Absorption studies of tripositive praseodymium and neodymium doped zinc fluoride borophosphate (ZFBP) glasses

Indian Journal of Engineering & Materials Sciences Vol. 12, February 2005, pp. 65-74 Absorption studies of tripositive praseodymium and neodymium doped zinc fluoride borophosphate (ZFB) glasses Y K Sharma a*, R Dubedi b, V Joshi c, K B Karnataka a & S S L Surana d a Department of hysics, Government ost Graduate College, Uttarkashi 249 193, India b Department of Chemistry, Amardeep Degree College, Firozabad 283 203, India c Department of Chemistry, Government College Dhakpathar, Vikashnagar, Dehradun, India d Department of hysics, J. N. V. University, Jodhpur 342 001, India Received 20 January 2004; accepted 8 October 2004 Zinc fluoride borophosphate (ZFB) glasses doped with tripositive praseodymium and neodymium ions have been prepared by melt quenching technique. The final composition of glasses in mol.% is 15.44 B 2 O 3 39.77 2 O 5-17.36 Na 2 O 16.67 Al 2 O 3 10.74 ZnF 2 R Ln (where R= 0.0, 0.1, 0.3 and 0.5 mol % and Ln = r 3+ and Nd 3+ ). The absorption spectra of doped ZFB glasses have been recorded at room temperature. Eight absorption bands have been observed from the ground state 3 H 4 to excited states 3 F 3 2, F 3 3, F 1 4, G 1 4, D 2, 3 0, 3 1 and 3 2 in r 3+ doped ZFB glasses and twelve absorption bands from the ground state 4 I 9/2 to excited states 4 F 3/2, 4 F 5/2, 4 F 7/2, 4 S 3/2, 4 F 9/2, 2 H 11/2, 4 G 5/2, 4 G 7/2, 4 G 9/2, 2 G 9/2, 4 G 11/2, and 2 1/2 in the case of Nd 3+ doped ZFB glasses. From these spectral data various energy interaction parameters like Slater-Condon parameter F k (k=2, 4, 6), Lande parameter ζ 4f and Racah parameters E k (k=1,2,3) have been computed. Nephelauxetic ratio (β ) and bonding parameters (b 1/2 ) have also been computed from these parameters to study the nature of bonding in doped glasses. The intensities of the f-f transitions in the absorption spectra have been analyzed by the application of the Judd-Ofelt theory. J-O parameters (Ω λ ) have been computed. Ω λ parameters of ZFB glasses have been compared with those of similar glasses to infer the nature of bonding in these glasses. IC Code: C03C 3/00 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. The absorption spectrum of a lanthanide ion doped into single crystal shows groups of many fine lines, resembling an atomic spectrum. In solutions and glasses, the line transitions within one group are broadened to form one band. However, the line width of this band is still much smaller than that in the absorption spectrum of transition metal ion. The peak positions of the spectral lines reveal the electronic structure of the 4f n configuration. The ligand field splitting gives information about the symmetry of the lanthanide site and the shape of the coordination polyhedron, and the intensities of spectral transitions reflect the interaction between the lanthanide ion and its environment. In recent years, much attention has been focused on the search for new rare earth doped materials to be *For correspondence (E-mail: dryksharma@yahoo.com) used as hosts for optical devices. Rare earth doped heavy metal fluoride glasses are transparent from ultraviolet to the infrared region. They can be easily prepared and a range of concentration of transition metal and rare earth ions can be incorporated into the glass 2. Fluoride 3-5 glasses have smaller multiphonon emission rates and are chemically stable. They are also stable against atmospheric moisture 2-4,6-8. On the other hand, borate, phosphate and borophosphate glasses 1,9-17 are much useful as host materials for laser action and amplifiers. In order to combine the properties of both fluoride and borophosphate glasses, zinc fluoride borophosphate glasses have been prepared. They are expected to be good materials for optical devices such as lasers and optical fibres. In this paper, a new kind of trivalent praseodymium and neodymium doped zinc fluoride borophosphate (ZFB) glasses have been prepared. The absorption spectra of trivalent praseodymium and neodymium ions doped ZFB glasses have been investigated. From these spectral data various energy interaction parameters like Slater-Condon parameter F k (k=2,4,6), Lande parameter ζ 4f and Racah parameters E k (k=1,2,3) have been computed. From these parameters

