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5D0→7F0 transitions of Sm2+ in SrMgF4:Sm2+
Journal of Alloys and Compounds 374 (2004) 194–196
5
D0 → 7F0 transitions of Sm2+ in SrMgF4 :Sm2+ H. Hagemann a,∗ , F. Kubel a,b , H. Bill a , F. Gingl c
b
a Dépt. de Chimie Physique, Université de Genève, 30 quai E. Ansermet, 1211 Geneva 4, Switzerland Institute for Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164, 1060 Vienna, Austria c Lab. de Cristallographie, Université de Genève, 30 quai E. Ansermet, 1211 Geneva 4, Switzerland
Abstract Temperature-dependent emission spectra of Sm2+ -doped SrMgF4 have been obtained in the temperature range from 50 to 300 K. At 50 K, six bands are observed for the very strong 5 D0 → 7 F0 transition, in agreement with the reported sixfold crystal superstructure. The overall splitting of more than 70 cm−1 highlights the important structural differences of the six Sr sites. Upon heating progressively to room temperature, the spectra change progressively with a more pronounced change between 270 and 300 K. These observations suggest the possibility of a complex structural behavior for SrMgF4 which will require new experiments. © 2003 Elsevier B.V. All rights reserved. Keywords: Alkaline earth fluorides; Samarium; Luminescence spectroscopy; f-f Transitions; Crystal structure
1. Introduction Inorganic fluorides and fluorohalides have been of interest to us for some time already, in part because of their potential interest as host materials for rare earth ions for optical applications [1–3]. In particular, we have studied the luminescence of Sm2+ in BaMgF4 and in the solid solutions Ba1−x Srx MgF4 with 0 ≤ x ≤ 0.6 [4]. As an extension of this study, we have also prepared Sm2+ -doped SrMgF4 , but at that time, no single crystal X-ray structure of this compound was available. The crystal structure of SrMgF4 has been published recently [5] and reveals a complex sixfold superstructure of the basic Cmcm lattice initially reported by Banks et al. [6]. Very recently, Abrahams [7] reanalyzed these new structural data and suggested the presence of a phase transition with a Curie temperature of ca. 450 K. Our luminescence data reported below provide additional information in relation to these structural data. 2. Experimental Samples were prepared using solid state techniques similar to those reported by Gingl [8]. Sm was introduced as ∗ Corresponding author. Tel.: +41-22-379-65-39; fax: +41-22-379-61-03. E-mail address: [email protected] (H. Hagemann).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.091
the metal with a nominal concentration of 0.2% with respect to Sr. Small orange-colored crystals (maximum size 0.3 mm × 0.3 mm × 1 mm) were obtained in addition to transparent MgF2 and greenish-blue SrF2 (doped with Sm). These samples are not very moisture sensitive. A crystal was chosen and placed in a glass capillary to make handling more easy, which was then introduced in an Oxford Instruments Helium flow cryostat for temperature dependent measurements. Emission spectra were excited with the 488 nm argon-ion laser line and analyzed using a Spex 1403 double monochromator (focal length 85 cm, aperture f/7.8) equipped with a Burley C31034-A20 PM tube. The 100 m slits used in the experiments provide a spectral resolution of ca. 1.2 cm−1 . 3. Results Sm2+ substitutes easily for divalent Sr2+ and Ba2+ [1,4], and therefore its emission spectrum can be used as a probe of the local environment of the substituted divalent ion. In particular, the non-degenerate 5 D0 → 7 F0 transition shown in Fig. 1 illustrates this behavior for Sm2+ in four different crystals, namely BaMgF4 [4], Ba2 Mg3 F10 , Ba0.75 Sr0.25 MgF4 [4] and SrAlF5 [9]. In BaMgF4 , there is only one Ba site in the crystal structure, while there are two in Ba2 Mg3 F10 [8]. In the case of Ba0.75 Sr0.25 MgF4 with the BaMgF4 structure, the substitutional disorder on the Ba site induces a significant inhomogeneous broadening of the
H. Hagemann et al. / Journal of Alloys and Compounds 374 (2004) 194–196
195
3
200x10
3
160x10 140 SrAlF5
Emission Intensity [a.u.]
Emission intensity [a.u.]
