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A structural interpretation of the Pb2+, Eu3+ and Nd3+ optical spectra in doped sodium borate glasses
Journal of Non-Crystalline Solids 69 (1985) 221-229 North-Holland, Amsterdam
221
A S T R U C T U R A L I N T E R P R E T A T I O N O F T H E P b 2+, Eu 3+ A N D N d 3+ O P T I C A L S P E C T R A IN D O P E D S O D I U M B O R A T E G L A S S E S
M. Z A H I R , R. O L A Z C U A G A , C. P A R E N T , G. L E F L E M a n d P. H A G E N M U L L E R Laboratoire de Chimie du Solide du CNRS, 351 Cours de la Lib&ation, 33405 Talence Cedex. France
Received 10 February 1984 Revised manuscript received 27 June 1984
The study of sodium borate glasses over the compositional range 10-35 mol.% Na20 by using Pb2+ , Eu3÷ and Nd 3+ as local sounds shows the existence of two possible situations depending on whether the Na20 content is lower or higher than about 25 mol.%. In the first composition range the simultaneous increase of optical basicity and of modifier ion site asymmetry implies rather a 2D-character of the network former. The properties of the glasses with high Na20 content involve more 3D-character for the former sublattice.
1. Introduction T h e e l a b o r a t i o n of rare earth b a s e d glasses for technological uses as laser o r solar energy c o n c e n t r a t o r s requires an accurate k n o w l e d g e of the influence of the glass c o m p o s i t i o n on the l u m i n e s c e n t properties. T h e J u d d - O f e l t theory for crystal field i n d u c e d electric-dipole transitions b e t w e e n 4f states allows the p h e n o m e n o l o g i c a l p a r a m e t e r s related to radiative p r o p e r t i e s for a p a r t i c u l a r glass c o m p o s i t i o n to be d e t e r m i n e d [1,2]. Largely used it has p r o v e d to b e a p o w e r f u l tool for the systematic investigation a n d selection of glasses with specific purposes. T h e first studies were m a i n l y d e v o t e d to the influence of the n e t w o r k f o r m e r in materials with c o m p o s i t i o n s close to c o m m e r c i a l glasses [3]. M o r e recently the role of m o d i f i e r ions ( n a t u r e a n d c o n c e n t r a t i o n ) has been c o n s i d e r e d for silicate a n d p h o s p h a t e glasses, b u t the i n t e r p r e t a t i o n of the results is quest i o n e d [4-8]. In the scope o f a general s t u d y of the glasses a b l e to occur in the M 2 0 - M 3 P O 4 - B 2 0 3 ( M = Li, N a ) systems, this p a p e r presents a structural investigation o f s o d i u m b o r a t e glasses b a s e d on a s i m u l t a n e o u s analysis o f the a b s o r p t i o n p r o p e r t i e s of N d 3 ÷ a n d Pb 2 + a n d the emission p r o p e r t i e s of Eu 3 + used as local structural probes. T h e results are c o m p a r e d with those recently o b t a i n e d b y T. I z u m i t a n i et al. for p h o s p h a t e a n d silicate glasses [7]. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M. Zahir et al. / Structural interpretation of optical spectra
222
2. I n v e s t i g a t i o n m e t h o d
2.1. Calculation of the Judd-Ofelt parameters from neodymium absorption measurements The line strength Sea of the forced electric-dipole transition between the initial J manifold I(SL)J >1 and the terminal manifold I(S'L')J')I of the neodymium 4f states can be expressed by: Sea =
~
~2xI((SL)JIIUta~II(S'L')J')I 2
~ 2,4,6
where: - the three terms (IJUtX)ll) are the doubly reduced tensor operators in the intermediate coupling approximation, - the Judd-Ofelt parameters 12z, 124, 126 contain two terms: (i) the crystal field parameters A,.p (with t odd) which are the odd parity terms in the crystal field expansion depending on the site symmetry of the probe ion in a given glass, (ii) the E(t, ~,) parameter which contains the radial part of the 4f wave functions and the energy denominator between 4f and admixing levels with opposite parity. In other words E(t, ?~) measures the covalency rate of the rare earth ligand bond (nephelauxetic effect). Sea is related to the integrated absorbance of an electric-dipole transition fk(X) dX by the equation:
8~3e27,
(
1 (n 2 + 2) 2 Sed
]dk(Xda , = N3hc(Zj + 1) n
9
'
(1)
where: N is the rare earth ion concentration, the average wavelength of the absorption band, n the bulk refractive index at the ~ wavelength. Once the 12x parameters have been determined, the spontaneous emission probability from a I(S'L')J')[ state to a I(SL)J > I state may be obtained via:
A[(S'L')J';(SL)J] = 64 "/r4e 2 n ( n 2 q- 2) 2 Sed 3 h ( 2 J ' + 1)X3 9 and the radiative lifetime is given by:
"tad.~--- EA[(S'L')J';(SL)J]
= A -rad '
SLJ
where A tad is the corresponding radiative deexcitation probability. In order to separate the respective contributions of A,. e and E(t, ~) two types of complementary measurements were carried out on the same glasses: the absorption spectra of Pb z+ and the europium fluorescence spectra.