66 INDIAN J. ENG. MATER. SCI., FEBRUARY 2005 nephelauxetic ratio (β ) and bonding parameters (b 1/2 ) have been computed to study the nature of bonding in doped glasses. Intensities of the f-f transitions in the absorption spectra have been analysed by the application of Judd-Ofelt theory. Experimental rocedure The zinc fluoride borophosphate glasses of the final composition in mol % 15.44 B 2 O 3 39.77 2 O 5-17.36 Na 2 O 16.67 Al 2 O 3 10.74 ZnF 2 R Ln (where R= 0.0, 0.1, 0.3 and 0.5 mol.% and Ln = r 3+ and Nd 3+ ) have been prepared by the melt quenching technique 18 from reagents of analytical grade in 10 g batches. The lanthanide oxides (99.99% pure) were obtained from the Indian Rare Earth Ltd. Batches containing 0.0, 0.1, 0.3 and 0.5 mol.% of r 3+ and Nd 3+ in the base glass materials were mixed in an agate pestle mortar for 2 h and were thermally treated for 6 h in an alumina crucible up to 950±25 C. Homogeneity of the melt was ensured by stirring the melt with an alumina rod from time to time. The melt was quenched by pouring it into a rectangular shaped brass mould placed on a pre-heated (200 o C) heavy copper plate. The glass specimens so prepared were taken away after 24 h and annealed for 3 h at 300 C so as to remove stresses and to give them thermal stability and strength. Samples of the size 15 10 1.5 mm 3 were cut and polished on all sides to make the faces flat and parallel. The initial and final polishing of the samples were done with the help of fine powder of cerium oxide. These samples were again annealed at 250 C for further removing mechanical stresses developed during polishing. The glass samples so prepared were of good optical quality and were transparent. X-ray diffractogram of the glass sample was studied with Rigaku X-ray diffractometer Geigerflex D/max-B system. The X-ray diffractogram showed no prominent peaks indicating the absence of a crystalline phase. The absorption spectra in the spectral range 300-900 nm and 800-2400 nm were recorded on a double beam UV-VIS/NIR Spectrophotometer Hitachi model 330. The absorption spectra have been recorded in terms of relative absorption (I o /I) versus wavelength (nm), where I o is the intensity of the radiation transmitted through the undoped glass specimen and I is the intensity of the radiation transmitted through the doped glass specimen of the same thickness. The density ρ of the glass samples was determined by Archimedeś displacement method, using xylene as the immersion liquid and sensitive ADAIR DUTT analytical microprocessor based electronic single pan digital top loading balance Model AD 180. It has max. capacity of 180 g with an accuracy of 0.1 mg. The refractive index of glass specimens were measured on an Abbé refractometer (ATAGO 3T). An ordinary lamp was used as the light source. The path length was measured to the nearest of 0.01 cm with the help of a micrometer. Results and Discussion The representative absorption spectra of r 3+ and Nd 3+ doped ZFB glass specimens have been presented in Figs 1-3 in terms of relative absorption (I o /I) versus wavelength (nm), where I and I o are intensities of the radiation transmitted through doped specimens and undoped specimens of equal thickness. The various physical properties have been collected in Table 1. The absorption spectra of lanthanide ions lie in the visible and near infrared regions and correspond to Fig. 1 Absorption spectrum of praseodymium (0.3 mol%) doped ZFB glass specimen in the visible region Fig. 2 Absorption spectrum of praseodymium (0.3 mol%) doped ZFB glass specimen in near I R region

