- Email: [email protected]
Excited states of Sm2+ in chloride host lattices
Journal of Luminescence 94–95 (2001) 127–132
Excited states of Sm2+ in chloride host lattices Claudia Wickleder* Institut fur . zu Koln, . Greinstrabe 6, 50939 Koln, . Germany . Anorganische Chemie, Universitat
Abstract Emission spectra of Sm2+ at different temperatures in several chloride host lattices after excitation into the 4f55d1 levels are presented and compared. SrZnCl4 : Sm2+ shows broad 4f55d1-4f6 emission at room temperature while additional 5D0-7FJ emission is observed at lower temperatures. 4f55d1-4f6 as well as 5D0-7FJ emission is observed in BaZnCl4-I : Sm2+ at room temperature, investigations at lower temperatures show only 5D0-7FJ emission. In spite of the very similar [BaCl8] polyhedra in the crystal structures of BaZnCl4-I and BaZnCl4-II, the maximum of the 4f55d1-4f6 transition at room temperature is shifted to about 400 cm 1 in BaZnCl4-II. Again, 5D0-7FJ emission is observed in addition. At lower temperatures, the 5D1-7FJ emission is only detected. Finally, NaBaLaCl6 : Sm2+ shows only f-f transitions: 5D0-7FJ at room temperature and 5D1-7FJ at lower temperatures. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Divalent lanthanides; Samarium; Emission; Chlorides
1. Introduction One interesting feature of the electronic properties of Sm2+ ions with the ground-state configuration 4f6 is the low-lying 4f55d1 level. Because of the excitation of one electron to a d state, the energy of the transition depends very strongly on the host lattice. In most chlorides, the broad f–d transition starts in the green region so that the compounds appear red. In contrast, f–f transitions, which occur at comparable energies, are nearly independent of the surrounding. Therefore, emission from a 4f55d1 or 4f6 level or both can be observed. The position of the excited 4f55d1 level *Fax: +49-221-470-5083. E-mail address: [email protected] (C. Wickleder).
depends on several parameters like crystal field strength, point symmetry, coordination number and covalency of the bond. In general, some of these parameters are changed in different host lattices. In order to evaluate the influence of a single parameter, it is necessary to investigate host lattices, which are very similar. The chlorides studied in this work, are indeed very similar with respect to Sm2+ coordination. Previous investigations of Sm2+ in chlorides have been carried out in simple host lattices like SrCl2 [1], BaCl2 [2], SrFCl [3–6] and BaFCl [6–9]. Note that the present contribution aims to summarize and to compare the emission spectra of Sm2+ in SrZnCl4, two modifications of BaZnCl4 (BaZnCl4-I and BaZnCl4-II) and NaBaLaCl6. A detailed presentation of all experimental results and a comprehensive discussion is given elsewhere [10–12].
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 8 7 - 8
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2. Experimental Pure SmCl2 was prepared by the reduction of SmCl3 with Sm metal according to the procedure given in Ref. [13]. All educts were sublimed in high vacuum to guarantee high purity of the products. The ternary chlorides were synthesized by melting the respective binary chlorides at 6001C (SrZnCl4), 6001C (BaZnCl4-I), 4801C (BaZnCl4-II) and 5501C (NaBaLaCl6) followed by slow cooling (11C/h) using the Bridgman technique. As a dopant, 0.05 mol% of SmCl2 was added. To avoid oxidation of Sm2+ the reactions were carried out in , sealed tantalum containers. The compounds are extremely moisture-sensitive and must be handled under inert conditions. The purity of all products was checked by powder X-ray diffraction (Stoe and Cie., Stadi P) (Table 1).
Emission spectra of SrZnCl4 : Sm2+ and BaZnCl4-I : Sm2+ were recorded using a frequency-doubled Nd : YAG laser (Spectra Physics, GCR 11), a 0.27 m single monochromator and a photomultiplier (Hamamatsu, R2949) for detection. The signal was preamplified (Stanford, SR445) and measured with a photon-counting system (Stanford, SR440). Cooling was achieved by a closed-cycle cryostat (Air Products). The measurements of the emission of BaZnCl4II : Sm2+ and NaBaLaCl6 : Sm2+ were carried out on a SPEX DM3000F spectrofluorometer equipped with two 0.22 m SPEX 1680 double monochromators, a 450 W xenon lamp and a liquid helium flow cryostat (Oxford LF 205). All spectra were corrected for photomultiplier sensitivity.
