Tb3+ Luminescence as a Tool to Study Clustering of Lanthanide ...

Journal of Non-Crystalline Solids 325 (2003) 29–33www.elsevier.com/locate/jnoncrysolTb 3þ luminescence as a tool to study clusteringof lanthanide ions in oxynitride glassesD. de Graaf * , S.J. Stelwagen, H.T. Hintzen, G. de WithLaboratory of Solid-State and Materials Chemistry, Eindhoven University of Technology,P.O. Box 513, 5600 MB Eindhoven, The NetherlandsReceived 30 September 2002; received in revised form 26 May 2003AbstractThe 5 D 3 and 5 D 4 emission of Tb 3þ under UV-excitation in the 4f 7 5d band has been measured as a function of the Tbcontent in Y–Si–Al–O–N glasses. A strong increase of the 5 D 4 emission over the 5 D 3 emission was observed at highTb 3þ contents. This increase can be attributed to cross-relaxation between two neighbouring Tb 3þ ions. The feedingratio of the 5 D 3 and 5 D 4 levels (0.11) and the interaction distance for cross-relaxation (1.43 nm) excludes thepresence of Tb 3þ clusters, pairs or other abnormalities (like phase separation) in the Tb 3þ activated materials.Ó 2003 Elsevier B.V. All rights reserved.1. IntroductionRare-earth containing Si–Al–O–N glasses havebeen extensively studied with respect to their mechanicalproperties. It is shown that the presenceof chemically incorporated nitrogen in these glassesleads to a considerable improvement of thechemical and mechanical durability. The hardness[1], elastic modulus [2] and slow crack growth resistance[3] are higher than for the correspondingoxide glass. The choice of a small trivalent lanthanideion as a modifier cation further improves themechanical resistance of these glasses [4,5].A number of attempts have been made to explainthese properties from the structure of the* Corresponding author. Tel.: +31-40 247 5031; fax: +31-40244 5619.E-mail address: d.de.graaf@tue.nl (D. de Graaf).glass. Using tools such as XPS [6] and 27 Al/ 28 SiNMR [7,8] a clearer picture has been produced ofthe network structure and the location and role ofnitrogen in that structure. There is, however, littledetailed information available on how the modifiercations are incorporated in the glass matrix.Recent investigations have shown that bystudying the luminescence of lanthanides in theseglasses, valuable information on the local coordinationof the lanthanide ion can be obtained [9].These studies have stressed the issue of possiblelanthanide pair formation or clustering in theglasses. Lanthanide clustering has been suspectedto play a role in many lanthanide-containingglasses [10], although it is experimentally hard toverify.The luminescence characteristics of Tb 3þ canprovide an elegant solution for this problem. It ispossible to evaluate lanthanide clustering in theseglasses by studying the luminescence properties of0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0022-3093(03)00324-7

D. de Graaf et al. / Journal of Non-Crystalline Solids 325 (2003) 29–33 31then allowed to cool down to room temperaturein air.The oxide scale, which had formed duringcooling, was removed by grinding the surface. Thedensity of the glasses was measured using the Archimedesmethod in distilled water. From thedensity the average volume per Tb ion (V Tb )andthe number of Tb ions per unit volume (1=V Tb ) canbe calculated using Eq. (1):V Tb ¼ 1 M glass;ð1ÞN Av x Tb q glasswherein N Av is the number of Avogadro, x Tb is thefraction of Tb, M glass is the molar weight of theglass, q glass is the measured density of the glass.The glass was crushed to a fine powder prior tothe spectroscopic analysis. The luminescencecharacteristics of the glasses were measured usinga fluorophotospectrometer equipped with a Xeflashlamp.The glasses were measured in reflection,excitation and emission mode. For reflection thespectra were corrected for lamp and detectorcharacteristics and system transmission using theknown spectra of BaSO 4 and black felt as standards.The emission spectrum was calibrated usinga tungsten lamp as standard. For calibration of theexcitation spectra rhodamine was used as a standardin combination with a second photomultiplier.3. ResultsThe measured density increases with the Tbcontent (Fig. 2), which can be expected from thesignificantly larger atomic mass and the somewhatlarger ionic radius of Tb as compared to Y.The reflection measurements for low Tb contents(Fig. 3) show a strong absorption at lowwavelengths, which can be attributed to the glassmatrix absorption. With an increasing Tb concentrationadditional regions of enhanced absorptioncan be discriminated. These Tb 3þabsorption bands also show up in the excitationspectrum (Fig. 4) and can be attributed to thevarious transitions from the 7 F 6 ground state tohigher states (f ! d and f ! f transitions). In thisspectrum it can be clearly seen that the glassρ gl ass [g cm -3 ]65.554.543.530 5 10 15x [-]Fig. 2. Density of Y 15:2 x Tb x Si 14:7 Al 8:7 O 54:1 N 7:38 glasses as afunction of the Tb content (x).Reflection [-]f-d transitionf-f transitionsx = 0.025x = 15.2250 300 350 400 450Wavelength [nm]Fig. 3. Reflection spectra of Y 15:2 x Tb x Si 14:7 Al 8:7 O 54:1 N 7:38glasses (x ¼ 0:025 and x ¼ 15:2). Arrows denote terbium absorption.Host4f 7 5d5 D 15 D 2 , 5 L 9, 5 L 10, 5 D 35 D 4200 250 300 350 400 450 500wavelength [nm]Fig. 4. Excitation spectrum of Tb 15:2 Si 14:7 Al 8:7 O 54:1 N 7:38 glass(k mon ¼ 545 nm, 5 D 4 ! 7 F 5 ).