SHARMA et al.: ZINC FLUORIDE BOROHOSHATE GLASSES 67 transitions from the ground state to various excited states of 4f N configuration. The relevant theoretical background along with the interpretation of the observed absorption spectra of r 3+ and Nd 3+ doped ZFB glasses in terms of the energy interaction parameters and Judd-Ofelt intensity parameters is given below. Energy interaction parameters The absorption spectra of lanthanide ions in the visible and near infra-red regions are known to arise due to 4f- 4f transitions 19. The energy level structure of 4f-configuration may be considered to arise from the electrostatic and magnetic interactions between the 4f electrons. Taylor series 20 expansion may be used to express the energy of the jth electronic level by the equation: E E F E F F E 0 0 j j j ( k, ζ 4f ) = oj ( k,ζ 4f ) + Δ k + Δζ4f k = 2,4,6 Fk ζ4f (1) where E oj is the zero-order energy of the jth level and ΔF k and Δζ 4f are the small changes in the corresponding parameters. The values of ΔF k and Δζ 4f may be evaluated using observed energy values (E j ), reported values 20 of zero order energies (E 0j ) and partial derivatives E j / F k and E j / ζ 4f by partial regression method 21,22. The values of Slater-Condon, F k, (k=2,4,6) and spinorbit interaction, ζ 4f, parameters are then evaluated using equations F k = F k o + ΔF k ζ 4f = ζ 4f o + Δζ 4f (2) where ΔF k <<F o k, Δζ 4f <<ζ 4f o and F k o and ζ 4f o are the zero-order values of Slater-Condon and spin orbit interaction parameters as reported by Wong 20. The Racah parameters E k (k=1, 2, 3) are related to F k parameters by the following relations: E 1 = (1/9) (70F 2 + 231F 4 + 2002F 6 ) Fig. 3 Absorption spectrum of neodymium (0.3 mol%) doped ZFB glass specimen in the visible region E 2 = (1/9) (F 2 3F 4 + 7F 6 ) E 3 = (1/3) (5F 2 + 6F 4-91F 6 ) (3) and have been evaluated using these equations. The absorption bands, on the basis of energy separation between the energy levels of r 3+ free ion and the observed intensities of the bands, have been assigned to transitions from the ground state 3 H 4 to different energy levels. Eight absorption bands have been observed from the ground state 3 H 4 to the excited states 3 F 2, 3 F 3, 3 F 4, 1 G 4, 1 D 2, 3 0, 3 1 and 3 2 in r 3+ doped ZFB glass specimens. The experimental and calculated energy of various bands are given in Table 2. The observed band positions of r 3+ ion in Table 1 hysical properties of various undoped and praseodymium and neodymium doped ZFB Glass specimens with dopent concentrations (mol %) in paranthesis Dopant ions Specimen code athlength (cm) Refractive index Density (g/cm 3 ) Concentration, dopant (mol %) r 3+ Nd 3+ ZFB(0.0) 0.301 1.476 2.558 undoped ZFB(0.1) 0.322 1.482 2.643 0.1 ZFB(0.3) 0.312 1.481 2.651 0.3 ZFB(0.5) 0.325 1.488 2.648 0.5 ZFB(0.0) 0.293 1.476 2.558 undoped ZFB(0.1) 0.310 1.487 2.653 0.1 ZFB(0.3) 0.322 1.485 2.649 0.3 ZFB(0.5) 0.315 1.484 2.651 0.5

68 INDIAN J. ENG. MATER. SCI., FEBRUARY 2005 Table 2 Experimental energy ( ) and calculated energy ( ) with their differences ( ) for various absorption bands for the different doping concentrations (mol %) of r 3+ doped ZFB glass specimens Absorption bands ZFB (0.1) ZFB (0.3) ZFB (0.5) 3 H 4 3 F 2 5142 4875 266 5128 4848 279 5134 4854 279 3 F 3 6535 6256 278 6514 6263 250 6506 6278 227 3 F 4 6785 6909 124 6802 6932 130 6795 6948 153 1 G 4 9843 10115 272 9852 10209 357 9862 10235 373 1 D 2 16961 17329 368 16898 17250 352 16949 17270 321 3 0 20855 20911 56 20661 20749 88 20661 20768 107 3 1 21258 21526 268 21163 21398 235 21217 21426 209 3 2 22378 22759 381 22344 22655 311 22430 22687 257 r.m.s. deviation (σ) ± 272 ± 267 ± 254 different glasses are best fitted with energy levels observed in the case of free ion. The band positions agree with previously reported band positions of r 3+ ion in other glass systems 9,21. The values of Slater- Condon (F k ), Racah (E k ) and Lande (ζ 4f ) parameters have been computed by using the observed energies of the bands, the values of zero order energies (E 0j ) and partial derivatives with the help of partial regression method 22. The values of F k, E k and ζ 4f parameters have been given in Table 3. In the ZFB glass specimens, the relation among different F k parameters