3. Results and discussion Table 1 Crystallographic data of the host lattices used in this work Compound
SrZnCl4 BaZnCl4-I BaZnCl4-II NaBaLaCl6
Space group Z a (pm) b (pm) c (pm) Site symmetry of Sm2+ Ref.
I41/a 4 650.4 650.4 1437.0 S4
Pnna 4 724.1 986.3 047.7 C2
Pbcn 4 650.4 681.9 1536.3 C2
P63/m 1 710.7 710.7 766.5 C3h
[14]
[14]
[11]
[15]
SrZnCl4 Point symmetry S4 Coordination number 8 2+ Sr -Cl : 297.1 and 305.6pm
NaBaLaCl6 Point symmetry C3h Coordination number 9 2+ Ba -Cl : 304.1 and 305.1pm
Because of the similar ionic radii of Sr2+ (140 pm [14,15]) and Sm2+ (141 pm [14,15]), Sm2+ occupies the crystallographic sites of Sr2+ rather than the Zn2+ site (74 pm [14,15]) in SrZnCl4 : Sm2+ with low dopant concentrations. This site possesses the coordination number eight and the site symmetry S4 [16] (Fig. 1, Table 1). Fig. 2a shows the Sm2+ emission after excitation at 532 nm at different temperatures. At room temperature, only broad 4f55d1-4f6(7FJ) emission
BaZnCl4-I Point symmetry C2 Coordination number 8 2+ Ba -Cl : 314.0- 319.1pm
BaZnCl4-II Point symmetry C2 Coordination number 8 2+ Ba -Cl : 315.9- 319.7pm
Fig. 1. Coordination polyhedra of chloridic host lattices together with coordination numbers, pointsymmetries and M2+–Cl distances (M = Sr, Ba). The point symmetries are determined by X-ray structure data and may be lower in reality (see text).
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energy
excitation
2+
5
SrZnCl4:Sm 5
5
7
D0
F1
7
D0
5
7
D0
1
4f 5d 5
F0
D0
20K
5
F2
5
D0
7
4f 5d
1
7
FJ
FJ
298K 5
D0
7
13000
F3
7
14000
(a)
15000
16000
(b)
FJ
configuration coordinate
-1
Energy/cm
Fig. 2. (a) Emission spectra of SrZnCl4 : Sm2+ at room temperature and at 20 K upon excitation at 18796 cm 1. 5D0-7FJ transitions are assigned. (b) Configurational curve diagram for SrZnCl4 : Sm2+. For clarity, only one 7FJ level is drawn.
Table 2 Emission of Sm2+ in different chlorides Host lattice
Emission at 298 K
Maximum (cm 1)
Emission at lower temperature
Maximum (cm 1)
SrZnCl4
4f55d1-7FJ
14685
4f55d1-7FJ 5 D0-7FJ
14609 14580 (5D0-7F0?)
BaZnCl4-I
4f55d1-7FJ 5 D0-7FJ
15400
4f55d1-7FJ 5 D0-7FJ
15810 14588 (5D0-7F0)
5
14545 (5D0-7F0)
5
BaZnCl4-II
NaBaLaCl6
5
D0-7FJ
D0-7FJ
14567 (5D0-7F0)
D1-7FJ
15926 (5D1-7F0)
D1-7FJ
15870 (5D1-7F0)
5
with a maximum at 14685 cm 1 is observed while at lower temperature, additional sharp emission appears which can be assigned to 5D0-7FJ transitions (Table 2). The observation of a peak which can be assigned to the 5D0-7F0 transition is in contradiction with the S4 point symmetry. Further investigations are necessary to clarify whether this is an artefact of the measurement or due to the lowering of the point symmetry of the Sr2+ site. Because of the decreasing occupation of vibrational excited states, the full-width at halfmaximum of the broad band decreases at lower temperature, and the maximum is blue shifted. The shape of the broad band as well as the number of
