Optical properties of Tb3+-doped Rb2KInF6 elpasolite

Optical properties of Tb3+-doped Rb2KInF6 elpasolite

Optical Materials 13 (1999) 211±223 Optical properties of Tb3‡ -doped Rb2KInF6 elpasolite M.A. Bu~ nuel a b a,* , L. Lozano b, J.P. Chaminade b, B...

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Optical Materials 13 (1999) 211±223

Optical properties of Tb3‡ -doped Rb2KInF6 elpasolite M.A. Bu~ nuel a b

a,* ,

L. Lozano b, J.P. Chaminade b, B. Moine a, B. Jacquier

a

Laboratoire de Physico-Chimie des Mat eriaux Luminescents, UMR n° 5620, CNRS, Universit e, Lyon l, B^ at 205, 69622 Villeurbanne C edex, France Institut de Chimie de la Mati ere Condens ee de Bordeaux (ICMCB)-CNRS [UPR 9048], Universit e Bordeaux 1, 33608 Pessac C edex, France Received 20 October 1998; accepted 13 November 1998

Abstract Crystals of Rb2 KInF6 :Tb3‡ with elpasolite structure have been prepared by the Bridgman method. Absorption, wide range luminescence, high resolution emission, excitation and luminescence decay measurements have been carried out at low and room temperature in order to study the environments of Tb3‡ ions and their optical properties. An Oh site of Tb3‡ ions (octahedral TbF3ÿ 6 ; units) has been found and the identi®cation and assignments of the pure magnetic-dipole transitions split out of the Tb3‡ 5 D4 ® 7 F6 , 7 F5 , 7 F4 , 7 F3 and 5 D3 ® 7 F6 , 7 F5 , 7 F4 transitions have been achieved assuming a thermal equilibrated set of crystal-®eld sublevels and comparing with the literature data in chloride and ¯uoride elpasolites; a C2v distortion of the octahedral symmetry has been detected at low temperature. Some experimental results could suggest the presence of a small concentration of other sites of Tb3‡ with a symmetry lower than Oh . Energy transfer between the two sites could also be suggested. Finally, an up-conversion process by means of sequential two-photon absorption (7 F6 ® 5 D4 and 5 D4 ® above 5 D3 ) has been demonstrated. Ó 1999 Elsevier Scince B.V. All rights reserved. Keywords: Elpasolite; Rare earth; Optical properties; Light emission PACS: 81.10.Jt; 71.55.Ht; 78.55.Hx

1. Introduction Photoionization processes have received great attention recently due to their technological applications (optical memories, scintillators, etc.) and also from a basic research viewpoint (photorefractive e€ect). In this way an investigation has been recently performed on Ce3‡ -doped ¯uoroindates and particularly in the elpasolite ¯uoroindates A2 BInF6 (A ˆ K, Rb; B ˆ Na, K) which

* Corresponding author. Tel.: 33 4 72 44 80 00 30 64; fax.: 33 4 72 43 11 30; e-mail: [email protected]

appear attractive ionic host crystals for rare earth. In a recent work in Rb2 KInF6 :Ce3‡ [1] a reversible photoionization process between In3‡ and Ce3‡ has been demonstrated; nevertheless the authors have found that Ce3‡ ions occupy three di€erent positions in the matrix (in indium, potassium and rubidium sites) which makes dicult the spectroscopical study of the photoionization process. Due to its smaller size, Tb3‡ is a more suitable ion to go just in indium site in this kind of material; this fact signi®cantly enhances the possibilities of carrying out unambiguous spectroscopical investigations of the possible photoinization process between Tb3‡ and In3‡ ions. Moreover the presence of the

0925-3467/99/$ - see front matter Ó 1999 Elsevier Scince B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 8 5 - 8

