25). About 6–10 g of the batch composition was thoroughly mixed and ground in an agate mortar. This homogeneous mixture was taken into crucible and heated in an electric furnace at a temperature of 900 °C– 1000 °C for 1–2 h. Then the melt was quenched between two well polished preheated brass plates and glass samples in a size of 1 cm diameter and 1 mm thickness were obtained. These samples were polished for optical measurements. Refractive index measurements were performed using an Abbe refractometer at sodium wavelength (589.3 nm) with monobromonaphthalene as contact liquid. The density of the glasses was determined by Archimedes’ method using xylene as immersion liquid. Absorption spectra were recorded on a JASCO-V570 spectrophotometer in the wavelength regions 350–1650 nm (for Sm3+) and 400–1800 nm (for Dy3+) with a spectral resolution of 1 nm. The luminescence spectra were ob-
Figs. 1 and 2 show the absorption spectra of Sm3+ and Dy3+ ions doped lithium–sodium chloroborate glasses for different x values in the glass matrix. Except for small variations in spectral intensities of a few bands, the absorption spectra of these two ions in lithium–potassium glass matrix are similar to the lithium–sodium glass matrix. Hence the spectra are not shown. In the present work, nine absorption transitions are observed for Sm3+ ion and seven absorption transitions are observed for Dy3+ ion in the two glass matrices. These absorption transitions start from 6H5/2 in the case of Sm3+ ion and 6H15/2 in the case of Dy3+ ion. The Judd–Ofelt theory [24,25] has been applied to analyze the spectral characteristics of the two ions in these glass matrices. The best set of three Judd–Ofelt parameters (X2, X4 and X6) is obtained from the experimental spectral intensities and the doubly reduced matrix elements using the procedure explained in Ref. [26]. These values are presented in Table 1 for both Sm3+ and Dy3+ ions in all the glass matrices studied. It is observed that for Sm3+ ion, in lithium–sodium glass matrix, X2 and X6 parameters are minimum at x = 25 mol% (i.e., at lithium rich) indicating lower covalency of Sm–O bond and lower rigidity of glass matrix at x = 25 mol%. These values are maximum at x = 10 mol% in this glass matrix indicating higher covalency of Sm–O bond and higher rigidity of glass matrix. In lithium–potassium glass matrix, X2 and X6 parameters are minimum at x = 15 mol% (i.e. at equal mol% of lithium and potassium) indicating lower covalency and lower rigidity of the glass matrix at this composition.
Fig. 1. Absorption spectra of Sm3+ doped lithium–sodium chloroborate glasses (x in mol%) (all the absorption transitions start from the ground state, 6H5/2).
Fig. 2. Absorption spectra of Dy3+ doped lithium–sodium chloroborate glasses (x in mol%) (all the absorption transitions start from the ground state, 6H15/2).
2. Experimental
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C. Venkateswarlu et al. / Optical Materials 33 (2011) 799–806 Table 1 Judd–Ofelt intensity parameters (Xk 1020) (cm2) of Sm3+ and Dy3+ doped chloroborate glasses. S. No.
