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Cationic disorder effects in complex oxide laser materials and phosphors
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Optical Materials 30 (2008) 1677–1681 www.elsevier.com/locate/optmat
Cationic disorder effects in complex oxide laser materials and phosphors A. Lupei a, V. Lupei a,*, C. Gheoghe a, L. Gheorghe a, G. Aka b, D. Vivien b a
Institute of Atomic Physics – INFLPR, Laboratory of Solid-State Quantum Electronics, Bucharest 077125, Romania b Ecole Nationale Superieure de Chimie, LCAES, CNRS-UMR 7574, 75231 Paris, France Available online 31 December 2007
Abstract A comparative analysis of the high-resolution spectra of the Nd3+ and Pr3+ in strontium lanthanum hexa-aluminates, charge compensated with magnesium Sr1 x(Nd, Pr)yLax–yMgxAl12 xO19, on a large composition range is presented. The spectra are mainly dependent on the composition parameter x and show two families of centers for x < 0.5 and three for x P 0.8, with distinct spectral characteristics. Based on spectral data, composition dependence and crystal structure, tentative structural models for the these centers are proposed. It is inferred that this complex picture results from the interplay between the effects determined by the anionic coordination of the centers and the perturbative effects caused by the multiple occupation of specific sites by cations of different valence. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Optical spectroscopy; Nd3+; Pr3+; Strontium lanthanum aluminates; Disordered crystals
1. Introduction The strontium lanthanum hexa-aluminates doped with rare earth (RE3+) ions (especially Nd3+ or Pr3+) and charge compensated with Mg2+, Sr1 xLax–yREyMgxAl12 xO19 0 6 x < 1, y 6 x (known as ASL or SAM for low x and LMA for x = 1) have been investigated as laser materials especially for Nd3+ near infrared emission [1–7] or for Pr3+ visible emission [8,9] and for Pr3+ photon cascade/ quantum cutting emission [10–14]. The co-doping with the optically inert La3+ ions is used to improve the crystals quality (congruent melting has been observed for x > 0.2) and to avoid RE3+ emission quenching at high concentrations [5–7,14]. At low x values (<0.4) these systems have hexagonal magnetoplumbite structure (space group P63/mmc) in which the Sr2+ and La3+ ions as well as the dopant RE3+ ions occupy a unique (2d) site of ideal D3h symmetry, with 12O2 coordination and with similar cation–oxygen distances. The (2d) sites are placed in the mirror planes (per*
Corresponding author. E-mail addresses: [email protected]fim.ro, [email protected] (V. Lupei). 0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.11.009
pendicular on the optical axis c), separated by two spineltype groups where Al3+ can occupy octahedral, tetrahedral and five-fold coordinated bipyramidal sites [3]. The charge compensation is made with Mg2+ ions that replace Al3+ ions. The X-ray investigations [3] have suggested that for large x (x > 0.4) some of the lanthanide ions could occupy also sites of lower symmetry (C2v). Structural X-ray investigations [15] on crystals with x = 1, LaMgAl11O19 – LMA have revealed a more complex structure, close to magnetoplumbite, but with additional structural (6h) sites for the La3+ or RE3+ cations similar to the (2d) ones, but of lower symmetry (C2v or lower). The high oxygen coordination and the presence of small highly charged Al3+ ions sharing the oxygens with the RE3+ ions determine reduced covalence and low nephelauxetic effect leading to high barycenters of the energy manifolds as well as to moderate crystal field effects. Thus, the 4F3/2 ? 4I9/2 Nd3+ laser emission wavelength (900 nm) is one of the shortest reported for this transition and the lowest Pr3+ 4f5d level is above 1S0 (4f2), making the quantum cutting emission possible. Although the early investigations of Nd3+ or Pr3+ spectra in crystals with x < 0.4 have revealed a complex composition dependence, they have been interpreted in terms of a unique spectroscopic center [1–13]. However, at the end of