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metastable level 5 D4 could allow photoionization through a two-visible-photon instead of ultraviolet excitation, which is interesting from a technological and basic viewpoint. Structure of ¯uorides with general formula A2 BMF6 (M ˆ In, Y) derived from that of the el3m pasolite (K2 NaAlF6 prototype structure, Fm  with Z ˆ 4). The arrangement corresponds to the perovskite with an additional cationic ordering between the smaller monovalent cations B and trivalent cations M in an octahedral sites [2,3]. Elpasolites represent ideal lattices for the study of transition metal and rare earth ions in octahedral environment [4,5] and consequently during the last 20 years a number of spectroscopic studies have been reported for lanthanide ions, particularly Tb3‡ , residing in these octahedral sites. Additionally, several members of the elpasolite family exhibit structural phase transitions [6,7]. However one-photon f±f transition is electric-dipole forbidden and the absorption cross-section of vibronic transitions is low; therefore emission data are used to determine the energies of the levels. Consequently one- and two-photon luminescence data of Tb3‡ have been obtained and studied in Cs2 NaTbCl6 [8±11], Cs2 NaYCl6 :Tb3‡ [4,10], Cs2 NaTbF6 [11,12], Cs2 KYF6 :Tb3‡ [13] and Cs2 NaTbBr6 [11] crystals. This work is the ®rst one of the series which deals with the study of a reversible photoionization process between Tb3‡ and In3‡ ions in Rb2 KInF6 . The purpose of this work is the optical characterisation of Tb3‡ -doped Rb2 KInF6 elpasolite and the study of some two-photon optical properties. The crystal structure of Rb2 KInF6 is such that the ®rst-neighbour coordination for the In3‡ ion is an octahedron of ¯uoride ions, the second-neighbour coordination shell is a simple cube of rubidium ions, and the third-shell coordination is an octahedron of potassium ions. The In3‡ ions appear again only in the ®fth-neighbour shell. Given this structure if Tb3‡ goes in the In3‡ site, pure electric-dipole (f±f) transitions are strongly forbidden and one may expect to observe only vibronically induced (f±f) electric-dipole transitions and pure magnetic-dipole transitions. Furthermore, one may expect the f±f excited states to be characterised by relatively long relaxation

times and long-lived emissions. Additionally two phase transitions have been detected in this material: a second order phase transition at 283 K which gives a quadratic structure I4/m and a ®rst order one at 269 K which gives a monoclinic structure P21 /n [14]. 2. Experimental details 2.1. Crystal growth The alkali ¯uorides, KF and RbF were commercial products which were dehydrated under vacuum. Nitrates of terbium or indium were ®rst dissolved in distilled water and then poured into a 40% HF solution. The co-precipitate obtained after centrifugation was washed with water and immersed in a small amount of 40% HF which was eliminated by evaporation. In order to remove OH groups in the initial powder, a thermal treatment was accomplished under argon in the presence of a ¯uorinated atmosphere, HF ¯ow up to 500°C or under a F2 stream up to 600°C, respectively. Rb2 KInF6 :Tb3‡ compounds have been prepared by solid state reaction from stoichiometric mixtures of composition 2RbF + KF + (1 ÿ x)InF3 + xTbF3 at about 700°C in sealed gold tubes. All the manipulations were carried out in a dry box containing dried and deoxidized Ar gas. The Bridgman method has been selected for crystal growth. The crystal growth equipment was made of two independent furnaces separated by an insulating zone. The temperature of each furnace has been separately programmed. The powder was introduced in a platinum 10% rhodium biconical shaped crucible which was sealed under a dry argon atmosphere and set in the crystal growth apparatus initially heated to T ˆ TF + 50 K in the upper furnace. The crucible was moved down to the cooler furnace at a rate of 0.5 mm/h in a thermal gradient of 2.5 K/mm. The temperature was then lowered to room temperature at a rate of 50 K/h. After sawing o€ the upper part of the platinum crucible, crystals were removed mechanically. Transparent colourless single crystals of Rb2 KIn1ÿx Tbx F6 with x ˆ 0.001 and 0.01 compositions around 5 ´ 5 ´ 5 mm3 were obtained