D. de Graaf et al. / Journal of Non-Crystalline Solids 325 (2003) 29–33 33of 1.43 nm, which is roughly the same as the valuesreported for YAG:Tb 3þ (1.15 [14], 1.34 [15]). Boththe feeding ratio and the interaction distance correspondto an ideal dispersion of Tb 3þ in the glass.It is apparent that over the whole measured rangethe behaviour of the (Tb,Y)–Si–Al–O–N glass issomewhat different from that of YAG:Tb, i.e.lower values at low Tb concentrations and highervalues at high Tb concentrations. It must howeverbe noted that this effect is relatively insignificant tothat caused by cluster or pair formation, whichenhances the green emission by a few orders ofmagnitude over the whole Tb concentration range[11]. These effects are clearly absent, showing thatthe distribution of the terbium ions is normal.This raises the question whether these observationscan be extended to predict the distributionof other lanthanides in Si–Al–O–N glasses. For thesmall trivalent lanthanides (high-Z) this seemslikely because of the chemical similarity betweenthese ions. However, for the large ions (low-Z) itisrecommended to be determined independently.5. ConclusionsThe strong increase of the Tb 3þ 5 D 3 emissionover the 5 D 4 emission in Y–Si–Al–O–N:Tb glasseswhen lowering the Tb concentration has shownbeyond doubt that Tb 3þ pair formation, clustering,and phase separation are absent in theseglasses. Both the interaction distance for 5 D 4 / 5 D 3cross-relaxation as well as the feeding ratio ( 5 D 4 /5 D 3 ) corresponds to an ideal Tb 3þ dispersion.References[1] D. de Graaf, M. Bracisiewicz, H.T. Hintzen, M. Sopicka-Lizer, G. de With, J. Mater. Sci., submitted for publication.[2] R.E. Loehman, in: Treatise on Materials Science andTechnology, vol. 26 glass IV, Academic Press, 1985, p. 119.[3] D.N. Coon, J. Non-Cryst. Solids 226 (1998) 281.[4] R. Ramesh, E. Nestor, M.J. Pomeroy, S. Hampshire,J. Eur. Ceram. Soc. 17 (1997) 1933.[5] Y. Menke, V. Peltier-Baron, S. Hampshire, Preparationand characterisation of rare-earth sialon glasses and glassceramics, in: Conf. and Exh. of the Eur. Ceram. Soc. Proc.,2000, p. 363.[6] T. Hanada, N. Ueda, N. Soga, J. Ceram. Soc. Jpn. Int. Ed.96 (1988) 281.[7] S. Sakka, J. Non-Cryst. Solids 181 (1995) 215.[8] A. Nordmann, Y.-B. Cheng, M.E. Smith, Chem. Mater. 8(1996) 2516.[9] D. de Graaf, H.T. Hintzen, S. Hampshire, G. de With,J. Eur. Ceram. Soc. 23 (2003) 1093.[10] M.C. Wilding, A. Navrotsky, J. Non-Cryst. Solids 265(2000) 238.[11] A.M.A. van Dongen, J. Non-Cryst. Solids 139 (1992) 271.[12] C. Armellini, M. Ferrari, M. Montagna, G. Pucker, C.Bernard, A. Monteil, J. Non-Cryst. Solids 245 (1999) 115.[13] L.G. van Uitert, L.F. Johnson, J. Chem. Phys. 44 (9)(1966) 3514.[14] W.F. van der Weg, Th.J.A. Popma, A.T. Vink, J. Appl.Phys. 57 (12) (1985) 5450.[15] D.J. Robbins, B. Cockayne, B. Lent, J.L. Glasper, SolidState Commun. 20 (1976) 673.