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the series (x = 1, La1 yREyMgAl11O19 - LMA), the spectroscopic data show clearly three centers for Pr3+ [16] and Nd3+ [17,18]. The recent high-resolution optical spectroscopy revealed the presence of at least two families of structural centers of Nd3+ [19–21] or Pr3+ [22] in ASL crystals for small and intermediate values of composition parameter x < 0.5. This paper presents a comparative investigation of the composition dependence of Nd3+ and Pr3+ spectral characteristics in hexa-aluminate crystals, for a large composition range, in the attempt to clarify the distribution of the cations in the various lattice sites of these disordered crystals and to assess the optimal compositions for various applications. 2. Experimental Various Sr1 xLax–yREyMgxAl12 xO19 single crystals with compositions 0.01 < x 6 1, doped with Nd3+ or with 0.05 6 x 6 0.3 doped with Pr3+, grown by Czochralski method in iridium crucibles, in N2 atmosphere, were investigated. High-resolution absorption, selective excitation and emission kinetics, obtained at 15 or 300 K, with setups described elsewhere [19] are analyzed. Detection was made with a one-meter GDM double monochromator and a photon counting system in connection with a MCS multi-channel analyzer. A closed cycle He refrigerator was used for low temperature measurements. 2.1. Nd3+ spectral behavior in hexa-aluminates In various regions of the Nd3+ absorption of Sr1 xLax–yNdyMgxAl12 xO19 (0.01 6 x 6 1) crystals, the spectra show significant changes when the structural parameter x varies. The spectra are transition-dependent and practically independent on parameter y, the Nd3+ content. For x < 0.5 two families of centers, denoted as C1 and C2 [19], were observed. The lines are well separated in the 4 I9/2 ? 4F3/2 transition, as illustrated in Fig. 1 with 15 K absorption spectra for samples of different compositions, taken in unpolarized light with propagation along c-axis. C2
3+
: 4I
9/2
- 2P1/2
3
C2
C1
10
Nd
I
y=0.05
C1
k(cm-1)
x=0.1 x=0.2
k(cm-1)
4 4 I9/2 - F3/2
The center denoted by C2 is prevailing at very low x (<0.1), while C1 is dominant at intermediate x values (>0.4). The 4F3/2 splitting is about twice larger for C1 than for C2 (90–110 cm 1 for C1 and 40–50 cm 1 for C2 function of parameter x). The C2 lines are fairly sharp (2–5 cm 1), while the C1 lines are asymmetric and much broader (10– 25 cm 1). For the same transition, the absorption spectra for high x = 0.8–1 (LMA: Nd3+) samples, each 4I9/2 ? 4 F3/2 line is split into two components (I–II), as observed previously [17,18]. Such a structure is resolved in other transitions too. An interesting composition dependence was observed in the 4I9/2 ? 2P1/2 Nd3+ absorption at 15 K. Whereas for very low x two peaks, separated by about 2–3 cm 1, were resolved, at higher x (>0.2) only an asymmetric line is observed. At the end of series, x = 0.8 and 1, the spectra show three main lines (I–III), as illustrated in Fig. 2. The lines of centers II and III are shifted from those of center I by 30 cm 1 and respectively 200 cm 1, on lower energies. The lines are asymmetric and inhomogeneously broadened (widths of 10–15 cm 1 for center I, 25–30 cm 1 for center II, and 15–25 for center III) indicating a large structural disorder in the environment of the Nd3+ ions. The intensity of centers II and III increase with x. The lines of center III are very well resolved in the hypersensitive Nd3+4I9/2 ? 4G5/2, 2G7/2 transition, having negligible intensity at x = 0.5 as observed in (Fig. 3). In the 4I9/2 ? 4F3/2 absorption spectra the lines of this center are unclear even for x = 1, but they have been detected by selective excitation [18]. The mean splittings of 4F3/2 are 160 cm 1 for centers I and II and 200 cm 1 for center III, slightly different in stochiometric and congruent grown crystals [18]. The selective excitation in broad absorption lines especially of the C1 center in ASL [19] and I, II, III centers in LMA [18] have shown the existence of different sub-sets of centers in the composition of these lines.
II
III
x=0.5
I
2 x=0.8
x=0.5 0
II 11500
II
I
11600
x=1
x=1
1
I
11700
E(cm-1) Fig. 1. Composition dependence of Nd3+4I9/2 ? 4F3/2 15 K absorption of Sr1 xLax–yNdyMgxAl12 xO19 samples.
23250
23400
23550
E(cm-1) Fig. 2. The 4I9/2 ? 2P1/2 absorption spectra at 15 K of ASL:Nd for x = 0.5, y = 0.05 and x = 0.8, 1; y = 0.1.
A. Lupei et al. / Optical Materials 30 (2008) 1677–1681
Nd3+ : 4I
20
9/2
3+ Pr : 3 3 H - P 4 2 C
- 4G
,2 5/2 G7/2
2
I, II
k(cm-1)
1679
3+ Pr : 3 3 H - P 4 0
C1
x=0.5
III
x=0.05,y=0.05 x=0.1,y=0.1
x=y=0.1
x=0.8
10
x=0.2,y=0.05 x=y=0.2 x=0.3,y=0.05
x=y=0.05
x=1 17000
17200
17400
17600
-1
E(cm ) 4
4
2
22400
22600
E(cm-1)
22800
20750
20800
20850
E(cm-1)
Fig. 4. Composition dependence of Pr3+ 15 K absorption.
Fig. 3. The I9/2 ? G5/2, G7/2 absorption spectra at 15 K of Sr1 xLax–yNdyMgxAl12 xO19 for x = 0.5, y = 0.05 and x = 0.8, 1, y = 0.1.