Parameter
Lithium–sodium x=5
Samarium 1 2 3 Dysprosium 1 2 3
Lithium–potassium
x = 10
x = 15
x = 20
x = 25
x=5
x = 10
x = 15
x = 20
x = 25
X2 X4 X6
9.20 4.73 3.97
13.14 9.68 7.57
11.09 8.38 6.15
8.56 6.13 5.23
1.72 1.94 1.40
7.36 5.97 4.63
3.36 3.79 2.29
3.35 3.77 1.86
7.47 6.24 4.56
6.34 6.05 3.95
X2 X4 X6
18.13 4.79 4.48
13.79 3.80 3.59
6.15 1.82 2.29
26.63 7.75 6.63
11.70 3.22 3.07
9.73 1.69 3.37
15.46 4.75 3.45
5.03 1.34 1.78
9.13 3.09 1.94
12.21 3.45 4.01
For Dy3+ ion, in lithium–sodium glass matrix, X2 and X6 parameters are minimum at x = 15 mol% (at equal mol% of lithium and sodium) indicating lower covalency of Dy–O bond and lower rigidity of the glass matrix. These parameters are maximum at x = 20 mol% indicating higher covalency and higher rigidity of the glass matrix. In lithium–potassium glass matrix also, X2 and X6 parameters are minimum at x = 15 mol% (at equal mol% of lithium and potassium) indicating lower covalency of Dy–O bond and lower rigidity of the glass matrix. Variation of X2, X4 and X6 parameters with x in Sm3+ and Dy3+ doped lithium–sodium and lithium–potassium chloroborate glass matrices is shown in Figs. 3 and 4 respectively. It is observed that for Sm3+ ion, in the case of lithium–sodium glass matrix, X2, X4 and X6 parameters increased at x = 10 mol%
and then decreased for x = 15, 20 and 25 mol% while they are minimum at x = 25 mol% (i.e. at higher lithium content). But in the case of lithium–potassium glass matrix, these parameters decreased for x = 5, 10 and 15 mol% and then increased at x = 20 mol%. For Dy3+ ion, in the case of lithium–sodium glass matrix, X2, X4 and X6 parameters are decreased for x = 5, 10 and 15 mol% and then increased at x = 20 mol%. For lithium–potassium glass matrix these parameters increased at x = 10 mol% and then decreased at x = 15 mol% and further increased for x = 20–25 mol%. Xk parameter can also be written as [27]
Xk ¼ ð2k þ 1Þ
X
jAt;p j2 N2 ðt; kÞð2t þ 1Þ1 ;
k ¼ 2; 4; 6
ð1Þ
t;p
Fig. 3. Variation of Xk (k = 2, 4 and 6) parameter with the variation of x in Sm3+ doped (a) lithium–sodium (LSCB) and (b) lithium–potassium (LPCB) chloroborate glasses.
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Fig. 4. Variation of Xk (k = 2, 4 and 6) parameter with the variation of x in Dy3+ doped (a) lithium–sodium (LSCB) and (b) lithium–potassium (LPCB) chloroborate glasses.
where At,p are the components of the crystal field operator and depend on the symmetry of the crystal field around rare earth ions. N(t, k) is a function of radial integral and depends reciprocally on the energy separations of the 4f level and admixing levels e.g. 5d, 5g. The sum over k includes only the even values 2, 4 and 6 whereas the sum over t includes only the odd values 1, 3 and 5. It has been suggested by Reisfeld [27] that N is related to the nephelauxetic parameter b which indicates the degree of covalency of RE–O bond.
3.2. Hypersensitive transitions The band positions and spectral intensities of certain transitions of rare earth ions are found to be very sensitive to the environment of rare earth ion. These transitions follow the selection rules, |DS| = 0, |DL| P 0 and |DJ| P 0 and are called hypersensitive transitions. 6H5/2 ? 6F1/2 is the hypersensitive transition for Sm3+ ion and 6 H15/2 ? 6F11/2, 6H9/2 is the hypersensitive transition for Dy3+ ion.
Table 2 Radiative lifetimes (sR) (ls) of certain excited states of Sm3+ and Dy3+ doped chloroborate glasses. S. No.