crystal field components of each 7FJ state confirm that Sm2+ occupies only one crystallographic site. In SrZnCl4 : Sm2+, the 4f55d1 state is located at slightly higher energy than the 5D0 state (Fig. 2b), so it is thermally occupied at all temperatures. At higher temperatures, only 4f55d1-4f6 emission is observed because the parity-allowed transitions are much more probable than the parity-forbidden f-f transitions. At lower temperatures, the degree of thermal occupation is lower, and the less probable 5D0-7FJ transitions are also observed. Because of the diameter of Ba2+ (156 pm [14,15]), Sm2+ ions will also occupy the alkaline earth site in the barium compounds. The
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coordination of Sm2+ in BaZnCl4-I : Sm2+ is again eightfold, but the point symmetry is lowered to C2 and the Ba2+–Cl distances increase when compared with SrZnCl4 [16] (Fig. 1, Table 1). Accordingly, the crystal field splitting is lower and shifts the lowest 4f55d1 state to higher energy as compared to SrZnCl4. This decreases the degree of thermal occupation of the 4f55d1 state (Fig. 3b), so that 4f55d1-4f6 and 5D0-7FJ emission can be
5
7
D0
F2
detected at room temperature (Fig. 3a). The maximum of the broad band is located at 15400 cm 1 with a red shift of more than 700 cm 1 when compared to SrZnCl4. The crystal field splitting of the 7FJ states could not be resolved in most cases but the strong 5D0-7F0 emission is remarkable. At lower temperatures, no thermal occupation of the 4f55d1 state occurs, so only 5D0-7FJ emission is observed. Again, the
BaZnCl4-I:Sm 5
7
D0
F1
2+
energy
20K
excitation
5
1
4f 5d 5
5
5
7
F4 D0
7
D0
F3
5
D0
7
D0
F0
5
5 5
7
D0
7
D0
F1 5D 0
F2
7
295K
F0
5
D0
7 5
D0
5
7
7
F 4 D0
12000
4f 5d
7
1
7
FJ
FJ
FJ
F3
13000
14000 -1 Energy/cm
(a)
15000
16000
configuration coordinate
(b)
Fig. 3. (a) Emission spectra of BaZnCl4-I : Sm2+ at room temperature (below) and at 20 K (above) upon excitation at 18796 cm 1. 5 D0-7FJ transitions are assigned. (b) Configurational curve diagram for BaZnCl4-I : Sm2+. For clarity, only one 7FJ level is drawn.
excitation
F1 7
7
7
D1 5
D0
5
F0
5
4f
6
1
7
FJ
7
FJ
D0
1
4f 5d
5
D0 13000
7
D0 F J at 295K
7
D0
7
7
D0
5
F3
7 5
F4
7
D0 5
12000
(a)
7
D1 F J at 4K
5
5
4f 5d
1
F0
D1
7
5
5
F1
F2
5
295K
5
4f 5d
5
5
D1
D1
5
D1
D1
5
F4
5
D1
7
7
F5
F2
F3
energy 2+
BaZnCl4-II:Sm 20K
14000
15000 -1
Energy/cm
16000
configuration coordinate
(b)
Fig. 4. (a) Emission spectra of BaZnCl4-II : Sm2+ at room temperature (below) and at 4 K (above) upon excitation at 18868 cm 1. 5 DJ-7FJ transitions are assigned. (b) Configurational curve diagram for BaZnCl4-II : Sm2+. For clarity, only one 7FJ level is drawn.