Polarized absorption data show a complex behavior for Nd3+ C1 and C2 non-equivalent centers in ASL. The C2 center spectra satisfy the selection rules of the D3h ideal local symmetry of (2d) sites [23], while those of C1 center indicate a small lowering in symmetry [21]. In the case of Nd3+ centers I and II (x = 0.8–1 samples), the observed polarization data in the 15 and 300 K absorption can be rather well interpreted in terms of the selection rules of D3h group. However, the spectrum of center III in the 4 I9/2 ? 4G5/2 transition has three lines (not two in r spectrum and one in p as expected for D3h local symmetry) indicating a clear lower symmetry than D3h. 2.2. Pr3+ spectra in hexa-aluminates The high-resolution 15 K absorption spectra of Sr1 xLax–yPryMgxAl12–xO19 crystals with low x parameter (0.05 6 x 6 0.3, y 6 0.2) show composition and transition-dependent features, similar to Nd3+. A clear two-line structure (denoted again with C1 and C2) with the components separated by 25 cm 1 is observed in the first line of the 15 K 3H4 ? 3P2 absorption (Fig. 4). The C1 center intensity increases with x and is dominant at x = 0.3. The widths of these lines are quite different (30 cm 1 for C1, and 15 cm 1 for C2 center in the x = 0.1, y = 0.1 sample). Similar to the 2P1/2 Nd3+ level, the positions of the 3P0 3+ Pr level for the two centers are very close, so that a single asymmetric line is observed in the 3H4 ? 3P0 absorption (Fig. 4). A slight deviation from D3h symmetry was remarked for C1 center from polarization data. 3. Discussion The spectroscopic investigation (high-resolution absorption, selective excitation or emission decay) of a large variety of Sr1 xLax–yNdyMgxAl12 xO19 samples reveals the presence of several non-equivalent Nd3+ centers that prevail in different composition ranges. These centers present
distinct spectral characteristics: energy levels, emission kinetics, etc. In the attempt to elucidate the nature of Nd3+ centers in Sr1 xLax–yNdyMgxAl12 xO19, the structural (X-ray), optical spectroscopy and EPR data could be used. Thus, the X-ray data have suggested that for x > 0.4 these crystals show departure from the ideal magnetoplumbite structure and the RE3+ ion could occupy sites of lower symmetry [3,15]. The optical spectra show essential changes with composition parameter x: two families of centers at low x and three at very high x were observed. The EPR spectra for very low x values present a single line, corresponding to identical isolated ions, while for larger x = y values (60.2) this line is accompanied by a structure associated to pairs of slightly different centers [24,25]. In case of x = 1 two resonance lines corresponding to two different types of centers were reported [17]. Some information about the nature of these centers could be obtained from the barycenters position of the 2S+1 LJ manifolds, defined as DE(2S+1LJ) = E(2S+1LJ) 4 E( I9/2). By using the energy levels of different Nd3+ centers from selective excitation data [18,19] a barycentre curve can be drawn (Fig. 5). Since the barycenters are influenced by nephelauxetic effect, their position depends on many structural and compositional factors, such as the nature of surrounding ions, the coordination number (increase with it), the size of the ion to be replaced by Nd3+ in the lattice host, the geometry of the first anionic sphere, etc. However, this diagram suggests that C1, C2 centers could have a similar ionic 12O2 environment for Nd3+ ions. Since the barycenters of centers I and II at x = 1 are close to those of C1 and C2 centers, one could assume that they have also the same anionic vicinity 12O2 , the lowering of their barycenters can be associated with the size of the ˚ replaced ions (ionic radii of La3+ and Sr2+ are 1.32 A ˚ and respectively 1.4 A) and a small shrinkage of the anionic vicinity. The barycenters of center III are lower, it could mean a lower coordination or a large distortion of the first anionic coordination sphere. This would alter severely the transition probabilities, explaining the high intensity in of center III lines in the hypersensitive transition.
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Δ E(cm
-1
2
) P 1/2
23200
C 1, C 2
ASL
I
C1
C2
II
23100
III
I, II, III LMA
23000
22900
YAG 11200
11300
ΔE(cm-1)
11400
4F 3/2
Fig. 5. The barycenters positions of the 4F3/2 and 2P1/2 manifolds, for Nd3+ doped in different crystals.
The difference between the 4F3/2 level splittings for C1 and C2 centers (about twice) evidence different second order crystal field parameters. These parameters depend stronger on the second order neighbors than the higher order parameters, so one can assume a different cationic vicinity for these centers. The cations of different electric charges that occupy the same sites: Sr2+ and (La3+, RE3+) in (2d) sites or Al3+ and Mg2+, could determine the crystal field dissimilarity. A given (2d) site in ASL is surrounded by six (2d) sites ˚ , the next ones being situated in the mirror plane at 5.56 A ˚ . There are many Al3+ sites at distances larger than 9.62 A ˚ there are three close to a given (2d) site. From 3.2 to 6 A bypiramidal, 24 octahedral and six tetrahedral Al3+ sites. The positions occupied by Mg2+ ions are not clear, they could be Al3+ tetrahedral sites [26], but probably also other Al3+ sites at high x. For low x, the Mg2+ crystal field effects are expected to be much lower than those of trivalent ions, since the relative concentration with respect to the