M. Zahir et al. / Structural interpretation of optical spectra
223
2.2. Absorption spectra of Pb: + In glasses the concept of optical basicity, introduced by Duffy and Ingram, is connected to the ability of the former network oxygen atoms to donate some of their negative charge to the modifier metal ion [10]. In the presence of lead it can be deduced from the position of the wavelength of the absorption band maximum corresponding to the 1S0 --~3P 1 transition of Pb 2+. This band is red shifted if the host glass basicity increases. As pointed out by Reisfeld, such a shift can be correlated indeed to the increasing covalency of the P b - O bond and consequently of the F~(t, X) term [5].
2.3. Europium fluorescence spectra When Eu 3+ occupies sites with low symmetry, which is generally the case in glasses, its emission spectrum may include dipole-electric emission lines such as 5D 0 ----~7F0,2,4,6. Their relative intensities give the degree of asymmetry of the probe environment. But on the other hand the 5D0 ---,7F 1 transition which is of the magnetic-dipole type is always observed and its intensity is independent of the site symmetry. Therefore, it is generally admitted that the ratio of the emission intensities R-
'o0 -'7F2 5 D 0 ----~7 F 1
is an asymmetry parameter for the Eu 3+ sites [11,12]. The variation of R versus glass composition reflects the variation of the A,. e term. However one can emphasize that the covalency degree of the E u - O bond brings its own contribution to the 5D0 --, 7F2 transition intensity by modulating the admixture of the 4 f ' 5 d states into the 4f 6 states.
3. Experimental procedure The glasses were prepared by melting at l l 0 0 ° C in Au, Pt, Rh crucibles a mixture with suitable proportions of Na2CO 3 (Merck 99.5%), Nd203, Eu203 (Rh6ne-Poulenc 99.99%) PbO (Merck 99.9%) and BzO3 (Merck 99.5%). After quenching in air the samples were systematically annealed at 20°C lower than the vitreous transition temperature for 15 h and slowly cooled down. This heat treatment removes the internal tensions of thermal origin. The Nd 3+ absorption was studied on 1 mm thick samples containing 1 mol.% Nd203 which were carefully polished with diamond powder ( = 8 ~m). The spectra were recorded between 400 and 920 nm at room temperature using a Cary 17 spectrometer. For each sample the surfaces of the absorption bands found between these wavelengths and originating from t h e 419/2 ground state were measured.
224
M. Zahir et al. / Structural interpretation of optical spectra
Index of refraction measurements were made using a Pulfrich refractometer. The 122, 124, ~'26 w e r e calculated by a computerized least square program describing eq. (1). The Pb 2+ absorption spectra were recorded with a Cary 17 spectrometer on small thin glasses ( < 0.5 mm). The fluorescence of Eu 3+ under a xenon lamp excitation at 392 nm was recorded using a Jobin-Yvon monochromator and a Varian VPM 159 A photomultiplier on glasses containing 1 mol.% E u 2 0 3. The spectra were corrected by taking into account the specific response curve of the photomultiplier.
4. Experimental
results
Table 1 gives a listing of glass compositions, refractive indexes and densities. Fig. 1 represents the variation of the wavelength due to the ISo----~3P1 transition as a function of N a 2 0 content in the sodium borate glasses. This evolution is compared to: - the experimental values previously obtained by Duffy and Ingram [10], - the theoretical curves calculated by those authors on the basis of the optical basicity concept for Na 2 0 - B 2 0 3 , N a 2 0 - S i O 2 and Na20-P205 systems. The calculation takes into account the Pauling electronegativities of Na, B, Si and P and the glass composition. In the NaaO-B203 system (b' curve) a marked increase of the P b - O covalency is observed for 15 to 25 mol.% Na20. By further increasing of the N a 2 0 content this trend becomes less important. The obtained values are slightly different from those measured by the previous authors, but the general tendency is similar (a' curve). Fig. 2 gives the variation of the intensity ratio R of the 5D o ---,TF2 to the 5D o ---*TF1 fluorescence of E u 3+ v e r s u s N a 2 0 content. The results are compared to those previously given by Gallagher [12]. If the variation of R differs by an almost constant quantity, the trend of the two curves is similar. The