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without visible inhomogeneities. Single crystals were orientated by X-ray di€raction, cut into slices along di€erent crystallographic orientations with a diamond wire and polished with alumina powder in an alcoholic solution. 2.2. Optical measurements Absorption spectra were obtained in a CARY 2300 from VARIAN spectrophotometer for a spectral range between 160 and 3500 nm. Wide range emission and excitation spectra were performed by exciting the samples with a 1000 W ORIEL 66187 tungsten lamp followed by a double 0.5 m SPEX 1680 B monochromator and detecting the ¯uorescence through a 0.5 m Jarrell-Ash monochromator with a Hamamatsu R-928 photomultiplier tube; the spectral resolution was 1 nm; the measurements have been corrected from the system response. High resolution luminescence measurements, emission and excitation spectra and lifetime, were obtained by exciting the samples with a tuneable pulsed dye laser; the pulse duration was 5 ns and the spectral line about 3 cmÿ1 ; the energy per pulse was 50 and 500 lJ for exciting the Stokes and anti-Stokes spectra, respectively. The dye laser was pumped by a Lumonics Excimer 5000 laser; ¯uorescence was detected through a single 1 m Higer & Watts monochromator with an RCA AsGa photomultiplier tube and an EG&G ORTEC 9315 photon counter system which was gated to count 1 ls after the laser pulse for 40 ms, with no background channel subtraction; the spectral resolution was 0.1 nm. Lifetime measurements were carried out using a Stanford Research Systems SR430 multi-channel scaler triggered by the laser. Low-temperature measurements were taken in a liquid helium optical cryostat. 3. Experimental results 3.1. Absorption and wide range luminescence spectra Fig. 1 shows the evolution with temperature of the absorption spectrum of a sample with 1% of Tb3‡ . Three features with very di€erent intensities

Fig. 1. Y axis (left): absorption coecient of the low energy spectral range, Y2 axis (right): absorption coecient of the high energy spectral range of the sample with 1% of Tb3‡ . Labels are explained in the text.

are observed. The absorption coecient of the most intense one is presented in Y2 axis (right); it is a wide band without structure detected at about 46 270 cmÿ1 (A) which broadens when the temperature increases. The rest of the spectrum is presented in the Y axis (left); a structure of three peaks at about 39 400 cmÿ1 (B) is detected; these three peaks broaden when temperature increases; a very small peak is also observed at about 26 570 cmÿ1 (C). No other features have been detected out of the shown spectral range. The ultraviolet features A and B are assigned, although their very di€erent intensities, to the electric-dipole allowed transitions from the lowest Stark component of the 4f8 con®guration to the eg and t2g components of the excited 4f7 5d con®guration, respectively; the three crystal-®eld splitting 3t2g +2eg could be indicated by the three observed peaks in the B absorption feature [15,16]. The small feature C is assigned to the 7 F6 ® 5 D3 intracon®gurational (4f8 ® 4f8 ) transition of Tb3‡ ions, in agreement with the literature [12]. The other intracon®gurational transitions are not detected due to their very well-known forbidden electric-dipole character. As we will see later the emission spectrum of Tb3‡ ions in our crystals is made by several bands in the visible-ultraviolet spectral range due to the transitions from 5 D3 and 5 D4 levels toward the 5 FJ multiplets. In order to have more information about the position of the absorption transitions, the excitation spectra at 10 K (continuous line)

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and RT (dotted line) shown in Fig. 2 have been carried out by detecting the green emission at 18 483 cmÿ1 on a sample of 0.1% of Tb3‡ ions. Three main features, with very di€erent intensities, are detected. At 10 K the most intense one, centered at about 39 400 cmÿ1 (B) is a structure of three peaks. The two other features are shown expanded ´ 10; the ®rst one has a peak at about 26 570 cmÿ1 (C) and a structure of some small peaks in its high energy side; the second one is a peak at about 20 718 cmÿ1 (D). All the excitation features broaden when the temperature increases but they do not change their spectral position. The positions and the relative intensities of the features B and C correspond very well to those of the previously described absorption features B and C and the same assignments can be made. The peak D is assigned to the 7 F6 ® 5 D4 transition of Tb3‡ ions according to the literature [8±13]. Fig. 3 shows the emission spectrum at RT taken by exciting the sample of 0.1% of Tb3‡ ions at 27 700 cmÿ1 , slightly above the 7 F6 ® 5 D3 transition. Ten main structures are observed. The most intense ones are a doublet at about 26 480 and 25 860 cmÿ1 and another one at about 18 475 and 18 220 cmÿ1 . This emission spectrum is known to be due to Tb3‡ ions [8±13]. The excitation decays non-radiatively to the 5 D3 and 5 D4 levels and the emission occurs from these levels to the 7 FJ manifolds. Following the literature [8±13] the emission structures are assigned to the transitions from 5 D3