available crystallographic sites is x for trivalent lanthanides (La3+ or RE3+) and x/12 in the case of Mg2+. Based on these data it was proposed that C2 centers, prevailing at low x (x < 0.1), could correspond to Nd3+ in a (2d) site with only Sr2+ in nearby six (2d) sites and charge compensated with Mg2+, probably in tetrahedral Al3+ sites at different distances [20]. The C1 lines are composite of various structural centers of Nd3+ in a (2d) site with 12O2 around and one or more of the six (2d) nearest neighbor sites could be occupied by La3+ or Nd3+. This structural model for the C1 and C2 families of centers is consistent with the data on the composition dependence of the relative intensity of C1 center and with the emission decays measurements [19]. The relative intensity of the C1 Nd3+ center lines to the total intensity in the 4I9/2(1) ? 4 F3/2(1) absorption, function on composition parameter x, increases as 1 (1 x)6 with a saturation at x 0.5 [21]. This law corresponds to the calculated global probability that one up to six nearby (2d) sites to be occupied by trivalent La3+ or Nd3+ ions assuming a random occupa-
tion. The emission decays for the two centers differs only at early times up to 200 ls, being faster for C1, and then they run in parallel. It was inferred that this difference originates from the energy transfer inside the first Nd3+–Nd3+ pairs that, according to this structural model, are possible only for C1 center. This fact is in accord with EPR data [24,25] that show only a line assigned to isolated ions and pairs for the x = y 6 0.2 crystals. The charge compensator Mg2+ could have a correlated distribution with the dopant and its effects are assumed to contribute mainly to linewidths. The X-ray investigations of LMA (x = 1): Nd crystals are not conclusive as concerns the crystal structure The lanthanide ions could occupy two types of sites of (2d) or distorted (6h) of lower symmetry (C2v), Al3+ in the mirror plane has been also localized not in a five-fold coordination, but in a tetrahedral – like environment [15,27]. These crystals show departures from stoichiometry, they have been found defective in La3+, Mg2+ [15,18,27] or oxygen. The probability of having Mg2+ ions near the Nd3+ ions and substituting other Al3+ sites closer to Nd3+ increases, and a non-random distribution is very likely. Thus, it is difficult to give definite models for these centers from the optical spectroscopy data. The centers I and II could be linked to a weak distortion of D3h symmetry, while the center III corresponds to a strongly distorted site of lower symmetry, probably of C2v, as is sustained by the polarized absorption. The spectral data of Pr3+ in Sr1 xLax–yPryMgxAl12 xO19 for x 6 0.3 show a similar two center C1 and C2 structure, resolved in some transitions. One could assume that as for Nd3+, the C2 center corresponds to a Pr3+ ion in a (2d) site surrounded only by Sr2+ in nearby sites and with a far away charge compensation, while C1 center could have some trivalent lanthanide ions in nearby (2d) sites. The electric charge difference effects induce crystal field perturbations that could also influence the position of the energy levels of the excited 4f5d configuration. Similar perturbing effects coming from cationic sites occupied by acceptor ions in sensitised quantum cutting systems could be also expected.
4. Conclusions This study demonstrates that the mixed hexa-aluminates represent an interesting class of materials in which the perturbed RE3+ centers could dominate the normal centers of the basic lattice. This can be used to control the absorption and emission properties of solid-state laser materials or luminescent phosphors. Thus, in the case of Sr1 xLax–yNdyMgxAl12 xO19, efficient 901 nm Nd3+ laser emission was obtained for x = 0.4, where C1 perturbed center is prevailing. The reported parameters for quasi-three-level Nd3+ emission are: a slope efficiency of 74% in absorbed power at 792.2 nm pumping in 4F5/2 [28] and 84% for direct pumping at 865 nm in 4F3/2 [29]. The presence of the perturbed centers can influence both absorption and emission proper-
A. Lupei et al. / Optical Materials 30 (2008) 1677–1681
ties for visible laser emission, or the quantum cutting properties of Pr:ASL. References [1] Kh.S. Bagdasarov, A.A. Kaminskii, L. Kevorkov, T.A. Tevosyan, Sov. Phys. Dock. 19 (1974) 350. [2] S. Alablanche, R. Collongues, M. Leduc, J. Thery, D. Vivien, J. de Phys. IV (1) (1991), C 7–275. [3] S. Alablanche, A. Kahn-Harari, J. Thery, B. Viana, D. Vivien, J. Dexpert-Ghys, M. Faucher, J. Solid-State Chem. 98 (1992) 10. [4] H.R. Verdun, R. Wortman, C.A. Morrison, J.L. Bradshaw, Opt. Mater. 7 (1997) 117. [5] V. Delacarte, J. Thery, D. Vivien, J. de Phys. IV (C4) (1994) 361. [6] V. Delacarte, J. Thery, D. Vivien, J. Lumin. 62 (1994) 237. [7] V. Delacarte, J. Thery, J.M. Benitez, D. Vivien, C. Borel, R. Templier, C. Wyon, OSA Proc. Adv. Solid-State Lasers 24 (1995) 123. [8] L.D. Merkle, B. Zhandi, R. Moncorge, Y. Guyot, H.L. Verdun, B. McIntoch, J. Appl. Phys. 79 (1996) 1849. [9] B. Zhandi, L.D. Merkle, J.B. Gruber, D.E. Wortman, C.A. Morrison, J. Appl. Phys. 81 (1997) 1047. [10] A.M. Srivastava, W.W. Beers, J. Lumin. 71 (1997) 285. [11] S. Huang, X. Wang, R. Meltzer, A.M. Srivastava, A.A. Setlur, W.M. Yen, J. Lumin. 94–95 (2001) 119. [12] S. Huang, X. Wang, B. Chen, D. Jia, W.M. Yen, J. Lumin. 102–103 (2003) 344. [13] F. Liu, J. Zhang, S. Lu, S. Liu, S. Huang, X. Wang, J. Lumin. 117– 118 (2006) 492.