Table 1 Experimental values of Nd 3+ concentration, density and refractive index in ( 1 0 0 - x ) B 2 0 3 - x N a 2 0 glasses Glasses
Nd 3 + concentration N(102° cm - 3 )
Density d ___10 -2
Refractive index n _+10 -3
90B203 - 10Na 20 85B203-15Na20 80B203-20Na 2° 75B203-25Na 20 70B203-30NazO 65B203-35Na 20
3.77 3.84 3.91 4.02 4.11 4.22
2.24 2.27 2.30 2.35 2.39 2.44
1.495 1.500 1.502 1.508 1.516 1.518
M. Zahir et al. / Structural interpretation of optical spectra
225
observed difference may be explained by the recording conditions. Table 2 gives values of the ~2x ( X = 2, 4, 6) parameters for Nd 3+ as a function of Na2 O content and the corresponding radiative deexcitation probability Af~d from the Nd 3+ nF3/2 level. The orders of magnitude are close to those already found for glasses with nearly identical composition e.g. Af~j(20 Na20-15 BAO-65 B 2 0 3 ) = 2670 s -1, Arid(15 K20-18 BAO-67 B203)= 2386s 1 [13,14]. Fig. 3 represents the variation of A~ o versus Na20 content for borate glasses. It reveals an almost linear increase up to about 25 tool.% Na20. At Table 2 Intensity parameters #x (~ = 2, 4, 6) and radiative deexcitation probability Arad,F~/2 ( s - i ) for Nd 3+ in (100- x ) B 2 0 3 - x N a 2 0 glasses GLasses
#2( 10-20 crn2)
#4 (10 20 cm 2)
#6 ( 10-2° cm2)
Arad,~v2
90B203-10Na20 85B203-15Na20 80B203-20Na 20 75B203-25Na 20 70B203-30Na 20 65BEO3-35Na20
3.4_+0.2 3.7_+0.4 4.4_+0.1 5.0_+0.3 5.1 _+0.3 5.1 _+0.2
2.9_+0.1 3.5_+0.4 3.7_+0.2 4.4-+0.3 4.34-0.4 4.7_+0.4
4.3_+0.1 4.5_+0.3 4.8-+0.3 5.0-+0.1 4.8-+0.3 5.0_+0.2
1894 2146 2270 2524 2485 2665
l
X (nm)
//
/
/
/
//
(a)
(aq .o(b')
/ / / / / / ~ ( b )
230
~ -//"
210 ~'
/
./
~
i
o
'
1;
'
2'0
'
3'o
"/.N.d
Fig. 1. Wavelength of the 1S0 --, 3P1 transition of Pb2+ versus Na20 content. Broken curve, theoretical curves for glasses: (a) (100-x)SiO2-xNa20, (b) (100-x)B203-xNa20, (c) (t00x)P2Os-xNa20. Full curve, experimental curves for glasses (100-x)B203-xNa20. (a') Ref. [10], (b') this work.
'
10 '
'
2'0
'
'
'
10 '
.~'---,~" C /
?
'
~
'
2'o
[14] '
30 '
'
(100-x) SiO,., z _ x Na20 _ 0.3°1.Nd,'zO3
~
I~8 -~~-c'~(B5"x)5i 0~;]15BaO- x Na20
{~" '
this work
,,oo -x) B203 _ x Na20
(90-x) P20S_10 At203_ x Na20 [7]
% N~ o"
Fig. 3. Radiative deexcitation of probability versus NazO content in phosphate, borate and silicate glasses.
Fig. 2. Intensity fluorescenceratio R of the 5Do -~ 7F2 to the 5Do --~ 7Fi transition versusNa~O content for (]00-x)B203-xNa20 |% Eu203-
0
0
% Na2~
i
i
50 '
1500
2
AGHER [12]
2000
•.
25o
3
4
work
A rad (4F3,,2)(s-l)
E
c~
;:m
W,
M. Zahir et al. / Structural interpretation of optical spectra
227
higher sodium concentration this relative increase becomes much smoother. The results for phosphate and silicate glasses with similar N a 2 0 content are given for comparison and they will be discussed below.
5. Discussion
For the sodium borate properties illustrated in figs. 1, 2 and 3 one may notice a modification of their respective evolution at an almost similar sodium concentration ( = 25% N a 2 0 ). Therefore the question of the possible selection of specific sites by the probes can be ruled out and the three measured parameters can be assumed to reflect an effective average of the properties of the glasses.
5.1. Analysis of Pb 2 + absorption spectra The red shift of the Pb 2. absorption band with N a 2 0 content corresponds to rising covalency of the P b - 0 bonds. The values are always included between those calculated for sodium silicate and sodium phosphate glasses. In other words the B - O bonds are less covalent than the P - O bonds and more covalent than the Si-O ones. Consequently the covalency of P b - O antagonist bonds increases in the reverse order: phosphate, borate, silicate. Moreover, in borate glasses the experimental curve is upward shifted apart from the theoretical one for compositions between 15 and 25 mol.% Na20. The factors able to increase the glass basicity are the number of tetrahedral IBO41 units and of non-bridging oxygens [10]. The proportion of IBO4/ groups detected by N M R in the Na20-B203 systems rapidly rises actually at a concentration between 10 and 25 mol.% N a 2 0 [15]. The number of non bridging oxygens appearing in the different possible 3-coordinated or 4coordinated borate units cannot be estimated. Its influence on the quick increase of optical basicity in this region is nevertheless probable.