Fig. 3. Emission spectrum measured at RT of the sample with 0.1% of Tb3‡ . The assignments of the transitions have been written.

to 7 F6 (26 480 and 25 860 cmÿ1 ), 7 F5 (24 240 and 23 860 cmÿ1 ), 7 F4 (22 950 and 22 370 cmÿ1 ), 7 F3 (21 750 cmÿ1 ) levels and from 5 D4 to 7 F6 (20 586 and 20 040 cmÿ1 ), 7 F5 (18 475 and 18 220 cmÿ1 ), 7 F4 (17 100 and 16 765 cmÿ1 ), 7 F3 (16 040 cmÿ1 ), 7 F2 (15 200 cmÿ1 ) and 7 F1 (14 690 cmÿ1 ) levels. The 5 D3 ® 7 F2 , 7 F1 and 7 F0 transitions are masked by the 5 D4 ® 7 F6 emission that is localised at the same position. In order to understand the Tb3‡ environments, a detailed study of the most intense of these emissions has been performed in the most concentrated sample in Tb3‡ ions. 3.2. Luminescence spectra of 5 D4 level 5

D4 ® 7 F5 transition: Fig. 4 (top) shows the D4 ® 7 F5 emission spectrum at RT (dotted line) excited at 20 718 cmÿ1 (directly at the 5 D4 level). It shows two main doublets not well resolved. The ®rst one shows two maxima peaked at 18 510 and 18 480 cmÿ1 and the second one shows two maxima at 18 185 and 18 085 cmÿ1 . The emission spectrum does not depend on the excitation energy into the 7 F6 ® 5 D4 transition. The emission spectra at 16 K excited at 20 718 cmÿ1 (continuous line) and at 20 603 cmÿ1 (short dotted line, expanded ´ 60) are also shown at the top of the ®gure. The former presents a very intense group (in the following we will use numbers 0  in parentheses to indicate the labels of transitions 5

Fig. 2. Excitation spectrum at RT (left axis) and 16 K (right axis) of the sample with 0.1% of Tb3‡ . The low spectral range is shown expanded ´ 10. Labels are explained in the text.

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Table 1 Assignments of the labelled transitions of continuous line spectra Transition 5

Fig. 4. Top: emission spectra of the 5 D4 ®7 F5 transition measured at RT (left axis) and 16 K (right axis) and excited at 20 718 and 20 603 (expanded ´ 65) cmÿ1 . Bottom: expansion ´ 20 of the emission spectrum excited at 20 718 cmÿ1 at 16 K. Labels are explained in the text and the assignments are shown in Tables 1 and 2.

7

5

assigned in Table 1; 3 in this case) of three narrow peaks located at 18 489, 18 483 and 18 476 cmÿ1 almost with the same intensity and three other much less intense structures at 18 508 (2), 18 185 (5) and 18 083 (6) cmÿ1 also with almost the same intensity. Fig. 4 (bottom) presents an expansion of the continuous line spectrum of the top of the ®gure. Some other less intense peaks are localised at 18 552 (1), 18 218 (4), 18 052 (7), 18 016 (8) and 17 973 (9) cmÿ1 . The same spectrum is obtained for an excitation energy higher than 20 718 cmÿ1 into the 5 D4 level. The short dotted line spectrum shows the same above-described features, and some new ones at 18 463, 18 432, 18 362 (in the following we will use numbers 0  in parentheses to indicate the labels of transitions assigned in Table 2; 10 in this case) and 18 130 (20 ) cmÿ1 . The emission spectra for other excitation energies (not shown here) show that the intensity of the features associated with the continuous line spectrum increases with respect to that of the features associated with the short dotted line one when the excitation energy goes from 20 603 to 20 718 cmÿ1 and it decreases when the excitation energy is lower than 20 603 cmÿ1 . The emission spectrum at 6 K (not shown here) just shows structure 3. The emission spectrum taken at RT, described above, shows a growing of the intensity of the structures