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[14] E. Van der Kolk, P. Dorenbos, C.W.E. van Eijk, J. Phys.: Condens. Matter 13 (2001) 5471. [15] A. Kahn, A. Lejus, J. Thery, D. Vivien, J.C. Brenier, J. Appl. Phys. 52 (1981) 6864. [16] J.P.M. Van Vliet, G. Blasse, Chem. Phys. Lett. 143 (1988) 221. [17] D. Saber, J. Dexpert-Ghys, P. Caro, A.M. Lejus, D. Vivien, J. Chem. Phys. 82 (1985) 5648. [18] C. Garapon, L. Lou, R. Moncorge, J. Lumin. 79 (1998) 161. [19] A. Lupei, V. Lupei, C. Gheorghe, L. Gheorghe, D. Vivien, G. Aka, J. Appl. Phys. 96 (2004) 3057. [20] A. Lupei, V. Lupei, C. Gheorghe, L. Gheorghe, D. Vivien, G. Aka, Phys. Status Solidi 2 (c) (2005) 276. [21] A. Lupei, V. Lupei, C. Gheorghe, L. Gheorghe, D. Vivien, G. Aka, E. Antic-Fidancev, J. Phys.: Condens. Matter 18 (2006) 597. [22] V. Lupei, A. Lupei, C. Gheorghe, L. Gheorghe, G. Aka, D. Vivien, in: Fourth International Symposium on Laser, Sxcintillator and Nonlinear Optical Materials, Prague-Czech 2006. [23] S. Hufner, Optical Spectra of Transparent Rare Earth Compounds, Academic Press, 1978. [24] O. Guillot-Noel, Ph. Goldner, P. Higel, D. Gourier, Chem. Phys. Lett. 380 (2003) 563. [25] O. Guillot-Noel, Ph. Goldner, P. Higel, D. Gourier, J. Phys.: Condens. Matter 16 (2004), R1. [26] L. Xie, A.N. Cormack, J. Solid-State Chem. 83 (1989) 282. [27] R. Collongues, D. Gourier, A. Khan-Harari, A.M. Lejus, J. Thery, D. Vivien, Annu. Rev. Mater. Sci. (1990) 51. [28] G. Aka, D. Vivien, V. Lupei, Appl. Phys. Lett. 85 (2004) 2685. [29] V. Lupei, G. Aka, D. Vivien, Opt. Lett. 31 (2006) 1064.
Optical Materials 30 (2008) 1677–1681 www.elsevier.com/locate/optmat
Cationic disorder effects in complex oxide laser materials and phosphors A. Lupei a, V. Lupei a,*, C. Gheoghe a, L. Gheorghe a, G. Aka b, D. Vivien b a
Institute of Atomic Physics – INFLPR, Laboratory of Solid-State Quantum Electronics, Bucharest 077125, Romania b Ecole Nationale Superieure de Chimie, LCAES, CNRS-UMR 7574, 75231 Paris, France Available online 31 December 2007
Abstract A comparative analysis of the high-resolution spectra of the Nd3+ and Pr3+ in strontium lanthanum hexa-aluminates, charge compensated with magnesium Sr1 x(Nd, Pr)yLax–yMgxAl12 xO19, on a large composition range is presented. The spectra are mainly dependent on the composition parameter x and show two families of centers for x < 0.5 and three for x P 0.8, with distinct spectral characteristics. Based on spectral data, composition dependence and crystal structure, tentative structural models for the these centers are proposed. It is inferred that this complex picture results from the interplay between the effects determined by the anionic coordination of the centers and the perturbative effects caused by the multiple occupation of specific sites by cations of different valence. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Optical spectroscopy; Nd3+; Pr3+; Strontium lanthanum aluminates; Disordered crystals
1. Introduction The strontium lanthanum hexa-aluminates doped with rare earth (RE3+) ions (especially Nd3+ or Pr3+) and charge compensated with Mg2+, Sr1 xLax–yREyMgxAl12 xO19 0 6 x < 1, y 6 x (known as ASL or SAM for low x and LMA for x = 1) have been investigated as laser materials especially for Nd3+ near infrared emission [1–7] or for Pr3+ visible emission [8,9] and for Pr3+ photon cascade/ quantum cutting emission [10–14]. The co-doping with the optically inert La3+ ions is used to improve the crystals quality (congruent melting has been observed for x > 0.2) and to avoid RE3+ emission quenching at high concentrations [5–7,14]. At low x values (<0.4) these systems have hexagonal magnetoplumbite structure (space group P63/mmc) in which the Sr2+ and La3+ ions as well as the dopant RE3+ ions occupy a unique (2d) site of ideal D3h symmetry, with 12O2 coordination and with similar cation–oxygen distances. The (2d) sites are placed in the mirror planes (per*
Corresponding author. E-mail addresses: [email protected]fim.ro, [email protected] (V. Lupei). 0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.11.009
pendicular on the optical axis c), separated by two spineltype groups where Al3+ can occupy octahedral, tetrahedral and five-fold coordinated bipyramidal sites [3]. The charge compensation is made with Mg2+ ions that replace Al3+ ions. The X-ray investigations [3] have suggested that for large x (x > 0.4) some of the lanthanide ions could occupy also sites of lower symmetry (C2v). Structural X-ray investigations [15] on crystals with x = 1, LaMgAl11O19 – LMA have revealed a more complex structure, close to magnetoplumbite, but with additional structural (6h) sites for the La3+ or RE3+ cations similar to the (2d) ones, but of lower symmetry (C2v or lower). The high oxygen coordination and the presence of small highly charged Al3+ ions sharing the oxygens with the RE3+ ions determine reduced covalence and low nephelauxetic effect leading to high barycenters of the energy manifolds as well as to moderate crystal field effects. Thus, the 4F3/2 ? 4I9/2 Nd3+ laser emission wavelength (900 nm) is one of the shortest reported for this transition and the lowest Pr3+ 4f5d level is above 1S0 (4f2), making the quantum cutting emission possible. Although the early investigations of Nd3+ or Pr3+ spectra in crystals with x < 0.4 have revealed a complex composition dependence, they have been interpreted in terms of a unique spectroscopic center [1–13]. However, at the end of