5.2. Analysis of the R factor The R variation reveals a significant increase of the asymmetry of europium oxygen sites up to about 25 mol.% N a 2 0 and as a consequence an increase of A,.p. Beyond that value R is almost independent of glass composition. But in this region the conclusion of section 5.1. implies an increase of the E u - O bond covalency, so that it is possible to deduce an A,.p decrease indicating a higher site symmetry for E u 3+ o v e r the compositional range 25-35 mol.% Na20.
5.3. Analysis of the radiative deexcitation probability A,~ a from the 4F3/2 level of neodymium The spontaneous emission probability A ra d (41:23/2) in sodium borate glasses is compared in fig. 3 to those previously obtained for silicate and phosphate
228
M. Zahir et al. / Structural interpretation of optical spectra
glasses with same N a 2 0 content. Their values are situated between the phosphate and silicate ones but their relative variation with composition is larger. As mentioned above, the weakest covalent P b - O bond occurs in phosphate glasses which nevertheless exhibit the highest A raJ values. As a consequence the phosphate properties are mostly controlled by A,.p. According to Izumitani et al. this can be explained by the 2D-character of the phosphate network former made up of IPO41, chains with a flexibility capable of creating a large variety of low ~ymmetry sites. On the other hand the former network of silicates has a marked 3D-character so that modifier ion sites are less easily deformed, which supposes a lower A,,p. Concerning the borate glasses, for N a 2 0 contents lower than 25 mol.%, the large Arad increase is governed by both At. p and E(t, ~,) parameters. The occurrence of IBO41 units within the boroxol network slightly alters its 2D-character as long as the non-bridging oxygen proportion is high. Beyond 25 mol.% the At, ~ increase is less pronounced. This evolution is directly related to the decrease of A,.p deduced from the R variations. As in the case of silicate glasses, the former sublattice appears as a more 3D-like network which can only result from a simultaneous increase in bridging oxygens. This conclusion is also in agreement with the smaller increase of optical basicity over this composition range.
6. Conclusion
The study of the sodium borate glasses over the compositional range 10-35 mol.% N a 2 0 by using the three structural local probes Pb 2+, Eu 3+ and Nd 3+ demonstrates the existence of two different situations, whether the N a 2 0 content is lower or higher than about 25 mol.%. In the first domain a simultaneous increase of optical basicity and of modifier ion site asymmetry implies a rather 2D-character of the network former. On the contrary the properties of glasses with higher Na20-content involve a stronger 3D-character for the former sublattice. According to the interpretation about the evolution of the inhomogeneous linewidth of the 5D 0---,TF0 transition of Eu 3+ in the same system [16], this 2D ~ 3D evolution seems to be related to the appearance of diborate groups.
References [1] [2] [3] [4] [5]
B.R. Judd, Phys. Rev. 127 (1961) 750. G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. F. Auzel, Ann. T616commun. 24 (1969) 5. W.F. Krupke, I.E.E.E., J. Quantum Electron. 4 (1974) 450. R. Reisfeld, Structure and Bonding 22 (Springer-Verlag, Berlin, Heidelberg, New York, 1975) p. 123. [6] M.J. Weber, J. Non-Crystalline Solids 47 (1982) 117.
M. Zahir et al. / Structural interpretation of optical spectra
229
[7] T. Izumitani, H. Toratani and H. Kuroda, J. Non-Crystalline Solids 47 (1982) 87. [8] N.E. Alexfieff, V.P. Ganotsief, M.E. Jabotincki, V.B. Kravtchen and Y.P. Roudnitski, Lasernie phosphatnie stekla (Nauka, Moscow~ 1980). [9] A. Levasseur, R. Olazcuaga, M. Kbala, M. Zahir, P. Hagenmuller and M. Couzi, Solid St. lonics 2 (1981) 205. [10] J.A. Duffy and M.D. Ingram, J. Non-Crystalline Solids, 21 (1976) 373. [11] C. Linares, M. Blanchard and F. Gaume-Mahn, Proc. 7th. Rare Earth Symp. (Nauka, Moscow, 1972). [12] P.K. Gallagher, C.R. Kurkjian and P.M. Bridenbaugh, Phys. Chem. Glasses 6 (1965) 95. [13] Gan Fuxi, Chen Shuchun and Hu Hefang, Scienta Sinica 14 (1981) 1096: 1102. [14] R.R. Jacobs and M.J. Weber, I.E.E.E.J. Quantum Electron. 2 (1978) 102. [15] P.J. Bray and J.G. Okeefe, Phys. Chem. Glasses 4 (1963) 37. [16] J.R. Morgan, E.P. Chock, W.D. Hopewell, M.A. El°Sayed and R. Orbach, J. Phys. Chem. 85 (1981) 747.