is found to be F 2 > F 4 > F 6. It is interesting to note that the observed values of F 4 /F 2 ~ 0.138 and F 6 /F 2 ~ 0.015 (Table 3) are nearly same as calculated considering radial eigen functions to be hydrogenic (F 4 /F 2 ~ 0.14 and F 6 /F 2 ~0.015) 23. The E k parameters have been given in Table 3. The ratio of E 1 /E 3 and E 2 /E 3 for all the r 3+ doped glasses under study are about 9.890 and 0.052 respectively, which are almost equal to the ratios calculated for the hydrogenic wave functions 23. This implies that r 3+ ions at different doping concentrations are subjected to similar force fields. Bonding in the glasses can be inferred from the nephelauxetic ratio β defined 24,25 by the relation, β = F 2 (glass)/f 2 (free ion) (4) where F 2 (glass) is the Slater-Condon energy interaction parameter and F 2 (free ion) is the same parameter for free ion. If β is less than one, it indicates covalent bonding, while its value greater than one indicates ionic bonding. Henrie and Choppin 26 have defined another bonding parameter b 1/2 in terms of β, given by Table 3 Calculated values of Slater-Condon, Lande, Racah, Nephelauxetic ratio and bonding parameters for r 3+ doped ZFB glass specimens of different doping concentrations arameters Free ion ZFB (0.1) ZFB (0.3) ZFB (0.5) ZCB* F 2 322.09 311.61 308.86 309.01 310.09 F 4 44.46 43.01 42.63 42.65 42.81 F 6 4.867 4.708 4.667 4.66 4.68 ζ 4f 741.00 748.89 765.08 769.58 735.3 E 1 4728.92 4575.08 4534.76 4536.87 4552.6 E 2 24.75 23.93 23.72 23.73 23.81 E 3 478.10 462.56 458.48 458.69 460.29 F 4 /F 2 0.138 0.138 0.138 0.138 0.138 F 6 /F 2 0.015 0.015 0.015 0.015 0.015 E 1 /E 3 9.89 9.89 9.890 9.89 9.89 E 2 /E 3 0.052 0.051 0.052 0.052 0.052 β 0.967 0.958 0.959 0.962 b 1/2 0.127 0.143 0.142 0.136 *Zinc chloride borophosphate glass (Ref. 13) b 1/2 = [(1-β )/2] 1/2 (5) A real value of b 1/2 indicates covalent bonding. This parameter is very useful for comparative study of bonding between the central R.E. ion and the surrounding glass matrix. The values of nephlauxetic ratio (β ) and bonding parameter (b 1/2 ) for r 3+ ion in ZFB glasses are ~0.96 and ~(0.127-0.143) respectively (Table 3). Twelve absorption bands have been observed for different dopent concentrations of Nd 3+ ion ranging from 0.1 to 0.5 mol% in ZFB glasses. The assignment of these bands from the ground state 4 I 9/2 to the various excited states 4 F 3/2, 4 F 5/2, 4 F 7/2, 4 S 3/2, 4 F 9/2, 2 H 11/2, 4 G 5/2, 4 G 7/2, 4 G 9/2, 2 G 9/2, 4 G 11/2, and 2 1/2 are

SHARMA et al.: ZINC FLUORIDE BOROHOSHATE GLASSES 69 Table 4 Experimental energy ( ) and calculated energy ( ) with their differences ( ) for various absorption bands of Nd 3+ doped ZFB glass specimens of different doping concentrations Absorption level ZFB (0.1) ZFB (0.3) ZFB (0.5) 4 I 9/2 4 F 3/2 11329 11369 40.55 11406 11366 39.68 11344 11363 29.32 4 F 5/2 12455 12506 51.46 12500 12497 2.12 12545 12508 36.47 4 F 7/2 13369 13380 11.97 13369 13364 4.70 13408 13380 27.85 4 S 3/2 13531 13538 7.61 13519 13530 11.42 13518 13547 29.07 4 F 9/2 14837 14906 69.92 14837 14883 46.64 14861 14910 49.03 2 H 11/2 16008 16004 3.46 16008 15999 8.15 16017 16007 9.60 4 G 5/2 17235 17092 142.44 17218 17100 117.23 17241 17093 147.48 4 G 7/2 19026 19128 102.7 19011 19143 132.8 19011 19149 138.8 4 G 9/2 19639 19603 35.28 19520 19608 88.29 19623 19626 3.91 2 G 9/2 21231 21238 7.00 21222 21246 24.17 21222 21271 49.75 4 G 11/2 21997 21757 239.52 21988 21733 254.21 21987 21769 217.41 2 1/2 23105 23106 1.26 22957 23006 49.13 22956 23004 48.54 r.m.s. deviation(σ) ±90 ±96 ±91 observed in all the doped ZFB glasses. The experimental and calculated energy band positions are given in Table 4 for Nd 3+ doped ZFB glass specimens. The values of F k, E k and ζ 4f parameters have been computed by using the observed energies of the bands, E 0j and partial derivatives with the help of partial regression method 22 and given in Table 5. The relation among different F k parameters is found to be F 2 >F 4 >F 6 in Nd 3+ doped ZFB glass specimens. The value of F 2 ~ (327.46-330.68 cm -1 ) is nearly same as ~ 330.8 cm -1 for fluoro glasses 27,28. F 4 /F 2 ~ (0.143-0.147) and F 6 /F 2 ~(0.016 0.015) are nearly same as reported in other glasses 14. The ζ 4f value ~ (936.75-945.38 cm -1 ) is of the same order as is observed in the case of fluorophosphate glasses 27,28. However, the calculated value of ζ 4f by Blume et al. 29 is ~1130 cm -1. The higher calculated value may be due to the use of analytical non-relativistic Hartree-Fock wave functions for the f-orbitals. It may be pointed out that ζ 4f values in the case of Nd 3+ doped glasses are higher than that in r 3+ doped glasses. Further, the change in ζ 4f values is larger than the changes in F k values. This suggests that in doped glasses the spin-orbit coupling is affected more as compared to the electrostatic repulsion by the surrounding ligand field. The values of E 1, E 2, E 3 parameters have been given in Table 5. The ratio of E 1 /E 3 ~ (10.18-10.26) and E 2 /E 3 ~ (0.050-0.052) are found to remain almost Table 5 Calculated values of Slater-Condon, Lande, Racah, Nephelauxetic ratio and bonding parameters of Nd 3+ doped ZFB glass specimens of different doping concentrations arameters Free ion ZFB (0.1) ZFB (0.3) ZFB (0.5) ZCB* F 2 331.16 330.68 327.58 327.46 315.51 F 4 50.72 47.19 48.15 47.87 48.07 F 6 5.15 5.32 5.19 5.18 4.44 ζ 4f 884.0 944.6 936.7 945.4 924.01 E 1 5024.0 4966.7 4938.2 4928.9 4674.6 E 2 23.90 25.15 24.38 24.46 22.49 E 3 497.00 484.15 484.83 484.24 487.42 F 4 /F 2 0.153 0.143 0.147 0.146 0.152 F 6 /F 2 0.016 0.016 0.015 0.016 0.014 E 1 /E 3 10.11 10.26 10.18 10.18 9.60 E 2 /E 3 0.052 0.052 0.050 0.051 0.046 β 0.99 0.98 0.98 0.95 b 1/2 0.027 0.073 0.075 0.153 *Zinc chloride borophosphate glass (Ref. 14) constant over the entire range of Nd 3+ doping concentrations and are in good agreement with the corresponding hydrogenic ratios (Table 5). Using F 2 parameters, nephelauxetic ratio (β ) and bonding parameter (b 1/2 ) are found to be ~0.99 and 0.075 respectively for Nd 3+ ions in ZFB glasses. On comparing (Tables 3 and 5) energy interaction parameters and bonding parameters of present r 3+ and Nd 3+ ZFB glasses with those of zinc chloride borophosphate glasses, it is found that in the case of

70 INDIAN J. ENG. MATER. SCI., FEBRUARY 2005 doped glasses there is a little change in the bonding parameter but Nd 3+ doped glasses show that on addition of ZnF 2 to borophosphate glasses, a marked decrease in the % covalent character (or increase in % ionic character) between the central R.E. ion and its surrounding glass matrix is observed. Small values of r.m.s. deviation σ' between experimental energy ( ) and calculated energy ( ) of absorption levels in praseodymium and neodymium ions doped ZFB glass specimens justify the suitability of the use of Taylor series expansion method 20,30. r 3+ Judd-Ofelt intensity parameters The intensities of absorption bands under Gaussian approximation are measured in terms of experimental oscillator strength, exp, calculated by the relation 1 I = 4.60 10 log Δν (6) cl I 9 0 exp 1/2 where c is the molar concentration of the absorbing ion per unit volume, l is the path length and log (I 0 /I) is the absorptivity or optical density and Δν 1/2 is half band width. However, for a solid material it is generally expressed in terms of line strength S exp which is related to experimental oscillator strength exp by the relation 2 2 2 8 π mcν ( n + 2) exp = S 3 h (2J + 1) 9 n and S exp = S exp /e 2 exp (7) where ν is the average energy of the transition in cm -1, J is the total angular momentum of the initial level, the factor (n 2 + 2) 2 /9 n represents the local field correction for an ion embeded in a dielectric medium of refractive index, n, under the tight binding approximation and the other symbols have their usual meaning. Judd 31 and Ofelt 32 have worked out the theoretical background for the calculation of induced electric dipole transition moments. The odd part of the crystal field potential is considered as a perturbation for mixing states of opposite parity into the 4f N configuration. In