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the series (x = 1, La1 yREyMgAl11O19 - LMA), the spectroscopic data show clearly three centers for Pr3+ [16] and Nd3+ [17,18]. The recent high-resolution optical spectroscopy revealed the presence of at least two families of structural centers of Nd3+ [19–21] or Pr3+ [22] in ASL crystals for small and intermediate values of composition parameter x < 0.5. This paper presents a comparative investigation of the composition dependence of Nd3+ and Pr3+ spectral characteristics in hexa-aluminate crystals, for a large composition range, in the attempt to clarify the distribution of the cations in the various lattice sites of these disordered crystals and to assess the optimal compositions for various applications. 2. Experimental Various Sr1 xLax–yREyMgxAl12 xO19 single crystals with compositions 0.01 < x 6 1, doped with Nd3+ or with 0.05 6 x 6 0.3 doped with Pr3+, grown by Czochralski method in iridium crucibles, in N2 atmosphere, were investigated. High-resolution absorption, selective excitation and emission kinetics, obtained at 15 or 300 K, with setups described elsewhere [19] are analyzed. Detection was made with a one-meter GDM double monochromator and a photon counting system in connection with a MCS multi-channel analyzer. A closed cycle He refrigerator was used for low temperature measurements. 2.1. Nd3+ spectral behavior in hexa-aluminates In various regions of the Nd3+ absorption of Sr1 xLax–yNdyMgxAl12 xO19 (0.01 6 x 6 1) crystals, the spectra show significant changes when the structural parameter x varies. The spectra are transition-dependent and practically independent on parameter y, the Nd3+ content. For x < 0.5 two families of centers, denoted as C1 and C2 [19], were observed. The lines are well separated in the 4 I9/2 ? 4F3/2 transition, as illustrated in Fig. 1 with 15 K absorption spectra for samples of different compositions, taken in unpolarized light with propagation along c-axis. C2
3+
: 4I
9/2
- 2P1/2
3
C2
C1
10
Nd
I
y=0.05
C1
k(cm-1)
x=0.1 x=0.2
k(cm-1)
4 4 I9/2 - F3/2
The center denoted by C2 is prevailing at very low x (<0.1), while C1 is dominant at intermediate x values (>0.4). The 4F3/2 splitting is about twice larger for C1 than for C2 (90–110 cm 1 for C1 and 40–50 cm 1 for C2 function of parameter x). The C2 lines are fairly sharp (2–5 cm 1), while the C1 lines are asymmetric and much broader (10– 25 cm 1). For the same transition, the absorption spectra for high x = 0.8–1 (LMA: Nd3+) samples, each 4I9/2 ? 4 F3/2 line is split into two components (I–II), as observed previously [17,18]. Such a structure is resolved in other transitions too. An interesting composition dependence was observed in the 4I9/2 ? 2P1/2 Nd3+ absorption at 15 K. Whereas for very low x two peaks, separated by about 2–3 cm 1, were resolved, at higher x (>0.2) only an asymmetric line is observed. At the end of series, x = 0.8 and 1, the spectra show three main lines (I–III), as illustrated in Fig. 2. The lines of centers II and III are shifted from those of center I by 30 cm 1 and respectively 200 cm 1, on lower energies. The lines are asymmetric and inhomogeneously broadened (widths of 10–15 cm 1 for center I, 25–30 cm 1 for center II, and 15–25 for center III) indicating a large structural disorder in the environment of the Nd3+ ions. The intensity of centers II and III increase with x. The lines of center III are very well resolved in the hypersensitive Nd3+4I9/2 ? 4G5/2, 2G7/2 transition, having negligible intensity at x = 0.5 as observed in (Fig. 3). In the 4I9/2 ? 4F3/2 absorption spectra the lines of this center are unclear even for x = 1, but they have been detected by selective excitation [18]. The mean splittings of 4F3/2 are 160 cm 1 for centers I and II and 200 cm 1 for center III, slightly different in stochiometric and congruent grown crystals [18]. The selective excitation in broad absorption lines especially of the C1 center in ASL [19] and I, II, III centers in LMA [18] have shown the existence of different sub-sets of centers in the composition of these lines.
II
III
x=0.5
I
2 x=0.8
x=0.5 0
II 11500
II
I
11600
x=1
x=1
1
I
11700
E(cm-1) Fig. 1. Composition dependence of Nd3+4I9/2 ? 4F3/2 15 K absorption of Sr1 xLax–yNdyMgxAl12 xO19 samples.