221
A S T R U C T U R A L I N T E R P R E T A T I O N O F T H E P b 2+, Eu 3+ A N D N d 3+ O P T I C A L S P E C T R A IN D O P E D S O D I U M B O R A T E G L A S S E S
M. Z A H I R , R. O L A Z C U A G A , C. P A R E N T , G. L E F L E M a n d P. H A G E N M U L L E R Laboratoire de Chimie du Solide du CNRS, 351 Cours de la Lib&ation, 33405 Talence Cedex. France
Received 10 February 1984 Revised manuscript received 27 June 1984
The study of sodium borate glasses over the compositional range 10-35 mol.% Na20 by using Pb2+ , Eu3÷ and Nd 3+ as local sounds shows the existence of two possible situations depending on whether the Na20 content is lower or higher than about 25 mol.%. In the first composition range the simultaneous increase of optical basicity and of modifier ion site asymmetry implies rather a 2D-character of the network former. The properties of the glasses with high Na20 content involve more 3D-character for the former sublattice.
1. Introduction T h e e l a b o r a t i o n of rare earth b a s e d glasses for technological uses as laser o r solar energy c o n c e n t r a t o r s requires an accurate k n o w l e d g e of the influence of the glass c o m p o s i t i o n on the l u m i n e s c e n t properties. T h e J u d d - O f e l t theory for crystal field i n d u c e d electric-dipole transitions b e t w e e n 4f states allows the p h e n o m e n o l o g i c a l p a r a m e t e r s related to radiative p r o p e r t i e s for a p a r t i c u l a r glass c o m p o s i t i o n to be d e t e r m i n e d [1,2]. Largely used it has p r o v e d to b e a p o w e r f u l tool for the systematic investigation a n d selection of glasses with specific purposes. T h e first studies were m a i n l y d e v o t e d to the influence of the n e t w o r k f o r m e r in materials with c o m p o s i t i o n s close to c o m m e r c i a l glasses [3]. M o r e recently the role of m o d i f i e r ions ( n a t u r e a n d c o n c e n t r a t i o n ) has been c o n s i d e r e d for silicate a n d p h o s p h a t e glasses, b u t the i n t e r p r e t a t i o n of the results is quest i o n e d [4-8]. In the scope o f a general s t u d y of the glasses a b l e to occur in the M 2 0 - M 3 P O 4 - B 2 0 3 ( M = Li, N a ) systems, this p a p e r presents a structural investigation o f s o d i u m b o r a t e glasses b a s e d on a s i m u l t a n e o u s analysis o f the a b s o r p t i o n p r o p e r t i e s of N d 3 ÷ a n d Pb 2 + a n d the emission p r o p e r t i e s of Eu 3 + used as local structural probes. T h e results are c o m p a r e d with those recently o b t a i n e d b y T. I z u m i t a n i et al. for p h o s p h a t e a n d silicate glasses [7]. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M. Zahir et al. / Structural interpretation of optical spectra
222
2. I n v e s t i g a t i o n m e t h o d
2.1. Calculation of the Judd-Ofelt parameters from neodymium absorption measurements The line strength Sea of the forced electric-dipole transition between the initial J manifold I(SL)J >1 and the terminal manifold I(S'L')J')I of the neodymium 4f states can be expressed by: Sea =
~
~2xI((SL)JIIUta~II(S'L')J')I 2
~ 2,4,6
where: - the three terms (IJUtX)ll) are the doubly reduced tensor operators in the intermediate coupling approximation, - the Judd-Ofelt parameters 12z, 124, 126 contain two terms: (i) the crystal field parameters A,.p (with t odd) which are the odd parity terms in the crystal field expansion depending on the site symmetry of the probe ion in a given glass, (ii) the E(t, ~,) parameter which contains the radial part of the 4f wave functions and the energy denominator between 4f and admixing levels with opposite parity. In other words E(t, ?~) measures the covalency rate of the rare earth ligand bond (nephelauxetic effect). Sea is related to the integrated absorbance of an electric-dipole transition fk(X) dX by the equation:
8~3e27,
(
1 (n 2 + 2) 2 Sed
]dk(Xda , = N3hc(Zj + 1) n
9
'
(1)
where: N is the rare earth ion concentration, the average wavelength of the absorption band, n the bulk refractive index at the ~ wavelength. Once the 12x parameters have been determined, the spontaneous emission probability from a I(S'L')J')[ state to a I(SL)J > I state may be obtained via:
A[(S'L')J';(SL)J] = 64 "/r4e 2 n ( n 2 q- 2) 2 Sed 3 h ( 2 J ' + 1)X3 9 and the radiative lifetime is given by:
"tad.~--- EA[(S'L')J';(SL)J]
= A -rad '
SLJ
where A tad is the corresponding radiative deexcitation probability. In order to separate the respective contributions of A,. e and E(t, ~) two types of complementary measurements were carried out on the same glasses: the absorption spectra of Pb z+ and the europium fluorescence spectra.