simple form the line strength (S cal ) of an electric dipole transition between initial J manifold 4f N (α,s,l) J > and terminal J manifold 4f N (α,s,l ) J > is given by 31,32 S = e Ω < 4f (α, S, L) J U cal and 2 N (λ) λ λ = 2,4,6 N 2 4 f (α, S, L ) J > S cal =S cal /e 2 (8) where e stands for electron charge, 4f N (α,s,l)j > are the basis states in the LS coupling scheme and α represents an extra quantum number that might be necessary to describe the states completely, U (λ) are the unit tensor operators of rank λ which are doubly reduced to yield the matrix elements < U (λ) > in the intermediate coupling scheme 31 and Ω λ are the phenomenological Judd-Ofelt intensity parameters which specify the electric dipole moment between any two electronic levels and contain implicitly the odd symmetric crystal field terms, radial integrals, and perturbation denominators. An attempt has been made by Quimby and Mininscalco 33 to further modify the theory, though the basic consideration remains the same. In order to test the validity of Judd-Ofelt approach, used to interpret the spectral intensities, a reduced chisquare test between S exp and S cal values has been performed. The reduced chi-square is given 4 by Reduced chi-square = ( S cal S exp ) 2 /ξ 3 (9) where ξ is the number of absorption bands used in the computation. The low value of this statistical parameter is a measure of the goodness of fit. The experimental line strengths S exp (Tables 6 and 7) for different bands and reported values of unit tensor matrix elements < U (λ) > for different lanthanide ions 34, may be put in Eq. (8) to get as many equations as the number of bands. Then partial regression method 19 is used to obtain the Judd-Ofelt intensity parameters. The resulting values of Ω 2, Ω 4 and Ω 6 have been given in Tables 8 and 9 for different doping concentrations of r 3+ and Nd 3+ ions ranging from 0.1 to 0.5 mol.% in ZFB glass specimens. The computed values of Ω λ parameters are very important since they are used in the calculation of laser parameters. In the case of r 3+ and Nd 3+ doped ZFB glass specimens eight bands and eleven bands respectively in the visible and near IR regions have been used in the computation of Ω λ parameters. The calculated line strengths agree well with the experimental values. Reported values of Ω λ

SHARMA et al.: ZINC FLUORIDE BOROHOSHATE GLASSES 71 Table 6 Experimental (S exp ) and calculated line strength (S cal ) with their differences (Δś) for various absorption bands of r 3+ doped ZFB glass specimens for different doping concentrations Absorption ZFB (0.1) ZFB(0.3) ZFB(0.5) level S exp (10-20 ) S cal (10-20 ) Δś (10-20 ) S exp (10-20 ) S cal (10-20 ) Δś (10-20 ) S exp (10-20 ) S cal (10-20 ) Δś (10-20 ) cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 3 H 3 4 F 2 3.92 3.91 0.01 4.18 4.15 0.03 5.01 4.97 0.04 3 F 3 6.91 6.59 0.32 7.69 7.29 0.41 8.13 7.59 0.53 3 F 4 3.64 4.16 0.52 3.93 4.56 0.63 3.91 4.72 0.81 1 G 4 0.25 0.23 0.01 0.27 0.26 0.01 0.31 0.270 0.04 1 D 2 0.20 0.46 0.25 0.24 0.52 0.27 0.36 0.53 0.16 3 0 0.62 0.30 0.32 0.76 0.38 0.38 0.79 0.40 0.38 3 1 0.58 0.91 0.33 0.71 1.10 0.39 0.73 1.18 0.45 3 2 1.82 1.18 0.63 2.11 1.31 0.79 2.22 1.35 0.87 Goodness of fit 0.21 0.31 0.41 Table 7 Experimental (S exp ) and calculated line strength (S cal ) with their differences (Δś) for various absorption bands of Nd 3+ doped ZFB glass specimens for different doping concentrations Absorption level ZFB(0.1) ZFB(0.3) ZFB(0.5) S exp (10-20 ) S cal (10-20 ) Δś (10-20 ) S exp (10-20 ) S cal (10-20 ) Δś (10-20 ) S exp (10-20 ) S cal (10-20 ) Δś (10-20 ) cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 cm 2 4 I 9/2 4 F 3/2 0.25 0.29 0.04 0.43 0.19 0.24 0.41 0.13 0.29 4 F 2 5/2, H 9/2 1.78 1.05 0.73 1.02 0.67 0.35 0.91 0.60 0.31 4 F 4 7/2, S 3/2 1.24 1.41 0.17 0.90 0.91 0.01 0.89 0.85 0.04 4 F 9/2 0.28 0.06 0.22 0.24 0.04 0.20 0.13 0.04 0.09 2 H 11/2 0.03 0.02 0.01 0.09 0.01 0.08 0.13 0.01 0.12 4 G 2 5/2, G 7/2 2.86 2.69 0.17 1.61 1.47 0.14 2.06 1.90 0.16 2 K 13/2, 4 G 7/2 0.43 0.68 0.25 0.18 0.41 