150
Ba2 Mg3 F10 100
120 300K 100 270K 80 240K 60 40
Ba0.75 Sr0.25 MgF4
180K 120K
50
20 50K
0 14.60 0
14.60
14.65
14.70
14.75
14.80x10
Fig. 1. Sm2+ 5 D0 → 7 F0 emission bands (at 300 K) in different fluoride hosts.
Sm2+ emission bands. Finally, for SrAlF5 , four different bands (the third one appears as a shoulder of the central second band in Fig. 1) between 14 672 and 14 695 cm−1 confirm the four-fold superstructure refined using single crystal X-ray diffraction [9]. The lower resolution survey spectrum of Sm2+ in SrMgF4 at 50 K (shown in Fig. 2) is similar to the one observed for Sm2+ in BaMgF4 and the room temperature spectra of SrMgF4 samples doped with Sm prepared under reducing atmosphere [10]. No emission of Sm3+ was observed for our samples. The strongest emission bands correspond to the 5 D0 → 7 F transition, which are observed at a remarkably high fre0 quency around 14 700 cm−1 , similarly as in BaMgF4 (see Fig. 1 and [11]). In contrast to the isoelectronic Eu3+ spectra, where the 5 D0 → 7 F0 transition is very weak compared to other emission bands, a strong intensity for this transition can be observed for Sm2+ , as found experimentally for all samples in Fig. 1 or for Sm2+ doped into MFX compounds 3
200x10 Emission Intensity [a.u.]
3
14.75x10
2+ in SrMgF as a function Fig. 3. 0 → 0 emission spectra of Sm 4 of temperature (excitation wavelength 488 nm). 5D
Emission wavenumber [cm ]
150 100 50 0 12.5
14.70
Emission wavenumber [cm ] 3
-1
12.0
14.65
-1
BaMgF4
13.0
13.5
14.0
14.5
3
15.0x10
-1
Emission wavenumber [cm ]
Fig. 2. Emission spectrum of SrMgF4 :Sm2+ at 50 K (excitation wavelength 488 nm).
7F
(M = Ca, Sr, Ba; X = Cl, Br, I) [1]. This increase of intensity may be related to the close position of odd-parity states relative to 5 D0 and 7 F0 . Fig. 3 presents temperature dependent emission spectra of Sm2+ in SrMgF4 . At 50 K, one observes six different bands, three weaker bands (at 14 652, 14 659 and 14 690 cm−1 ) and three stronger ones (at 14 692, 14 707 and 14 724 cm−1 ). This observation is perfectly in agreement with the six different Sr sites reported in [5]. The overall spread of these bands (ca. 70 cm−1 ) is much larger than the one observed in SrAlF5 (see Fig. 1) and indicates large structural differences of the Sr–F coordination polyhedra. Indeed, the crystal structure data [5] show quite different envrionments. If one sets arbitrarily two Sr–F bond length limits at 2.7 and 3.0 Å, the coordinations around the six Sr ions can be summarized as follows: Sr1, five bonds <2.7 Å; three bonds between 2.7 and 3.0 Å; three bonds between 3.0 and 3.4 Å; in short 5 + 3 + 3. For Sr2, one obtains: 7 + 1 + 2; Sr3 and Sr4: 9 bonds <2.7 Å; Sr5: 6 + 1 + 3 and Sr6 5 + 5. Interestingly, these emission bands evolve with increasing temperature. Already at 120 K, the weak band seen as a shoulder at 50 K (at 14 690 cm−1 ) cannot be distinguished any more from the stronger emission band (at 14 692 cm−1 ). The two other weak bands have merged into one broader one at ca. 270 K. Between 270 and 300 K, there appears an important intensity redistribution of the three stronger emission bands, while the weaker large band shifts towards the other bands. These important spectral changes clearly indicate a dynamic evolution of the structure and strongly support the possibility of a higher temperature structural phase transition, as suggested by Abrahams [7]. We have attempted to reach this phase transition simply by laser heating of the small crystal, but even with laser powers of ca. 200 mW on the small crystal a spectrum similar to the one at 300 K resulted.