M. Zahir et al. / Structural interpretation of optical spectra
223
2.2. Absorption spectra of Pb: + In glasses the concept of optical basicity, introduced by Duffy and Ingram, is connected to the ability of the former network oxygen atoms to donate some of their negative charge to the modifier metal ion [10]. In the presence of lead it can be deduced from the position of the wavelength of the absorption band maximum corresponding to the 1S0 --~3P 1 transition of Pb 2+. This band is red shifted if the host glass basicity increases. As pointed out by Reisfeld, such a shift can be correlated indeed to the increasing covalency of the P b - O bond and consequently of the F~(t, X) term [5].
2.3. Europium fluorescence spectra When Eu 3+ occupies sites with low symmetry, which is generally the case in glasses, its emission spectrum may include dipole-electric emission lines such as 5D 0 ----~7F0,2,4,6. Their relative intensities give the degree of asymmetry of the probe environment. But on the other hand the 5D0 ---,7F 1 transition which is of the magnetic-dipole type is always observed and its intensity is independent of the site symmetry. Therefore, it is generally admitted that the ratio of the emission intensities R-
'o0 -'7F2 5 D 0 ----~7 F 1
is an asymmetry parameter for the Eu 3+ sites [11,12]. The variation of R versus glass composition reflects the variation of the A,. e term. However one can emphasize that the covalency degree of the E u - O bond brings its own contribution to the 5D0 --, 7F2 transition intensity by modulating the admixture of the 4 f ' 5 d states into the 4f 6 states.
3. Experimental procedure The glasses were prepared by melting at l l 0 0 ° C in Au, Pt, Rh crucibles a mixture with suitable proportions of Na2CO 3 (Merck 99.5%), Nd203, Eu203 (Rh6ne-Poulenc 99.99%) PbO (Merck 99.9%) and BzO3 (Merck 99.5%). After quenching in air the samples were systematically annealed at 20°C lower than the vitreous transition temperature for 15 h and slowly cooled down. This heat treatment removes the internal tensions of thermal origin. The Nd 3+ absorption was studied on 1 mm thick samples containing 1 mol.% Nd203 which were carefully polished with diamond powder ( = 8 ~m). The spectra were recorded between 400 and 920 nm at room temperature using a Cary 17 spectrometer. For each sample the surfaces of the absorption bands found between these wavelengths and originating from t h e 419/2 ground state were measured.
224
M. Zahir et al. / Structural interpretation of optical spectra
Index of refraction measurements were made using a Pulfrich refractometer. The 122, 124, ~'26 w e r e calculated by a computerized least square program describing eq. (1). The Pb 2+ absorption spectra were recorded with a Cary 17 spectrometer on small thin glasses ( < 0.5 mm). The fluorescence of Eu 3+ under a xenon lamp excitation at 392 nm was recorded using a Jobin-Yvon monochromator and a Varian VPM 159 A photomultiplier on glasses containing 1 mol.% E u 2 0 3. The spectra were corrected by taking into account the specific response curve of the photomultiplier.
4. Experimental
results
Table 1 gives a listing of glass compositions, refractive indexes and densities. Fig. 1 represents the variation of the wavelength due to the ISo----~3P1 transition as a function of N a 2 0 content in the sodium borate glasses. This evolution is compared to: - the experimental values previously obtained by Duffy and Ingram [10], - the theoretical curves calculated by those authors on the basis of the optical basicity concept for Na 2 0 - B 2 0 3 , N a 2 0 - S i O 2 and Na20-P205 systems. The calculation takes into account the Pauling electronegativities of Na, B, Si and P and the glass composition. In the NaaO-B203 system (b' curve) a marked increase of the P b - O covalency is observed for 15 to 25 mol.% Na20. By further increasing of the N a 2 0 content this trend becomes less important. The obtained values are slightly different from those measured by the previous authors, but the general tendency is similar (a' curve). Fig. 2 gives the variation of the intensity ratio R of the 5D o ---,TF2 to the 5D o ---*TF1 fluorescence of E u 3+ v e r s u s N a 2 0 content. The results are compared to those previously given by Gallagher [12]. If the variation of R differs by an almost constant quantity, the trend of the two curves is similar. The