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number of detected crystal field transitions reflects the C2 point symmetry of the Ba2+ site. In BaZnCl4-II, the Ba2+ ions again occupy sites with coordination number eight and point symmetry C2 [11] (Table 1). Only the distances Ba2+– Cl decrease slightly, approximately 1 pm (Fig. 1). Emission spectra of BaZnCl4-II : Sm2+ are depicted in Fig. 4a. As in BaZnCl4-I : Sm2+, 4f55d1-4f6 as well as 5D0-7FJ emission is observed (Table 2). The maximum of the broad band (15800 cm 1) shows a blue shift of 400 cm 1 as compared to the modification I. Note that a very small change in the Sm2+ coordination causes an unexpected large shift in the d-f emission. Additionally, the 5D0-7F0 transition is much less intense. Due to the lower thermal occupation of the 4f55d1 state as compared to BaZnCl4-I : Sm2+, the broad d-f emission is much smaller. At lower temperatures, there is again only f-f emission, but the emitting level is now mainly the 5D1 state (Fig. 4a, Table 2) and the 5D0-7F2 transition is detected as a very small peak. To the best of our knowledge, this is the first time that nearly only 5 D1-7FJ emission is detected for Sm2+. The explanation for this behaviour is as follows (Fig. 4b): The 4f55d1 state is now located at a higher energy than the 5D1 state. At room temperature, thermally induced transitions from the 5D1 to the 4f55d1 state occur, and the fast d-f
F1
2+
D1 5
D1
5
7
F0
F3
7
D1
D1
4f 5d
5
D0
F1 F0
7
7
5
7
D0 F J at 295K
D0
D0
5
D0
5
F3
5
7
7
F4
1
7
D1 FJ at 4K
5
D1
5 5
7
F2
D1
5
F4
7
F5
7
D1
5
FJ
5
5
D0
7
D0
excitation
5
5
295K
energy
7
NaBaLaCl6:Sm 20K
7
F2
emission is observed. Additionally, some radiationless relaxations to the 5D0 state take place, followed by 5D0-7FJ emission. At lower temperatures, the thermal occupation of the 4f55d1 state disappears. Multiphonon relaxation 5 D1-5D0 is not possible because the large gap between the states (1400 cm 1) cannot be bridged by the energy of a phonon, which has a maximum of 240 cm 1 [12]. Therefore, the only possibility for relaxation is 5D1-7FJ emission. NaBaLaCl6 is a much more complicated host lattice. There is only one crystallographic site, which is occupied by Ba2+ and La3+ simultaneously [17]. The coordination number is nine (Table 1). Unfortunately, it is not possible to determine the Ba2+–Cl distances by X-ray structure analysis because the mixed Ba/Laoccupation leads to an averaged value only. Additionally, each Ba/La site is different depending on the number of Ba2+ and La3+ ions in the neighbourhood. Therefore, the real microscopic point symmetry is lower than C3h, which is an average value determined by X-ray diffraction. Fig. 5a shows the emission spectra of NaBaLaCl6 : Sm2+. The optical behaviour is similar to that of BaZnCl4-II : Sm2+. At room temperature, only 5D0-7FJ emission is detected where the appearance of the 5D0-7F0 transition, which is forbidden for C3h symmetry, confirms the lower
13000
(a)
14000
-1
Energy/cm
15000
16000
(b)
configuration coordinate
Fig. 5. (a) Emission spectra of NaBaLaCl6 : Sm2+ at room temperature (below) and at 4 K (above) upon excitation at 19646 cm 1. 5 DJ-7FJ transitions are assigned. (b) Configurational curve diagram for NaBaLaCl6 : Sm2+. For clarity, only one 7FJ level is drawn.
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microscopic symmetry. At lower temperature, only 5 D1-7FJ emission is observed. In contrast to the other chlorides, 4f55d1-4f6 emission could not be detected (Table 2). The energy level diagram of BaZnCl4-II : Sm2+ is also valid in this case with small differences (Fig. 5a). Again, the 4f55d1 state is located at higher energy than the 5D1 state. Thermal population of this state also occurs but the nuclear equilibrium distance increases. Therefore, the overlap of the 4f55d1 and the 5D0 states is different, the nonradiative relaxation to the lowest 4f6 state is much faster and no 4f55d1-4f6 emission can be observed.
4. Conclusion Emission spectra of Sm2+ doped in different chloride host lattices are presented and compared. It is shown that a small difference of the coordination sphere strongly effects the emission spectra. Configurational curve diagrams, which explain the shape of the spectra at room and lower temperatures, are presented.
Acknowledgements Generous support by Prof. Dr. G. Meyer, Institut fur . Anorganische Chemie, Universit.at zu
. Koln, Germany, and Prof. Dr. A. Meijerink, Debye Institute, University Utrecht, Netherlands, is gratefully acknowledged.