Samarium 1 2 3 4 5 6 7 Dysprosium 1 2 3 4 5
Excited level
4
G5/2 F11/2 F9/2 6 F7/2 6 F5/2 6 F3/2 6 F1/2 6 6
4
I15/2 F9/2 F3/2 6 F5/2 6 F11/2 4 6
Lithium–sodium
Lithium–potassium
x=5
x = 10
x = 15
x = 20
x = 25
x=5
x = 10
x = 15
x = 20
x = 25
1773 668 723 839 980 1039 1021
981 355 386 459 562 621 631
1159 430 466 548 666 731 747
1516 532 582 699 864 961 980
5543 1914 2131 2589 3267 3825 4103
1642 591 647 772 953 1071 1096
2966 1130 1221 1446 1786 2041 2144
3059 1259 1327 1540 1846 2065 2135
1601 588 636 756 928 1041 1064
1753 653 710 840 1031 1157 1206
750 351 366 329 340
958 455 464 413 446
1745 896 790 706 960
517 236 248 217 228
1123 534 536 480 521
1201 601 554 502 663
2144 905 415 449 395
2198 1132 986 916 1196
1552 700 756 674 645
952 471 431 388 489
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The shift in peak wavelength of the hypersensitive transition towards shorter wavelength with the increase of x in the glass matrix (because of nephelauxetic effect [27]) indicates decrease in covalency of RE–O bond. For the present work, the peak wavelengths (kpeak) (nm) of the hypersensitive transitions and X2 (1020 cm2) parameters of Sm3+ and Dy3+ ions in lithium–sodium and lithium–potassium glass matrices are given below. Sm3+ x
Lithium– sodium
(in kpeak mol%) x=5 x = 10 x = 15 x = 20 x = 25
Dy3+
X2
Lithium– potassium kpeak
X2
Lithium– sodium kpeak
1523.2 9.20 1522.2 7.36 1255.2 1520.3 13.14 1524.6 3.36 1257.1 1521.6 11.09 1520.2 3.35 1256.8 1520.7 8.56 1520.0 7.47 1258.1 1523.6 1.72 1522.2 6.34 1254.3
Lithium– potassium
X2
kpeak
X2
18.13 13.79 6.15 26.63 11.70
1257.2 9.73 1255.0 15.46 1257.4 5.03 1254.5 9.13 1257.0 12.21
From the above data, it is observed that for Sm3+ ion, in lithium–sodium glass matrix, the peak wavelength of the hypersensitive transition shifts towards shorter wavelength for x = 5–10 mol%. But the X2 parameter increased from 9.20 to 13.14. Similarly for x = 20– 25 mol%, there is a shift in peak wavelength towards longer wavelength but the X2 parameter decreased largely (8.56–1.72). From Eq. (1) these changes may be due to structural changes that might take place at these compositions. The results of earlier studies on borate glasses [28] indicate that in alkali borate glasses, at x = 15– 20 mol%, concentration of tetra borate groups is maximum and boroxyl groups disappear from x = 20 mol% and formation of di-borates takes place. This peculiar and anomalous behavior is referred to as boron anomaly and was first explained in terms of the unique ability of boron to exist in two distinct co-ordination states, the trigonal and tetrahedral. These structural changes might be responsible for the variation of X2 parameter at x = 5–10 mol% and at x = 20–25 mol% in the above glass matrix. Similar type of variations are observed in the case of lithium–potassium glass matrix indicating the structural changes at x = 5–10 mol% and 20–25 mol%.
Table 3 Branching ratios (b) of certain transitions of Sm3+ and Dy3+ in lithium–sodium and lithium–potassium chloroborate glasses. S. No.
Samarium 1 2 3 4 5 Dysprosium 1 2 3 4 5
Transition
Lithium–sodium
Lithium–potassium
x=5
x = 10
x = 15
x = 20
x = 25
x=5
x = 10
x = 15
x = 20
x = 25
G5/2 ? 6H9/2 F7/2 ? 6H5/2 6 F5/2 ? 6H7/2 6 F3/2 ? 6H5/2 6 F1/2 ? 6H5/2
0.400 0.384 0.431 0.448 0.714
0.356 0.143 0.416 0.440 0.637
0.356 0.407 0.409 0.443 0.634
0.355 0.421 0.426 0.436 0.641
0.312 0.440 0.396 0.433 0.539
0.345 0.421 0.413 0.439 0.616
0.320 0.420 0.378 0.444 0.547
0.327 0.398 0.356 0.454 0.550
0.344 0.416 0.405 0.441 0.610
0.334 0.415 0.390 0.443 0.583
4
0.667 0.706 0.470 0.494 0.922
0.669 0.701 0.474 0.497 0.921
0.704 0.675 0.508 0.540 0.911
0.662 0.703 0.466 0.466 0.919
0.671 0.701 0.474 0.498 0.921
0.728 0.687 0.528 0.567 0.914
0.642 0.709 0.446 0.462 0.924
0.705 0.680 0.507 0.543 0.913
0.628 0.710 0.430 0.445 0.925
0.694 0.685 0.497 0.526 0.915
4 6
I15/2 ? 6H15/2 F9/2 ? 6H13/2 6 F3/2 ? 6H13/2 6 F5/2 ? 6H15/2 6 F11/2 ? 6H15/2 4
Fig. 5. Emission spectra of Sm3+ and Dy3+ doped lithium–sodium chloroborate glasses (x in mol%).