23250
23400
23550
E(cm-1) Fig. 2. The 4I9/2 ? 2P1/2 absorption spectra at 15 K of ASL:Nd for x = 0.5, y = 0.05 and x = 0.8, 1; y = 0.1.
A. Lupei et al. / Optical Materials 30 (2008) 1677–1681
Nd3+ : 4I
20
9/2
3+ Pr : 3 3 H - P 4 2 C
- 4G
,2 5/2 G7/2
2
I, II
k(cm-1)
1679
3+ Pr : 3 3 H - P 4 0
C1
x=0.5
III
x=0.05,y=0.05 x=0.1,y=0.1
x=y=0.1
x=0.8
10
x=0.2,y=0.05 x=y=0.2 x=0.3,y=0.05
x=y=0.05
x=1 17000
17200
17400
17600
-1
E(cm ) 4
4
2
22400
22600
E(cm-1)
22800
20750
20800
20850
E(cm-1)
Fig. 4. Composition dependence of Pr3+ 15 K absorption.
Fig. 3. The I9/2 ? G5/2, G7/2 absorption spectra at 15 K of Sr1 xLax–yNdyMgxAl12 xO19 for x = 0.5, y = 0.05 and x = 0.8, 1, y = 0.1.
Polarized absorption data show a complex behavior for Nd3+ C1 and C2 non-equivalent centers in ASL. The C2 center spectra satisfy the selection rules of the D3h ideal local symmetry of (2d) sites [23], while those of C1 center indicate a small lowering in symmetry [21]. In the case of Nd3+ centers I and II (x = 0.8–1 samples), the observed polarization data in the 15 and 300 K absorption can be rather well interpreted in terms of the selection rules of D3h group. However, the spectrum of center III in the 4 I9/2 ? 4G5/2 transition has three lines (not two in r spectrum and one in p as expected for D3h local symmetry) indicating a clear lower symmetry than D3h. 2.2. Pr3+ spectra in hexa-aluminates The high-resolution 15 K absorption spectra of Sr1 xLax–yPryMgxAl12–xO19 crystals with low x parameter (0.05 6 x 6 0.3, y 6 0.2) show composition and transition-dependent features, similar to Nd3+. A clear two-line structure (denoted again with C1 and C2) with the components separated by 25 cm 1 is observed in the first line of the 15 K 3H4 ? 3P2 absorption (Fig. 4). The C1 center intensity increases with x and is dominant at x = 0.3. The widths of these lines are quite different (30 cm 1 for C1, and 15 cm 1 for C2 center in the x = 0.1, y = 0.1 sample). Similar to the 2P1/2 Nd3+ level, the positions of the 3P0 3+ Pr level for the two centers are very close, so that a single asymmetric line is observed in the 3H4 ? 3P0 absorption (Fig. 4). A slight deviation from D3h symmetry was remarked for C1 center from polarization data. 3. Discussion The spectroscopic investigation (high-resolution absorption, selective excitation or emission decay) of a large variety of Sr1 xLax–yNdyMgxAl12 xO19 samples reveals the presence of several non-equivalent Nd3+ centers that prevail in different composition ranges. These centers present
distinct spectral characteristics: energy levels, emission kinetics, etc. In the attempt to elucidate the nature of Nd3+ centers in Sr1 xLax–yNdyMgxAl12 xO19, the structural (X-ray), optical spectroscopy and EPR data could be used. Thus, the X-ray data have suggested that for x > 0.4 these crystals show departure from the ideal magnetoplumbite structure and the RE3+ ion could occupy sites of lower symmetry [3,15]. The optical spectra show essential changes with composition parameter x: two families of centers at low x and three at very high x were observed. The EPR spectra for very low x values present a single line, corresponding to identical isolated ions, while for larger x = y values (60.2) this line is accompanied by a structure associated to pairs of slightly different centers [24,25]. In case of x = 1 two resonance lines corresponding to two different types of centers were reported [17]. Some information about the nature of these centers could be obtained from the barycenters position of the 2S+1 LJ manifolds, defined as DE(2S+1LJ) = E(2S+1LJ) 4 E( I9/2). By using the energy levels of different Nd3+ centers from selective excitation data [18,19] a barycentre curve can be drawn (Fig. 5). Since the barycenters are influenced by nephelauxetic effect, their position depends on many structural and compositional factors, such as the nature of surrounding ions, the coordination number (increase with it), the size of the ion to be replaced by Nd3+ in the lattice host, the geometry of the first anionic sphere, etc. However, this diagram suggests that C1, C2 centers could have a similar ionic 12O2 environment for Nd3+ ions. Since the barycenters of centers I and II at x = 1 are close to those of C1 and C2 centers, one could assume that they have also the same anionic vicinity 12O2 , the lowering of their barycenters can be associated with the size of the ˚ replaced ions (ionic radii of La3+ and Sr2+ are 1.32 A ˚ and respectively 1.4 A) and a small shrinkage of the anionic vicinity. The barycenters of center III are lower, it could mean a lower coordination or a large distortion of the first anionic coordination sphere. This would alter severely the transition probabilities, explaining the high intensity in of center III lines in the hypersensitive transition.