Table 1 Experimental values of Nd 3+ concentration, density and refractive index in ( 1 0 0 - x ) B 2 0 3 - x N a 2 0 glasses Glasses
Nd 3 + concentration N(102° cm - 3 )
Density d ___10 -2
Refractive index n _+10 -3
90B203 - 10Na 20 85B203-15Na20 80B203-20Na 2° 75B203-25Na 20 70B203-30NazO 65B203-35Na 20
3.77 3.84 3.91 4.02 4.11 4.22
2.24 2.27 2.30 2.35 2.39 2.44
1.495 1.500 1.502 1.508 1.516 1.518
M. Zahir et al. / Structural interpretation of optical spectra
225
observed difference may be explained by the recording conditions. Table 2 gives values of the ~2x ( X = 2, 4, 6) parameters for Nd 3+ as a function of Na2 O content and the corresponding radiative deexcitation probability Af~d from the Nd 3+ nF3/2 level. The orders of magnitude are close to those already found for glasses with nearly identical composition e.g. Af~j(20 Na20-15 BAO-65 B 2 0 3 ) = 2670 s -1, Arid(15 K20-18 BAO-67 B203)= 2386s 1 [13,14]. Fig. 3 represents the variation of A~ o versus Na20 content for borate glasses. It reveals an almost linear increase up to about 25 tool.% Na20. At Table 2 Intensity parameters #x (~ = 2, 4, 6) and radiative deexcitation probability Arad,F~/2 ( s - i ) for Nd 3+ in (100- x ) B 2 0 3 - x N a 2 0 glasses GLasses
#2( 10-20 crn2)
#4 (10 20 cm 2)
#6 ( 10-2° cm2)
Arad,~v2
90B203-10Na20 85B203-15Na20 80B203-20Na 20 75B203-25Na 20 70B203-30Na 20 65BEO3-35Na20
3.4_+0.2 3.7_+0.4 4.4_+0.1 5.0_+0.3 5.1 _+0.3 5.1 _+0.2
2.9_+0.1 3.5_+0.4 3.7_+0.2 4.4-+0.3 4.34-0.4 4.7_+0.4
4.3_+0.1 4.5_+0.3 4.8-+0.3 5.0-+0.1 4.8-+0.3 5.0_+0.2
1894 2146 2270 2524 2485 2665
l
X (nm)
//
/
/
/
//
(a)
(aq .o(b')
/ / / / / / ~ ( b )
230
~ -//"
210 ~'
/
./
~
i
o
'
1;
'
2'0
'
3'o
"/.N.d
Fig. 1. Wavelength of the 1S0 --, 3P1 transition of Pb2+ versus Na20 content. Broken curve, theoretical curves for glasses: (a) (100-x)SiO2-xNa20, (b) (100-x)B203-xNa20, (c) (t00x)P2Os-xNa20. Full curve, experimental curves for glasses (100-x)B203-xNa20. (a') Ref. [10], (b') this work.
'
10 '
'
2'0
'
'
'
10 '
.~'---,~" C /
?
'
~
'
2'o
[14] '
30 '
'
(100-x) SiO,., z _ x Na20 _ 0.3°1.Nd,'zO3
~
I~8 -~~-c'~(B5"x)5i 0~;]15BaO- x Na20
{~" '
this work
,,oo -x) B203 _ x Na20
(90-x) P20S_10 At203_ x Na20 [7]
% N~ o"
Fig. 3. Radiative deexcitation of probability versus NazO content in phosphate, borate and silicate glasses.
Fig. 2. Intensity fluorescenceratio R of the 5Do -~ 7F2 to the 5Do --~ 7Fi transition versusNa~O content for (]00-x)B203-xNa20 |% Eu203-
0
0
% Na2~
i
i
50 '
1500
2
AGHER [12]
2000
•.
25o
3
4
work
A rad (4F3,,2)(s-l)
E
c~
;:m
W,
M. Zahir et al. / Structural interpretation of optical spectra
227
higher sodium concentration this relative increase becomes much smoother. The results for phosphate and silicate glasses with similar N a 2 0 content are given for comparison and they will be discussed below.
5. Discussion
For the sodium borate properties illustrated in figs. 1, 2 and 3 one may notice a modification of their respective evolution at an almost similar sodium concentration ( = 25% N a 2 0 ). Therefore the question of the possible selection of specific sites by the probes can be ruled out and the three measured parameters can be assumed to reflect an effective average of the properties of the glasses.
5.1. Analysis of Pb 2 + absorption spectra The red shift of the Pb 2. absorption band with N a 2 0 content corresponds to rising covalency of the P b - 0 bonds. The values are always included between those calculated for sodium silicate and sodium phosphate glasses. In other words the B - O bonds are less covalent than the P - O bonds and more covalent than the Si-O ones. Consequently the covalency of P b - O antagonist bonds increases in the reverse order: phosphate, borate, silicate. Moreover, in borate glasses the experimental curve is upward shifted apart from the theoretical one for compositions between 15 and 25 mol.% Na20. The factors able to increase the glass basicity are the number of tetrahedral IBO41 units and of non-bridging oxygens [10]. The proportion of IBO4/ groups detected by N M R in the Na20-B203 systems rapidly rises actually at a concentration between 10 and 25 mol.% N a 2 0 [15]. The number of non bridging oxygens appearing in the different possible 3-coordinated or 4coordinated borate units cannot be estimated. Its influence on the quick increase of optical basicity in this region is nevertheless probable.