0.23 0.18 0.40 0.22 4 G 9/2 0.32 0.12 0.20 0.13 0.07 0.06 0.07 0.06 0.01 2 K 15/2, 2 G 9/2 0.45 0.10 0.35 0.34 0.06 0.28 0.28 0.06 0.22 ( 2 D, 2 ) 3/2, 4 G 11/2 0.09 0.05 0.04 0.06 0.03 0.03 0.07 0.02 0.05 2 1/2, 2 D 5/2 0.25 0.04 0.21 0.10 0.03 0.07 0.27 0.02 0.25 Goodness of fit 0.11 0.04 0.05 parameters for zinc chloride borophosphate 13,14, borate 16,17, phosphate 15,35, fluorophosphate 36,37, ZBLAN 3,38 and CLB 39 glasses have been given in Tables 8 and 9 for comparison. In all the three r 3+ doped ZFB glasses, the three J-O parameters follow the order Ω 4 < Ω 2 < Ω 6. Same ordering has been observed in the case of r 3+ doped zinc chloride borophosphate glasses 13, but in other glasses (Table 8) the ordering is different. This is because different glass matrixes have different covalent bonding between the central RE ion and the surrounding glass matrix. Rigidity of the glass also varies from matrix to matrix and it affects the ordering of Ω λ parameters. According to JØrgensen 40.41 Ω 2 is covalency dependent parameter and Ω 4 and Ω 6 are structure dependent parameters. Ratio Ω 4 /Ω 6 in the present r 3+ Table 8 Judd-Ofelt intensity parameters, Ω λ, for r 3+ doped ZFB glass specimens of different doping concentration and compare with other similar glasses Glass ZFB(0.1) ZFB(0.3) ZFB(0.5) ZCB Borate hosphate Fluorophosphate ZBLAN CLB Judd-Ofelt parameters Ω 2 Ω 4 Ω 6 (10-20 (10-20 (10-20 cm 2 ) cm 2 ) cm 2 ) 2.74 2.71 3.61 5.32 0.36 1.55 1.80 0.94 1.53 1.77 2.21 2.37 2.92 5.09 1.34 4.70 6.54 4.86 ZFB- Zinc fluoride borophosphate ZCB- Zinc chloride borophosphate CLB- Chloro borophosphate 8.31 9.09 9.36 5.39 6.34 1.03 5.20 3.84 2.77 Ω 4 /Ω 6 0.21 0.24 0.25 0.54 0.80 1.30 0.90 1.70 1.75 Ref. 14 16 35 37 38 39

72 INDIAN J. ENG. MATER. SCI., FEBRUARY 2005 glasses (Table 8) is ~0.25, which shows that they are less rigid as compared to other oxide glasses 13,15,17 and ZBLAN glass 3 in which the ratio Ω 4 /Ω 6 is ~1.70. The ZBLAN glass is well known for its rigidity. CLB glass 39 is also equally rigid. Nageno et al. 42 have also studied various alkali borate and phosphate glasses and reached to a similar conclusion that Ω 4 and Ω 6 parameters are related to the rigidity of the glass matrix. As far as covalency is concerned 43, values of Ω 2 parameters indicate that the degree of covalency in the present ZFB glasses is lower than the degree of covalency found in zinc chloride borophosphate glasses and ZBLAN glass is least covalent (% ionic character is highest) borate and phosphate glasses are known to have higher degree of covalency but this model many times 44 fails to predict so. In many r 3+ doped glasses it is found that the phenomenological Ω 2 parameter is negative which is very from definition 45 of the Ω λ parameters is not acceptable. This may be 45 due to the mixing of lowest excited configuration of opposite parity (4f 1 5d 1 ) into the ground configuration (4f 2 ). This configuration (4f 1 5d 1 ) is too close (~45,000 cm -1 ) to the ground configuration, invalidating the approximations 44 made in Judd-Ofelt theory. However, in Nd 3+ glasses where the energy separation between the lowest excited configuration and the ground configuration (~65,000 cm -1 ) is reasonably high, the predictions of this model are fairly accurate. In Nd 3+ doped ZFB glasses (Table 9) the ratio of Ω 4 /Ω 6 is ~0.8 which is comparable to that found in fluorophosphates glass (~0.86) but in zinc chloride borophosphate glass and ZBLAN glass the ratio is quite high. This shows that zinc chloride borophosphate glass is more rigid as compared to ZFB glass and other oxide glasses. Similarly Ω 2 parameter shows that the degree of covalency in the present glasses is lower than the degree of covalency present in zinc chloride borophosphate and other glasses and it is least in ZBLAN glass. The decrease in covalency as compared to oxide glasses may be due to the presence of mixed oxy-fluoride coordination shells 46 of 5+ and Zn 2+ cations in close vicinity of the central rare earth ion. These coordination shells are formed because