196
H. Hagemann et al. / Journal of Alloys and Compounds 374 (2004) 194–196
However, it is possible that the situation is even more complex, insofar as the reported crystal structure stemmed from a sample doped nominally with 0.06% of Ce [5]. It is not clear presently to which extent this heterovalent (Ce is trivalent, while Mg and Sr are bivalent) substitution stabilizes the reported crystal structure, and that there could be other structural varieties for pure or isovalently substituted SrMgF4 . Very recently, a superlattice structure was reported for Ce3+ -doped BaMgF4 [12], while BaMgF4 doped with Eu(II) or with Sm(II) did not show any sign of rare earth site multiplication by EPR (Eu) [2] and luminescence spectroscopy [11]. In this context, it may also be interesting to note that for BaAlF5 , four different crystal modifications have been reported [13] which differ by the stacking of the fluoride octahedral layers. Further structural and spectroscopic investigations on SrMgF4 are planned for the near future. Acknowledgements This work was supported by the Swiss National Science Foundation.
References [1] H. Bill, R. Jaaniso, H. Hagemann, D. Lovy, A. Monnier, M. Schnieper, Opt. Eng. 34 (1995) 2333. [2] J.M. Rey, H. Bill, D. Lovy, H. Hagemann, J. Alloys Compd. 268 (1998) 60. [3] F. Kubel, H. Bill, H. Hagemann, Z. Anorg. Allg. Chem. 625 (1999) 643. [4] F. Kubel, H. Hagemann, H. Bill, Mater. Res. Bull. 32 (1997) 263. [5] N. Ishizawa, K. Suda, B.E. Etschmann, T. Oya, N. Kodama, Acta Crystallogr. C 57 (2001) 784. [6] E. Banks, S. Nakashima, M. Shone, J. Electrochem. Soc. 127 (1980) 2234. [7] S.C. Abrahams, Acta Crystallogr. B58 (2002) 34. [8] F. Gingl, Z. Anorg. Allg. Chem. 623 (1997) 705. [9] F. Kubel, Z. Anorg. Allg. Chem. 624 (1998) 1481. [10] Z. Cao, C. Shi, J. Ni, J. Lumin. 55 (1993) 221–224. [11] F. Gaume, A. Gros, J.C. Bourcet, Rare Earth Mod. Sci. Technol. 3 (1982) 143. [12] M. Yamaga, K. Hattori, N. Kodama, N. Ishizawa, M. Honda, K. Shimamura, T. Fukuda, J. Phys. Condens. Matter 13 (2001) 10811– 10824. [13] M. Weil, E. Zobetz, F. Werner, F. Kubel, Solid State Sci. 3 (2001) 441.
5
D0 → 7F0 transitions of Sm2+ in SrMgF4 :Sm2+ H. Hagemann a,∗ , F. Kubel a,b , H. Bill a , F. Gingl c
b
a Dépt. de Chimie Physique, Université de Genève, 30 quai E. Ansermet, 1211 Geneva 4, Switzerland Institute for Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164, 1060 Vienna, Austria c Lab. de Cristallographie, Université de Genève, 30 quai E. Ansermet, 1211 Geneva 4, Switzerland
Abstract Temperature-dependent emission spectra of Sm2+ -doped SrMgF4 have been obtained in the temperature range from 50 to 300 K. At 50 K, six bands are observed for the very strong 5 D0 → 7 F0 transition, in agreement with the reported sixfold crystal superstructure. The overall splitting of more than 70 cm−1 highlights the important structural differences of the six Sr sites. Upon heating progressively to room temperature, the spectra change progressively with a more pronounced change between 270 and 300 K. These observations suggest the possibility of a complex structural behavior for SrMgF4 which will require new experiments. © 2003 Elsevier B.V. All rights reserved. Keywords: Alkaline earth fluorides; Samarium; Luminescence spectroscopy; f-f Transitions; Crystal structure