D4 ® 7 F5

F6 ® 5 D4 D4 ® 7 F6

Number 1 2 3

Assignment

Position (cmÿ1 )

Eg ® Tlga T1g ® T1ga A1g ® T1ga

18 552 18 508

(A1 ® A2 , B1 , B2 )

18 489 18 483 18 476

4 5 6 7 8 9

Eg ® T2g T1g ® T2g T1g ® Eg Eg ® T1gb T1g ® T1gb A1g ® Tlgb

18 218 18 185 18 083 18 052 18 016 17 973

10

A1g ® T1g

20 718

11

A1g ® T1g (A1 ® A2 , B1 , B2 )

5

D4 ® 7 F4

12

A1g ® T1g (A1 ® A2 , B1 , B2 )

5

D4 ® 7 F3

13 14 15

Eg ® T1g , T2g T1g ® T1g , T2g A1g ® T1g (A1 ® A2 , B1 , B2 )

5

5

5

D3 ® 7 F6

D3 ® 7 F5

16 17

20 656 20 638 20 586

17 203 17 172 17 161 16 022 15 993 15 985 15 975 15 961

A2g ® T2ga A2g ® T2gb

26 283

(A2 ® A1 , B1 , B2 )

25 697 25 680

18

A2g ® T2g

23 853

19

A2g ® T2g

22 329

7

D3 ® F4

2, 5 and 6 with respect to the structure 3, taking into account the broadening of the emission features with temperature.

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Table 2 Assignment of the labelled transitions of short dotted line spectra Transition 5

7

5

5

Number

Assignment

Position (cmÿ1 )

10 20

T2g ® T2g T2g ® Eg , T1gb

18 362 18 131

30 40

T1g ® T2g T2ga ® T2g

20 673 20 603

50 60

T2g ® T1g , T1g ® A1g E1g ® T1g , T1g ® T1g

17 304, 17 294 17 203

70 80

T2g ® T1g , T2g T2g ® A2g

16 114 15 921

7

D4 ® F5

F6 ® 5 D4

D4 ® 7 F4

D4 ® 7 F3

In order to clarify the two low-temperature spectra, luminescence decay measurements have been carried out. The luminescence decays at 16 K excited at 20 718 cmÿ1 and detected at every peak of the continuous line spectrum of Fig. 5 are exponential; the time constant is 18 ms for the detection at structure 3 and 17 ms at the structures 2, 5 and 6. The luminescence decays excited at 20 603 cmÿ1 and detected at every peak of the short dotted line not present in the continuous line spectrum are also exponential with a time constant of 7 ms. The luminescence decay excited at 20 603

Fig. 5. Evolution with temperature of the time constant of the luminescence decays excited at 20 718 cmÿ1 and detected at 18 483 cmÿ1 (solid squares) and excited at 20 603 cmÿ1 and detected at 18 362 cmÿ1 (solid circles). A linear ®tting is also shown in both cases.

cmÿ1 and detected at structure 3 is exponential with a time constant of about 18 ms and exhibits a short risetime of the order of 1 ms. The same risetime is found for the luminescence decays excited at 20 603 cmÿ1 and detected at the structures 1, 2 and 4±9. At RT the luminescence decays excited and detected anywhere are exponential with a time constant of 13 ms. To understand the luminescence decay behaviour with temperature, some measurements have been performed ranging the temperature between 16 K and RT. All the luminescence decays are exponential. The evolution of the time constant for excitation at 20 718 cmÿ1 and detection at the luminescence structure 3 is plotted in Fig. 5 (solid squares); a linear ®tting of the experimental data is also shown. The same evolution is also plotted for the excitation at 20 603 cmÿ1 and detection at structure 10 (solid circles); a good linear ®tting is also shown for these data. 7 F6 ® 5 D4 transition: Fig. 6 shows the excitation spectra at RT (dotted lines) detected at 18 483 cmÿ1 (5 D4 ® 7 F5 transition). It is a wide band with three large bumps at about 21 100, 20 800 and 20 650 cmÿ1 . This spectrum does not depend on the detection energy. The excitation spectra at 16 K detected at 18 483 cmÿ1 (continuous line) and 18 362 cmÿ1 (short dotted line, expanded ´ 500) are also shown in Fig. 6. The continuous line spectrum presents a complicated structure between 20 600 and 21 000 cmÿ1 with a very intense narrow peak at 20 718 cmÿ1 (10) and another structure