5.2. Analysis of the R factor The R variation reveals a significant increase of the asymmetry of europium oxygen sites up to about 25 mol.% N a 2 0 and as a consequence an increase of A,.p. Beyond that value R is almost independent of glass composition. But in this region the conclusion of section 5.1. implies an increase of the E u - O bond covalency, so that it is possible to deduce an A,.p decrease indicating a higher site symmetry for E u 3+ o v e r the compositional range 25-35 mol.% Na20.
5.3. Analysis of the radiative deexcitation probability A,~ a from the 4F3/2 level of neodymium The spontaneous emission probability A ra d (41:23/2) in sodium borate glasses is compared in fig. 3 to those previously obtained for silicate and phosphate
228
M. Zahir et al. / Structural interpretation of optical spectra
glasses with same N a 2 0 content. Their values are situated between the phosphate and silicate ones but their relative variation with composition is larger. As mentioned above, the weakest covalent P b - O bond occurs in phosphate glasses which nevertheless exhibit the highest A raJ values. As a consequence the phosphate properties are mostly controlled by A,.p. According to Izumitani et al. this can be explained by the 2D-character of the phosphate network former made up of IPO41, chains with a flexibility capable of creating a large variety of low ~ymmetry sites. On the other hand the former network of silicates has a marked 3D-character so that modifier ion sites are less easily deformed, which supposes a lower A,,p. Concerning the borate glasses, for N a 2 0 contents lower than 25 mol.%, the large Arad increase is governed by both At. p and E(t, ~,) parameters. The occurrence of IBO41 units within the boroxol network slightly alters its 2D-character as long as the non-bridging oxygen proportion is high. Beyond 25 mol.% the At, ~ increase is less pronounced. This evolution is directly related to the decrease of A,.p deduced from the R variations. As in the case of silicate glasses, the former sublattice appears as a more 3D-like network which can only result from a simultaneous increase in bridging oxygens. This conclusion is also in agreement with the smaller increase of optical basicity over this composition range.
6. Conclusion
The study of the sodium borate glasses over the compositional range 10-35 mol.% N a 2 0 by using the three structural local probes Pb 2+, Eu 3+ and Nd 3+ demonstrates the existence of two different situations, whether the N a 2 0 content is lower or higher than about 25 mol.%. In the first domain a simultaneous increase of optical basicity and of modifier ion site asymmetry implies a rather 2D-character of the network former. On the contrary the properties of glasses with higher Na20-content involve a stronger 3D-character for the former sublattice. According to the interpretation about the evolution of the inhomogeneous linewidth of the 5D 0---,TF0 transition of Eu 3+ in the same system [16], this 2D ~ 3D evolution seems to be related to the appearance of diborate groups.
References [1] [2] [3] [4] [5]
B.R. Judd, Phys. Rev. 127 (1961) 750. G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. F. Auzel, Ann. T616commun. 24 (1969) 5. W.F. Krupke, I.E.E.E., J. Quantum Electron. 4 (1974) 450. R. Reisfeld, Structure and Bonding 22 (Springer-Verlag, Berlin, Heidelberg, New York, 1975) p. 123. [6] M.J. Weber, J. Non-Crystalline Solids 47 (1982) 117.
M. Zahir et al. / Structural interpretation of optical spectra
229
[7] T. Izumitani, H. Toratani and H. Kuroda, J. Non-Crystalline Solids 47 (1982) 87. [8] N.E. Alexfieff, V.P. Ganotsief, M.E. Jabotincki, V.B. Kravtchen and Y.P. Roudnitski, Lasernie phosphatnie stekla (Nauka, Moscow~ 1980). [9] A. Levasseur, R. Olazcuaga, M. Kbala, M. Zahir, P. Hagenmuller and M. Couzi, Solid St. lonics 2 (1981) 205. [10] J.A. Duffy and M.D. Ingram, J. Non-Crystalline Solids, 21 (1976) 373. [11] C. Linares, M. Blanchard and F. Gaume-Mahn, Proc. 7th. Rare Earth Symp. (Nauka, Moscow, 1972). [12] P.K. Gallagher, C.R. Kurkjian and P.M. Bridenbaugh, Phys. Chem. Glasses 6 (1965) 95. [13] Gan Fuxi, Chen Shuchun and Hu Hefang, Scienta Sinica 14 (1981) 1096: 1102. [14] R.R. Jacobs and M.J. Weber, I.E.E.E.J. Quantum Electron. 2 (1978) 102. [15] P.J. Bray and J.G. Okeefe, Phys. Chem. Glasses 4 (1963) 37. [16] J.R. Morgan, E.P. Chock, W.D. Hopewell, M.A. El°Sayed and R. Orbach, J. Phys. Chem. 85 (1981) 747.