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Table 4 Emission cross-sections (rP 1021) (cm2) of the observed emission transitions of Sm3+ and Dy3+ doped chloroborate glasses. S. No.
Samarium 1 2 3 4 Dysprosium 1 2
Transition
Lithium–sodium
Lithium–potassium
x=5
x = 10
x = 15
x = 20
x = 25
x=5
x = 10
x = 15
x = 20
x = 25
G5/2 ? 6H5/2 G5/2 ? 6H7/2 4 G5/2 ? 6H9/2 4 G5/2 ? 6H11/2
0.511 0.697 1.315 0.140
1.074 1.601 2.034 0.442
0.939 1.313 1.772 0.355
0.728 1.022 1.471 0.287
0.220 0.312 0.331 0.084
0.665 0.907 1.239 0.395
0.387 0.498 0.598 0.222
0.371 0.496 0.579 0.167
0.717 1.017 1.307 0.320
0.688 0.928 1.188 0.295
4
0.667 6.296
0.563 5.074
0.345 2.435
1.059 9.788
0.481 4.328
0.518 4.282
0.569 5.652
0.272 2.057
0.319 3.413
0.687 5.381
4 4
F9/2 ? 6H15/2 F9/2 ? 6H13/2
4
Fig. 6a. Variation of emission cross-sections (rP) with the variation of x in Sm3+ doped lithium–sodium (LSCB) and lithium–potassium (LPCB) chloroborate glasses.
For Dy3+ ion, in lithium–sodium and lithium–potassium glass matrices, the variation in shift in peak wavelength of the HST and X2 parameter at x = 5–10 mol% is similar to the Sm3+ ion indicating structural changes, but at x = 20–25 mol%, the structural changes are not influencing the covalency of Dy–O bond in these two glass matrices. In addition to the above some more variations are observed at x = 10–15 mol% and 15–20 mol% in the case of lithium–potassium glass matrix. 3.3. Radiative lifetimes Using the three Judd–Ofelt intensity parameters (Xk), radiative transition probabilities (A) of different transitions of Sm3+ and Dy3+ ions, radiative lifetimes (sR) of the excited states 4G5/2, 6F11/2, 6F9/2, 6 F7/2, 6F5/2, 6F3/2 and 6F1/2 of Sm3+ and the excited states 4I15/2, 4F9/2, 6 F3/2, 6F5/2 and 6F11/2(6H9/2) of Dy3+ and branching ratios (bR) are calculated using the formulae given in our earlier article [26]. The estimated radiative lifetimes of the above excited states are presented in Table 2 for both the ions in the two glass matrices. It is observed that among various excited states of Sm3+ ion, 6F11/ 4 2 state has lowest lifetime and G5/2 state has highest lifetime values in the two glass matrices. It is also observed that in lithium–sodium glass matrix, the lifetime values decreased at x = 5 mol% to 10 mol% and increased with further increase of lithium content in the glass matrix. In lithium–potassium glass matrix, the lifetime
values increased with the increase of lithium content up to 15 mol% but decreased for further increase of x to 20 mol%. In the case of Dy3+ ion, among the five excited states, 4I15/2 and 6F5/2 shows highest and lowest radiative lifetimes for all the x values (except at x = 10 and x = 20 mol% in lithium–potassium glass matrix) in the two glass matrices. It is also observed that the lifetime values increased with the increase of lithium content up to 15 mol% but decreased at 20 mol% in lithium–sodium glass. In lithium–potassium glass matrix, there is no specific order. Table 3 gives branching ratios (bR) of certain transitions (which have higher in magnitudes among various transitions) of Sm3+ and Dy3+ ions for different x values in both lithium–sodium and lithium–potassium glass matrices. It is observed that the branching ratio values are higher for 6F1/2 ? 6H5/2 transition of Sm3+ at x = 5 mol% in both the glass matrices. In the case of Dy3+ ion, 4F11/2 ? 6H15/2 transition shows higher branching ratio at x = 5 mol% in lithium–sodium and at x = 20 mol% in lithium–potassium glass matrices. Hence these transitions may be suggested for laser excitation from the magnitude of branching ratio parameters. 3.4. Emission spectra Fig. 5 shows the emission spectra of Sm3+ and Dy3+ ions in lithium–sodium glass matrix for different x values in the glass matrix. Due to similarity of the emission spectra, the spectra of Sm3+ and
C. Venkateswarlu et al. / Optical Materials 33 (2011) 799–806
805
Fig. 6b. Variation of emission cross-sections (rP) with the variation of x in Dy3+ doped lithium–sodium (LSCB) and lithium–potassium (LPCB) chloroborate glasses.