References [1] J.D. Axe, P.P. Sorokin, Phys. Rev. 130 (1963) 945. [2] H.V. Lauer, Fong Jr., F.K. Fong, J. Chem. Phys. 65 (1976) 3108. [3] G. Grenet, M. Kibler, A. Gros, J.C. Souillat, J.C. G#acon, Phys. Rev. B 22 (1980) 5052. [4] R. Jaaniso, H. Bill, Phys. Rev. B 44 (1991) 2389. [5] Y.R. Shen, K.L. Bray, W.B. Holzapfel, J. Lumin. 72–74 (1997) 266. [6] Y. Shen, K.L. Bray, Phys. Rev. B 58 (1998) 11 944. [7] A.S.M. Alam, B. Di Bartolo, J. Chem. Phys. 47 (1967) 3790. [8] J.C. G#acon, G. Grenet, J.C. Souillat, M. Kibler, J. Chem. Phys. 69 (1978) 868. [9] J.C. G#acon, J.F. Marcerou, M. Bouazaoui, B. Jacquier, M. Kibler, Phys. Rev. B 40 (1989) 2070. [10] C. Wickleder, J. Alloys Compounds 300 (2000) 193. [11] C. Wickleder, J. Solid State Chem., in press. [12] C. Wickleder, in preparation. [13] G. Meyer, Inorg. Synth. 22 (1983) 1. [14] R.D. Shannon, C.T. Prettwitt, Acta Crystallogr. B 25 (1969) 925. [15] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [16] C. Wickleder, S. Masselmann, G. Meyer, Z. Anorg. Allg. Chem. 625 (1999) 507. [17] M. Wickleder, G. Meyer, Z. Anorg. Allg. Chem. 624 (1998) 1577.
Excited states of Sm2+ in chloride host lattices Claudia Wickleder* Institut fur . zu Koln, . Greinstrabe 6, 50939 Koln, . Germany . Anorganische Chemie, Universitat
Abstract Emission spectra of Sm2+ at different temperatures in several chloride host lattices after excitation into the 4f55d1 levels are presented and compared. SrZnCl4 : Sm2+ shows broad 4f55d1-4f6 emission at room temperature while additional 5D0-7FJ emission is observed at lower temperatures. 4f55d1-4f6 as well as 5D0-7FJ emission is observed in BaZnCl4-I : Sm2+ at room temperature, investigations at lower temperatures show only 5D0-7FJ emission. In spite of the very similar [BaCl8] polyhedra in the crystal structures of BaZnCl4-I and BaZnCl4-II, the maximum of the 4f55d1-4f6 transition at room temperature is shifted to about 400 cm 1 in BaZnCl4-II. Again, 5D0-7FJ emission is observed in addition. At lower temperatures, the 5D1-7FJ emission is only detected. Finally, NaBaLaCl6 : Sm2+ shows only f-f transitions: 5D0-7FJ at room temperature and 5D1-7FJ at lower temperatures. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Divalent lanthanides; Samarium; Emission; Chlorides
1. Introduction One interesting feature of the electronic properties of Sm2+ ions with the ground-state configuration 4f6 is the low-lying 4f55d1 level. Because of the excitation of one electron to a d state, the energy of the transition depends very strongly on the host lattice. In most chlorides, the broad f–d transition starts in the green region so that the compounds appear red. In contrast, f–f transitions, which occur at comparable energies, are nearly independent of the surrounding. Therefore, emission from a 4f55d1 or 4f6 level or both can be observed. The position of the excited 4f55d1 level *Fax: +49-221-470-5083. E-mail address: [email protected] (C. Wickleder).
depends on several parameters like crystal field strength, point symmetry, coordination number and covalency of the bond. In general, some of these parameters are changed in different host lattices. In order to evaluate the influence of a single parameter, it is necessary to investigate host lattices, which are very similar. The chlorides studied in this work, are indeed very similar with respect to Sm2+ coordination. Previous investigations of Sm2+ in chlorides have been carried out in simple host lattices like SrCl2 [1], BaCl2 [2], SrFCl [3–6] and BaFCl [6–9]. Note that the present contribution aims to summarize and to compare the emission spectra of Sm2+ in SrZnCl4, two modifications of BaZnCl4 (BaZnCl4-I and BaZnCl4-II) and NaBaLaCl6. A detailed presentation of all experimental results and a comprehensive discussion is given elsewhere [10–12].
0022-2313/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 3 8 7 - 8
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2. Experimental Pure SmCl2 was prepared by the reduction of SmCl3 with Sm metal according to the procedure given in Ref. [13]. All educts were sublimed in high vacuum to guarantee high purity of the products. The ternary chlorides were synthesized by melting the respective binary chlorides at 6001C (SrZnCl4), 6001C (BaZnCl4-I), 4801C (BaZnCl4-II) and 5501C (NaBaLaCl6) followed by slow cooling (11C/h) using the Bridgman technique. As a dopant, 0.05 mol% of SmCl2 was added. To avoid oxidation of Sm2+ the reactions were carried out in , sealed tantalum containers. The compounds are extremely moisture-sensitive and must be handled under inert conditions. The purity of all products was checked by powder X-ray diffraction (Stoe and Cie., Stadi P) (Table 1).