1. Introduction Inorganic fluorides and fluorohalides have been of interest to us for some time already, in part because of their potential interest as host materials for rare earth ions for optical applications [1–3]. In particular, we have studied the luminescence of Sm2+ in BaMgF4 and in the solid solutions Ba1−x Srx MgF4 with 0 ≤ x ≤ 0.6 [4]. As an extension of this study, we have also prepared Sm2+ -doped SrMgF4 , but at that time, no single crystal X-ray structure of this compound was available. The crystal structure of SrMgF4 has been published recently [5] and reveals a complex sixfold superstructure of the basic Cmcm lattice initially reported by Banks et al. [6]. Very recently, Abrahams [7] reanalyzed these new structural data and suggested the presence of a phase transition with a Curie temperature of ca. 450 K. Our luminescence data reported below provide additional information in relation to these structural data. 2. Experimental Samples were prepared using solid state techniques similar to those reported by Gingl [8]. Sm was introduced as ∗ Corresponding author. Tel.: +41-22-379-65-39; fax: +41-22-379-61-03. E-mail address: [email protected] (H. Hagemann).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.091
the metal with a nominal concentration of 0.2% with respect to Sr. Small orange-colored crystals (maximum size 0.3 mm × 0.3 mm × 1 mm) were obtained in addition to transparent MgF2 and greenish-blue SrF2 (doped with Sm). These samples are not very moisture sensitive. A crystal was chosen and placed in a glass capillary to make handling more easy, which was then introduced in an Oxford Instruments Helium flow cryostat for temperature dependent measurements. Emission spectra were excited with the 488 nm argon-ion laser line and analyzed using a Spex 1403 double monochromator (focal length 85 cm, aperture f/7.8) equipped with a Burley C31034-A20 PM tube. The 100 m slits used in the experiments provide a spectral resolution of ca. 1.2 cm−1 . 3. Results Sm2+ substitutes easily for divalent Sr2+ and Ba2+ [1,4], and therefore its emission spectrum can be used as a probe of the local environment of the substituted divalent ion. In particular, the non-degenerate 5 D0 → 7 F0 transition shown in Fig. 1 illustrates this behavior for Sm2+ in four different crystals, namely BaMgF4 [4], Ba2 Mg3 F10 , Ba0.75 Sr0.25 MgF4 [4] and SrAlF5 [9]. In BaMgF4 , there is only one Ba site in the crystal structure, while there are two in Ba2 Mg3 F10 [8]. In the case of Ba0.75 Sr0.25 MgF4 with the BaMgF4 structure, the substitutional disorder on the Ba site induces a significant inhomogeneous broadening of the
H. Hagemann et al. / Journal of Alloys and Compounds 374 (2004) 194–196
195
3
200x10
3
160x10 140 SrAlF5
Emission Intensity [a.u.]
Emission intensity [a.u.]
150
Ba2 Mg3 F10 100
120 300K 100 270K 80 240K 60 40
Ba0.75 Sr0.25 MgF4
180K 120K
50
20 50K
0 14.60 0
14.60
14.65
14.70
14.75
14.80x10
Fig. 1. Sm2+ 5 D0 → 7 F0 emission bands (at 300 K) in different fluoride hosts.
Sm2+ emission bands. Finally, for SrAlF5 , four different bands (the third one appears as a shoulder of the central second band in Fig. 1) between 14 672 and 14 695 cm−1 confirm the four-fold superstructure refined using single crystal X-ray diffraction [9]. The lower resolution survey spectrum of Sm2+ in SrMgF4 at 50 K (shown in Fig. 2) is similar to the one observed for Sm2+ in BaMgF4 and the room temperature spectra of SrMgF4 samples doped with Sm prepared under reducing atmosphere [10]. No emission of Sm3+ was observed for our samples. The strongest emission bands correspond to the 5 D0 → 7 F transition, which are observed at a remarkably high fre0 quency around 14 700 cm−1 , similarly as in BaMgF4 (see Fig. 1 and [11]). In contrast to the isoelectronic Eu3+ spectra, where the 5 D0 → 7 F0 transition is very weak compared to other emission bands, a strong intensity for this transition can be observed for Sm2+ , as found experimentally for all samples in Fig. 1 or for Sm2+ doped into MFX compounds 3
200x10 Emission Intensity [a.u.]
3
14.75x10
2+ in SrMgF as a function Fig. 3. 0 → 0 emission spectra of Sm 4 of temperature (excitation wavelength 488 nm). 5D
Emission wavenumber [cm ]
150 100 50 0 12.5
14.70
Emission wavenumber [cm ] 3
-1
12.0
14.65
-1
BaMgF4
13.0
13.5
14.0
14.5
3
15.0x10
-1
Emission wavenumber [cm ]
Fig. 2. Emission spectrum of SrMgF4 :Sm2+ at 50 K (excitation wavelength 488 nm).