in the presence of ZnF 2 a continuous structural break down of the polymeric phosphate chain and polymeric boroxol ring takes place. Therefore, in the present glasses Nd 3+ may have eight coordination number due to eight oxygens provided both by [O 4 ] 3- chains and non-bridging oxygens bonded only to a single boron atom. The presence of Table 9 Judd-Ofelt intensity parameters, Ω λ, for Nd 3+ doped ZFB glass specimens of different doping concentrations and compared with other similar glasses Glass ZFB(0.1) ZFB(0.3) ZFB(0.5) ZCB Borate hosphate Fluorophosphate ZBLAN Judd-Ofelt parameters Ω 4 (10-20 cm 2 ) Ω 2 (10-20 cm 2 ) 1.72 0.86 1.46 4.25 6.52 3.70 2.20 1.54 0.98 0.62 0.38 4.50 5.00 4.00 4.20 3.66 Ω 6 (10-20 cm 2 ) 1.22 0.79 0.75 3.99 6.16 4.40 4.90 3.50 Ω 4 /Ω 6 0.81 0.79 0.50 1.13 0.81 0.91 0.86 1.05 Ref. 13 17 15 36 3 non-bridging oxygens have been confirmed experimentally (fluorescence line narrowing and Raman scattering measurements) by an investigation of the phonon side bands in the case of Eu 3+ doped sodium borate glasses by Tanabe et al. 47 and by Gatterer et al. 48 in the case of Nd 3+ doped borate glasses. Ω 2 parameter involves the long range terms in the crystal field potential and is most sensitive 3,48,49 to the local structural changes. The intensity of the 3 H 4 3 F 2 (r 3+ ) and 4 I 9/2 4 G 5/2 (Nd 3+ ) transitions are the principal determiners of the Ω 2 parameter. This transition satisfies the selection rule ΔJ 2 and is known 40 as hypersensitive transition. The intensities of the hypersensitive bands in the present glasses (Tables 6 and 7) are comparable to those observed in fluoro phosphate glasses 36,44,50 but much smaller than those observed in oxide glasses 44 and zinc chloride borophosphate glass 13,14.That is why the value of Ω 2 parameter in present glasses is smaller than the value of the Ω 2 parameter observed in oxide glasses and zinc chloride borophosphate glass. A good agreement is found between the calculated and experimental line strengths (Tables 6 and 7). The low values of goodness of fit indicate 50 the validity of the Judd-Ofelt theory. Conclusions Optical absorption spectra of six r 3+ and Nd 3+ doped zinc fluoride borophosphate glasses have been measured at room temperature in UV-VIS/NIR spectral regions. Using the values of observed energy levels, energy interaction parameters have been evaluated. Nephelauxetic ratio and bonding parameter have also been calculated. A comparison with ZnCl 2

SHARMA et al.: ZINC FLUORIDE BOROHOSHATE GLASSES 73 borophosphate glass shows that addition of ZnF 2 to the borophosphate glass decreases the % covalent character (or increases the % ionic character) between the central R.E. ion and its surrounding glass matrix. Using the observed line strength Judd-Ofelt intensity parameters, Ω λ, have been evaluated and compared with the Ω λ parameters of other similar oxide glasses like borate, phosphate and borophosphate glasses. Statistical problems may arise in the calculation of Ω λ parameters from the observed line strengths in the case of r 3+ doped glasses. As far as the rigidity is concerned, zinc chloride borophosphate glass is better than ZFB glasses and other oxide glasses. A comparison of Ω 2 values also shows that the degree of covalency in the present glasses is lower than the degree of covalency present in zinc chloride borophosphate and other oxide glasses and it is least in the ZBLAN glass. The decrease in the covalency as compared to oxide glasses may be due to the presence of mixed oxy-fluoride coordination shells of 5+ and Zn 2+ cations surrounding the central RE ion. In the present glasses Nd 3+ may have overall coordination number eight. Acknowledgement The authors are thankful to rof. D C Dube, hysics Department, IIT, Delhi, for providing laboratory facilities to do the present work and rof. S Tandon for helpful discussions at various stages. One of the authors (YKS) is also thankful to UGC India for financial support in the form of a major research project. 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