Emission spectra of SrZnCl4 : Sm2+ and BaZnCl4-I : Sm2+ were recorded using a frequency-doubled Nd : YAG laser (Spectra Physics, GCR 11), a 0.27 m single monochromator and a photomultiplier (Hamamatsu, R2949) for detection. The signal was preamplified (Stanford, SR445) and measured with a photon-counting system (Stanford, SR440). Cooling was achieved by a closed-cycle cryostat (Air Products). The measurements of the emission of BaZnCl4II : Sm2+ and NaBaLaCl6 : Sm2+ were carried out on a SPEX DM3000F spectrofluorometer equipped with two 0.22 m SPEX 1680 double monochromators, a 450 W xenon lamp and a liquid helium flow cryostat (Oxford LF 205). All spectra were corrected for photomultiplier sensitivity.
3. Results and discussion Table 1 Crystallographic data of the host lattices used in this work Compound
SrZnCl4 BaZnCl4-I BaZnCl4-II NaBaLaCl6
Space group Z a (pm) b (pm) c (pm) Site symmetry of Sm2+ Ref.
I41/a 4 650.4 650.4 1437.0 S4
Pnna 4 724.1 986.3 047.7 C2
Pbcn 4 650.4 681.9 1536.3 C2
P63/m 1 710.7 710.7 766.5 C3h
[14]
[14]
[11]
[15]
SrZnCl4 Point symmetry S4 Coordination number 8 2+ Sr -Cl : 297.1 and 305.6pm
NaBaLaCl6 Point symmetry C3h Coordination number 9 2+ Ba -Cl : 304.1 and 305.1pm
Because of the similar ionic radii of Sr2+ (140 pm [14,15]) and Sm2+ (141 pm [14,15]), Sm2+ occupies the crystallographic sites of Sr2+ rather than the Zn2+ site (74 pm [14,15]) in SrZnCl4 : Sm2+ with low dopant concentrations. This site possesses the coordination number eight and the site symmetry S4 [16] (Fig. 1, Table 1). Fig. 2a shows the Sm2+ emission after excitation at 532 nm at different temperatures. At room temperature, only broad 4f55d1-4f6(7FJ) emission
BaZnCl4-I Point symmetry C2 Coordination number 8 2+ Ba -Cl : 314.0- 319.1pm
BaZnCl4-II Point symmetry C2 Coordination number 8 2+ Ba -Cl : 315.9- 319.7pm
Fig. 1. Coordination polyhedra of chloridic host lattices together with coordination numbers, pointsymmetries and M2+–Cl distances (M = Sr, Ba). The point symmetries are determined by X-ray structure data and may be lower in reality (see text).
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energy
excitation
2+
5
SrZnCl4:Sm 5
5
7
D0
F1
7
D0
5
7
D0
1
4f 5d 5
F0
D0
20K
5
F2
5
D0
7
4f 5d
1
7
FJ
FJ
298K 5
D0
7
13000
F3
7
14000
(a)
15000
16000
(b)
FJ
configuration coordinate
-1
Energy/cm
Fig. 2. (a) Emission spectra of SrZnCl4 : Sm2+ at room temperature and at 20 K upon excitation at 18796 cm 1. 5D0-7FJ transitions are assigned. (b) Configurational curve diagram for SrZnCl4 : Sm2+. For clarity, only one 7FJ level is drawn.
Table 2 Emission of Sm2+ in different chlorides Host lattice
Emission at 298 K
Maximum (cm 1)
Emission at lower temperature
Maximum (cm 1)
SrZnCl4
4f55d1-7FJ
14685
4f55d1-7FJ 5 D0-7FJ
14609 14580 (5D0-7F0?)