Dy3+ ions in lithium–potassium glass matrix have not been shown. In the emission spectra, four emission peaks are observed for Sm3+ ion and are designated as 4G5/2 ? 6H5/2, 4G5/2 ? 6H7/2, 4G5/2 ? 6H9/2 and 4G5/2 ? 6H11/2. In the case of Dy3+ ion, two emission peaks are observed and are designated as 4F9/2 ? 6H15/2 and 4F9/2 ? 6H13/2. Peak stimulated emission cross-sections (rP) are calculated for all the observed transitions in all the glass matrices using the formulae given in Ref. [26] and are presented in Table 4. It is observed that among the four transitions of Sm3+ ion, 4G5/2 ? 6H9/2 transition shows highest emission cross-sections for all the x values in the two glass matrices. Among the five x values in the glass matrix, this transition shows highest cross-section at x = 10 mol% in lithium–sodium glass matrix and at x = 20 mol% in lithium–potassium glass matrix. In the case of Dy3+ ion, 4F9/2 ? 6H13/2 transition shows highest emission cross-section for all the x values in the two glass matrices. Among the five glass compositions, this transition shows highest cross-sections at x = 20 mol% and at x = 10 mol% in lithium–sodium and lithium–potassium glass matrices respectively. Hence these two transitions are more useful for laser excitation at these compositions. Variation of emission cross-sections (rP) of different transitions with the variation of x in lithium–sodium and lithium–potassium glass matrices is shown in Figs. 6a and 6b for both the ions. 4. Conclusions The magnitudes of X2 and X6 parameters are minimum at x = 25 mol% (i.e. at lithium rich) in Sm3+ doped lithium–sodium chloroborate glass matrix indicating lower covalency of Sm–O bond and lower rigidity of the glass matrix. In lithium–potassium glass matrix, these parameters are minimum at x = 15 mol% (at equal mol% of lithium and potassium) indicating lower covalency of Sm–O bond and lower rigidity of the glass matrix at this composition. For Dy3+ ion, X2 and X6 parameters are minimum at x = 15 mol% in lithium–sodium and lithium–potassium glass matrices (at equal mol% of lithium, sodium and lithium, potassium) indicating lower covalency of Dy–O bond and lower rigidity of the glass matrix. From the variation of shift in peak wavelength of the hypersensitive transition and X2 parameter with x in the glass
matrix, it is concluded that there are structural changes at x = 5–10 mol% and at x = 20–25 mol% in Sm3+ doped lithium–sodium and lithium–potassium chloroborate glass matrices. In the case of Dy3+ ion, in both lithium–sodium and lithium–potassium glass matrices, structural changes are observed at x = 5–10 mol%, but at x = 20–25 mol% the structural changes do not influence the covalency of Dy–O bond. From the magnitude of peak emission cross-sections, the emission transition 4G5/2 ? 6H9/2 of Sm3+ ion at x = 10 mol% in lithium–sodium glass matrix and at x = 20 mol% in lithium–potassium glass matrix is useful for laser excitation. For Dy3+ ion, the emission transition, 4F9/2 ? 6H13/2 at x = 20 mol% and at x = 10 mol% in lithium–sodium and lithium–potassium glass matrices respectively are useful for laser excitation. Acknowledgements One of the authors (YCR) expresses his thanks to University Grants Commission for providing financial assistance in the form of a Major Research Project. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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