7F
(M = Ca, Sr, Ba; X = Cl, Br, I) [1]. This increase of intensity may be related to the close position of odd-parity states relative to 5 D0 and 7 F0 . Fig. 3 presents temperature dependent emission spectra of Sm2+ in SrMgF4 . At 50 K, one observes six different bands, three weaker bands (at 14 652, 14 659 and 14 690 cm−1 ) and three stronger ones (at 14 692, 14 707 and 14 724 cm−1 ). This observation is perfectly in agreement with the six different Sr sites reported in [5]. The overall spread of these bands (ca. 70 cm−1 ) is much larger than the one observed in SrAlF5 (see Fig. 1) and indicates large structural differences of the Sr–F coordination polyhedra. Indeed, the crystal structure data [5] show quite different envrionments. If one sets arbitrarily two Sr–F bond length limits at 2.7 and 3.0 Å, the coordinations around the six Sr ions can be summarized as follows: Sr1, five bonds <2.7 Å; three bonds between 2.7 and 3.0 Å; three bonds between 3.0 and 3.4 Å; in short 5 + 3 + 3. For Sr2, one obtains: 7 + 1 + 2; Sr3 and Sr4: 9 bonds <2.7 Å; Sr5: 6 + 1 + 3 and Sr6 5 + 5. Interestingly, these emission bands evolve with increasing temperature. Already at 120 K, the weak band seen as a shoulder at 50 K (at 14 690 cm−1 ) cannot be distinguished any more from the stronger emission band (at 14 692 cm−1 ). The two other weak bands have merged into one broader one at ca. 270 K. Between 270 and 300 K, there appears an important intensity redistribution of the three stronger emission bands, while the weaker large band shifts towards the other bands. These important spectral changes clearly indicate a dynamic evolution of the structure and strongly support the possibility of a higher temperature structural phase transition, as suggested by Abrahams [7]. We have attempted to reach this phase transition simply by laser heating of the small crystal, but even with laser powers of ca. 200 mW on the small crystal a spectrum similar to the one at 300 K resulted.
196
H. Hagemann et al. / Journal of Alloys and Compounds 374 (2004) 194–196
However, it is possible that the situation is even more complex, insofar as the reported crystal structure stemmed from a sample doped nominally with 0.06% of Ce [5]. It is not clear presently to which extent this heterovalent (Ce is trivalent, while Mg and Sr are bivalent) substitution stabilizes the reported crystal structure, and that there could be other structural varieties for pure or isovalently substituted SrMgF4 . Very recently, a superlattice structure was reported for Ce3+ -doped BaMgF4 [12], while BaMgF4 doped with Eu(II) or with Sm(II) did not show any sign of rare earth site multiplication by EPR (Eu) [2] and luminescence spectroscopy [11]. In this context, it may also be interesting to note that for BaAlF5 , four different crystal modifications have been reported [13] which differ by the stacking of the fluoride octahedral layers. Further structural and spectroscopic investigations on SrMgF4 are planned for the near future. Acknowledgements This work was supported by the Swiss National Science Foundation.
References [1] H. Bill, R. Jaaniso, H. Hagemann, D. Lovy, A. Monnier, M. Schnieper, Opt. Eng. 34 (1995) 2333. [2] J.M. Rey, H. Bill, D. Lovy, H. Hagemann, J. Alloys Compd. 268 (1998) 60. [3] F. Kubel, H. Bill, H. Hagemann, Z. Anorg. Allg. Chem. 625 (1999) 643. [4] F. Kubel, H. Hagemann, H. Bill, Mater. Res. Bull. 32 (1997) 263. [5] N. Ishizawa, K. Suda, B.E. Etschmann, T. Oya, N. Kodama, Acta Crystallogr. C 57 (2001) 784. [6] E. Banks, S. Nakashima, M. Shone, J. Electrochem. Soc. 127 (1980) 2234. [7] S.C. Abrahams, Acta Crystallogr. B58 (2002) 34. [8] F. Gingl, Z. Anorg. Allg. Chem. 623 (1997) 705. [9] F. Kubel, Z. Anorg. Allg. Chem. 624 (1998) 1481. [10] Z. Cao, C. Shi, J. Ni, J. Lumin. 55 (1993) 221–224. [11] F. Gaume, A. Gros, J.C. Bourcet, Rare Earth Mod. Sci. Technol. 3 (1982) 143. [12] M. Yamaga, K. Hattori, N. Kodama, N. Ishizawa, M. Honda, K. Shimamura, T. Fukuda, J. Phys. Condens. Matter 13 (2001) 10811– 10824. [13] M. Weil, E. Zobetz, F. Werner, F. Kubel, Solid State Sci. 3 (2001) 441.