BaZnCl4-I
4f55d1-7FJ 5 D0-7FJ
15400
4f55d1-7FJ 5 D0-7FJ
15810 14588 (5D0-7F0)
5
14545 (5D0-7F0)
5
BaZnCl4-II
NaBaLaCl6
5
D0-7FJ
D0-7FJ
14567 (5D0-7F0)
D1-7FJ
15926 (5D1-7F0)
D1-7FJ
15870 (5D1-7F0)
5
with a maximum at 14685 cm 1 is observed while at lower temperature, additional sharp emission appears which can be assigned to 5D0-7FJ transitions (Table 2). The observation of a peak which can be assigned to the 5D0-7F0 transition is in contradiction with the S4 point symmetry. Further investigations are necessary to clarify whether this is an artefact of the measurement or due to the lowering of the point symmetry of the Sr2+ site. Because of the decreasing occupation of vibrational excited states, the full-width at halfmaximum of the broad band decreases at lower temperature, and the maximum is blue shifted. The shape of the broad band as well as the number of
crystal field components of each 7FJ state confirm that Sm2+ occupies only one crystallographic site. In SrZnCl4 : Sm2+, the 4f55d1 state is located at slightly higher energy than the 5D0 state (Fig. 2b), so it is thermally occupied at all temperatures. At higher temperatures, only 4f55d1-4f6 emission is observed because the parity-allowed transitions are much more probable than the parity-forbidden f-f transitions. At lower temperatures, the degree of thermal occupation is lower, and the less probable 5D0-7FJ transitions are also observed. Because of the diameter of Ba2+ (156 pm [14,15]), Sm2+ ions will also occupy the alkaline earth site in the barium compounds. The
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coordination of Sm2+ in BaZnCl4-I : Sm2+ is again eightfold, but the point symmetry is lowered to C2 and the Ba2+–Cl distances increase when compared with SrZnCl4 [16] (Fig. 1, Table 1). Accordingly, the crystal field splitting is lower and shifts the lowest 4f55d1 state to higher energy as compared to SrZnCl4. This decreases the degree of thermal occupation of the 4f55d1 state (Fig. 3b), so that 4f55d1-4f6 and 5D0-7FJ emission can be
5
7
D0
F2
detected at room temperature (Fig. 3a). The maximum of the broad band is located at 15400 cm 1 with a red shift of more than 700 cm 1 when compared to SrZnCl4. The crystal field splitting of the 7FJ states could not be resolved in most cases but the strong 5D0-7F0 emission is remarkable. At lower temperatures, no thermal occupation of the 4f55d1 state occurs, so only 5D0-7FJ emission is observed. Again, the
BaZnCl4-I:Sm 5
7
D0
F1
2+
energy
20K
excitation
5
1
4f 5d 5
5
5
7
F4 D0
7
D0
F3
5
D0
7
D0
F0
5
5 5
7
D0
7
D0
F1 5D 0
F2
7
295K
F0
5
D0
7 5
D0
5
7
7
F 4 D0
12000
4f 5d
7
1
7
FJ
FJ
FJ
F3
13000
14000 -1 Energy/cm
(a)
15000
16000
configuration coordinate
(b)
Fig. 3. (a) Emission spectra of BaZnCl4-I : Sm2+ at room temperature (below) and at 20 K (above) upon excitation at 18796 cm 1. 5 D0-7FJ transitions are assigned. (b) Configurational curve diagram for BaZnCl4-I : Sm2+. For clarity, only one 7FJ level is drawn.
excitation
F1 7
7
7
D1 5
D0
5
F0
5
4f
6
1
7
FJ
7
FJ
D0
1
4f 5d
5
D0 13000
7
D0 F J at 295K
7
D0
7
7
D0
5
F3
7 5
F4
7
D0 5
12000
(a)
7
D1 F J at 4K
5
5
4f 5d
1
F0
D1
7
5
5
F1
F2
5
295K
5
4f 5d
5
5
D1
D1
5
D1
D1
5
F4
5
D1
7
7
F5
F2
F3
energy 2+
BaZnCl4-II:Sm 20K
14000
15000 -1
Energy/cm
16000
configuration coordinate
(b)
Fig. 4. (a) Emission spectra of BaZnCl4-II : Sm2+ at room temperature (below) and at 4 K (above) upon excitation at 18868 cm 1. 5 DJ-7FJ transitions are assigned. (b) Configurational curve diagram for BaZnCl4-II : Sm2+. For clarity, only one 7FJ level is drawn.