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Fig. 6. Excitation spectra of the 7 F6 ®5 D4 transition measured at RT (left axis) and 16 K (right axis) and detected at 18 483 and 18 362 (expanded ´ 500) cmÿ1 . Labels are explained in the text and the assignments are shown in Tables 1 and 2.

Fig. 7. Emission spectra of the 5 D4 ®7 F6 transition measured at RT (left axis) and 16 K (right axis) and excited at 21 500 cmÿ1 . Labels are explained in the text and the assignments are shown in Tables 1 and 2.

between 21 000 and 21 300 cmÿ1 . The same specta is obtained for the detection energy at the di€erent peaks of the continuous line spectrum of Fig. 4. The short dotted line spectrum presents two main peaks at 20 673 (30 ) and 20 603 (40 ) cmÿ1 and two wide structures at about 20 900 and 21 400 cmÿ1 . The same spectrum is obtained for the detection energy in the peaks of the short dotted line not present in the continuous line spectrum of Fig. 4. Despite the intensity di€erence at low temperature between continuous and short dotted line spectra, the excitation spectrum measured at RT is the convolution of both, taking into account the broadening of the structures with temperature. 5 D4 ® 7 F6 , 7 F4 and 7 F3 transitions: Figs. 7±9 show, respectively, the 5 D4 ® 7 F6 , 7 F4 and 7 F3 emission spectra at RT (dotted lines) excited directly at the 5 D4 level. The ®rst one shows two main features at about 20 600 and 20 000 cmÿ1 ; the second one shows two structures at about 17 150 and 16 700 cmÿ1 and the third one presents a complicated feature peaked at about 16 025 cmÿ1 . These emission spectra do not depend on the excitation energy into the 5 D4 level. The 5 D4 ® 7 F6 emission spectrum at 16 K excited at 21 500 cmÿ1 and the 5 D4 ® 7 F4 and 5 D4 ® 7 F3 emission spectra at 16 K excited at 20 718 cmÿ1 are shown in continuous lines in Figs. 7±9, respectively, and the 5 D4 ® 7 F4 and 7 F3 emission spectra at 16 K excited at 20 603 cmÿ1 are also shown in

Fig. 8. Emission spectra of the 5 D4 ®7 F4 transition measured at RT (left axis) and 16 K (right axis) and excited at 20 718 and 20 603 (expanded ´ 10) cmÿ1 . Labels are explained in the text and the assignments are shown in Tables 1 and 2.

short dotted lines in Figs. 8 and 9, respectively. The continuous line spectrum of Fig. 7 presents a very intense group of three narrow peaks (11) located at 20 656, 20 638 and 20 586 cmÿ1 almost with the same intensity and a complicated wide structure. The continuous line spectrum of Fig. 8 presents a very intense group of three narrow peaks (12) located at 17 172, 17 161 and 17 151 cmÿ1 almost with the same intensity and a complicated less intense structure with some peaks. The short dotted line spectrum (expanded ´ 10) shows, almost with the same intensity, the same

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Fig. 9. Emission spectra of the 5 D4 ®7 F3 transition measured at RT (left axis) and 16 K (right axis) and excited at 20 718 and 20 603 (expanded ´ 5) cmÿ1 . Labels are explained in the text and the assignments are shown in Tables 1 and 2.