1680
A. Lupei et al. / Optical Materials 30 (2008) 1677–1681
Δ E(cm
-1
2
) P 1/2
23200
C 1, C 2
ASL
I
C1
C2
II
23100
III
I, II, III LMA
23000
22900
YAG 11200
11300
ΔE(cm-1)
11400
4F 3/2
Fig. 5. The barycenters positions of the 4F3/2 and 2P1/2 manifolds, for Nd3+ doped in different crystals.
The difference between the 4F3/2 level splittings for C1 and C2 centers (about twice) evidence different second order crystal field parameters. These parameters depend stronger on the second order neighbors than the higher order parameters, so one can assume a different cationic vicinity for these centers. The cations of different electric charges that occupy the same sites: Sr2+ and (La3+, RE3+) in (2d) sites or Al3+ and Mg2+, could determine the crystal field dissimilarity. A given (2d) site in ASL is surrounded by six (2d) sites ˚ , the next ones being situated in the mirror plane at 5.56 A ˚ . There are many Al3+ sites at distances larger than 9.62 A ˚ there are three close to a given (2d) site. From 3.2 to 6 A bypiramidal, 24 octahedral and six tetrahedral Al3+ sites. The positions occupied by Mg2+ ions are not clear, they could be Al3+ tetrahedral sites [26], but probably also other Al3+ sites at high x. For low x, the Mg2+ crystal field effects are expected to be much lower than those of trivalent ions, since the relative concentration with respect to the available crystallographic sites is x for trivalent lanthanides (La3+ or RE3+) and x/12 in the case of Mg2+. Based on these data it was proposed that C2 centers, prevailing at low x (x < 0.1), could correspond to Nd3+ in a (2d) site with only Sr2+ in nearby six (2d) sites and charge compensated with Mg2+, probably in tetrahedral Al3+ sites at different distances [20]. The C1 lines are composite of various structural centers of Nd3+ in a (2d) site with 12O2 around and one or more of the six (2d) nearest neighbor sites could be occupied by La3+ or Nd3+. This structural model for the C1 and C2 families of centers is consistent with the data on the composition dependence of the relative intensity of C1 center and with the emission decays measurements [19]. The relative intensity of the C1 Nd3+ center lines to the total intensity in the 4I9/2(1) ? 4 F3/2(1) absorption, function on composition parameter x, increases as 1 (1 x)6 with a saturation at x 0.5 [21]. This law corresponds to the calculated global probability that one up to six nearby (2d) sites to be occupied by trivalent La3+ or Nd3+ ions assuming a random occupa-
tion. The emission decays for the two centers differs only at early times up to 200 ls, being faster for C1, and then they run in parallel. It was inferred that this difference originates from the energy transfer inside the first Nd3+–Nd3+ pairs that, according to this structural model, are possible only for C1 center. This fact is in accord with EPR data [24,25] that show only a line assigned to isolated ions and pairs for the x = y 6 0.2 crystals. The charge compensator Mg2+ could have a correlated distribution with the dopant and its effects are assumed to contribute mainly to linewidths. The X-ray investigations of LMA (x = 1): Nd crystals are not conclusive as concerns the crystal structure The lanthanide ions could occupy two types of sites of (2d) or distorted (6h) of lower symmetry (C2v), Al3+ in the mirror plane has been also localized not in a five-fold coordination, but in a tetrahedral – like environment [15,27]. These crystals show departures from stoichiometry, they have been found defective in La3+, Mg2+ [15,18,27] or oxygen. The probability of having Mg2+ ions near the Nd3+ ions and substituting other Al3+ sites closer to Nd3+ increases, and a non-random distribution is very likely. Thus, it is difficult to give definite models for these centers from the optical spectroscopy data. The centers I and II could be linked to a weak distortion of D3h symmetry, while the center III corresponds to a strongly distorted site of lower symmetry, probably of C2v, as is sustained by the polarized absorption. The spectral data of Pr3+ in Sr1 xLax–yPryMgxAl12 xO19 for x 6 0.3 show a similar two center C1 and C2 structure, resolved in some transitions. One could assume that as for Nd3+, the C2 center corresponds to a Pr3+ ion in a (2d) site surrounded only by Sr2+ in nearby sites and with a far away charge compensation, while C1 center could have some trivalent lanthanide ions in nearby (2d) sites. The electric charge difference effects induce crystal field perturbations that could also influence the position of the energy levels of the excited 4f5d configuration. Similar perturbing effects coming from cationic sites occupied by acceptor ions in sensitised quantum cutting systems could be also expected.
4. Conclusions This study demonstrates that the mixed hexa-aluminates represent an interesting class of materials in which the perturbed RE3+ centers could dominate the normal centers of the basic lattice. This can be used to control the absorption and emission properties of solid-state laser materials or luminescent phosphors. Thus, in the case of Sr1 xLax–yNdyMgxAl12 xO19, efficient 901 nm Nd3+ laser emission was obtained for x = 0.4, where C1 perturbed center is prevailing. The reported parameters for quasi-three-level Nd3+ emission are: a slope efficiency of 74% in absorbed power at 792.2 nm pumping in 4F5/2 [28] and 84% for direct pumping at 865 nm in 4F3/2 [29]. The presence of the perturbed centers can influence both absorption and emission proper-
A. Lupei et al. / Optical Materials 30 (2008) 1677–1681
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