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number of detected crystal field transitions reflects the C2 point symmetry of the Ba2+ site. In BaZnCl4-II, the Ba2+ ions again occupy sites with coordination number eight and point symmetry C2 [11] (Table 1). Only the distances Ba2+– Cl decrease slightly, approximately 1 pm (Fig. 1). Emission spectra of BaZnCl4-II : Sm2+ are depicted in Fig. 4a. As in BaZnCl4-I : Sm2+, 4f55d1-4f6 as well as 5D0-7FJ emission is observed (Table 2). The maximum of the broad band (15800 cm 1) shows a blue shift of 400 cm 1 as compared to the modification I. Note that a very small change in the Sm2+ coordination causes an unexpected large shift in the d-f emission. Additionally, the 5D0-7F0 transition is much less intense. Due to the lower thermal occupation of the 4f55d1 state as compared to BaZnCl4-I : Sm2+, the broad d-f emission is much smaller. At lower temperatures, there is again only f-f emission, but the emitting level is now mainly the 5D1 state (Fig. 4a, Table 2) and the 5D0-7F2 transition is detected as a very small peak. To the best of our knowledge, this is the first time that nearly only 5 D1-7FJ emission is detected for Sm2+. The explanation for this behaviour is as follows (Fig. 4b): The 4f55d1 state is now located at a higher energy than the 5D1 state. At room temperature, thermally induced transitions from the 5D1 to the 4f55d1 state occur, and the fast d-f
F1
2+
D1 5
D1
5
7
F0
F3
7
D1
D1
4f 5d
5
D0
F1 F0
7
7
5
7
D0 F J at 295K
D0
D0
5
D0
5
F3
5
7
7
F4
1
7
D1 FJ at 4K
5
D1
5 5
7
F2
D1
5
F4
7
F5
7
D1
5
FJ
5
5
D0
7
D0
excitation
5
5
295K
energy
7
NaBaLaCl6:Sm 20K
7
F2
emission is observed. Additionally, some radiationless relaxations to the 5D0 state take place, followed by 5D0-7FJ emission. At lower temperatures, the thermal occupation of the 4f55d1 state disappears. Multiphonon relaxation 5 D1-5D0 is not possible because the large gap between the states (1400 cm 1) cannot be bridged by the energy of a phonon, which has a maximum of 240 cm 1 [12]. Therefore, the only possibility for relaxation is 5D1-7FJ emission. NaBaLaCl6 is a much more complicated host lattice. There is only one crystallographic site, which is occupied by Ba2+ and La3+ simultaneously [17]. The coordination number is nine (Table 1). Unfortunately, it is not possible to determine the Ba2+–Cl distances by X-ray structure analysis because the mixed Ba/Laoccupation leads to an averaged value only. Additionally, each Ba/La site is different depending on the number of Ba2+ and La3+ ions in the neighbourhood. Therefore, the real microscopic point symmetry is lower than C3h, which is an average value determined by X-ray diffraction. Fig. 5a shows the emission spectra of NaBaLaCl6 : Sm2+. The optical behaviour is similar to that of BaZnCl4-II : Sm2+. At room temperature, only 5D0-7FJ emission is detected where the appearance of the 5D0-7F0 transition, which is forbidden for C3h symmetry, confirms the lower
13000
(a)
14000
-1
Energy/cm
15000
16000
(b)
configuration coordinate
Fig. 5. (a) Emission spectra of NaBaLaCl6 : Sm2+ at room temperature (below) and at 4 K (above) upon excitation at 19646 cm 1. 5 DJ-7FJ transitions are assigned. (b) Configurational curve diagram for NaBaLaCl6 : Sm2+. For clarity, only one 7FJ level is drawn.
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microscopic symmetry. At lower temperature, only 5 D1-7FJ emission is observed. In contrast to the other chlorides, 4f55d1-4f6 emission could not be detected (Table 2). The energy level diagram of BaZnCl4-II : Sm2+ is also valid in this case with small differences (Fig. 5a). Again, the 4f55d1 state is located at higher energy than the 5D1 state. Thermal population of this state also occurs but the nuclear equilibrium distance increases. Therefore, the overlap of the 4f55d1 and the 5D0 states is different, the nonradiative relaxation to the lowest 4f6 state is much faster and no 4f55d1-4f6 emission can be observed.
4. Conclusion Emission spectra of Sm2+ doped in different chloride host lattices are presented and compared. It is shown that a small difference of the coordination sphere strongly effects the emission spectra. Configurational curve diagrams, which explain the shape of the spectra at room and lower temperatures, are presented.
Acknowledgements Generous support by Prof. Dr. G. Meyer, Institut fur . Anorganische Chemie, Universit.at zu
. Koln, Germany, and Prof. Dr. A. Meijerink, Debye Institute, University Utrecht, Netherlands, is gratefully acknowledged.
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