Fig. 10. Anti-Stokes emission of 5 D4 level measured at RT (left axis) and 16 K (right axis) and excited at 20 718 cmÿ1 . The ®nal level of the assigned transition from the 5 D3 manifold is indicated for each emission feature. Numeric labels are explained in the text and assigned in Table 1.

three peaks described above and some new ones at 17 304, 17 294 (50 ) and 17 203 (60 ) cmÿ1 . The continuous line spectrum of Fig. 9 presents a very intense group of three narrow peaks (15) located at 15 985, 15 975 and 15 961 cmÿ1 almost with the same intensity and some other less intense structures at about 16 022 (13) and 15 993 (14) cmÿ1 . The short dotted line spectrum (expanded ´ 5) shows, almost with the same intensity, the same features described above (with di€erent relative intensity between structures 13 and 15) and some new peaks at 16 169, 16 114 (70 ), 16 081 and 15 921 (80 ) cmÿ1 . The same continuous line spectrum shown in Fig. 7 is obtained for an excitation energy in the 5 D4 level higher than 21 500 cmÿ1 ; the same continuous line spectra shown in Figs. 8 and 9 are obtained for excitation energies in the 5 D4 level higher than 20 718 cmÿ1 . The emission spectra measured at RT are the convolution of both the continuous and short dotted line spectra in Figs. 8 and 9, taking into account the broadening of the structures with temperature.

which is known to be due to the 5 D3 ® 7 FJ transitions of Tb3‡ ions. Four main features are observed; they have been assigned to the transitions from the 5 D3 level to 7 F6 (26 480 and 25 860 cmÿ1 ), 7 F5 (24 240 and 23 860 cmÿ1 ), 7 F4 (22 950 and 22 370 cmÿ1 ) and 7 F3 (21 785 cmÿ1 ) manifolds. Fig. 10 also shows the anti-Stokes emission spectrum at 16 K (continuous line) excited at 20 718 cmÿ1 . It shows four main peaks which have been assigned to the transitions from the 5 D3 level to 7 F6 (26 256 (16) and 25 692 (17) cmÿ1 ), 7 F5 (23 853 cmÿ1 (18)) and 7 F4 (22 330 cmÿ1 (19)) manifolds. Ought to try it no signal has been detected by exciting the sample at 20 603 cmÿ1 . The excitation spectrum detected at 16 K and at every peak of the continuous line spectrum of Fig. 10 is shown in short dotted line in Fig. 11 (right axis) together with the excitation spectrum detected at structure 3 (continuous line, left axis). They are essentially the same spectra with some di€erences in the relative intensities of their structures. Luminescence decays at 16 K excited at 20 718 cmÿ1 and detected anywhere on the continuous line spectrum are exponential with a time constant of 8 ms. 5 D3 ® 7 F6 transition: Fig. 12 shows the emission spectrum at 16 K taken by exciting the sample at 27 700 cmÿ1 . It shows a doublet at 25 697 and 25 680 cmÿ1 (17) and two less intense peaks at 26 450 and 26 283 (16) cmÿ1 . The luminescence decays measured by exciting the sample at 27 700

3.3. Luminescence spectra of 5 D3 level Anti-Stokes spectrum of 5 D4 level: Fig. 10 shows the anti-Stokes emission spectrum at RT (dotted line) excited at 20 718 cmÿ1 (directly at the 5 D4 level). This spectrum overlaps very well with the high energy range of the spectrum shown in Fig. 3

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219

Fig. 11. Excitation spectra detected at 16 K in the Stokes emission (left axis, continuous line) and in the anti-Stokes emission (right axis, dotted line) of 5 D4 level.

propriate for the crystal-®eld levels of the free electron manifolds as a function of crystal-®eld in Oh symmetry has been reported in a recent paper [12]. The 5 D4 level is split into four crystal-®eld levels named from lower to higher energy A1g , T1g , Eg and T2g and the 5 D3 level is split into three levels named A2g , T1g , and T2g . According to the literature [8,9,12], at low temperature no wide structures have been observed in the 5 D4 ® 7 F5 , 7 F3 and 5 D3 ® 7 F6 , 7 F4 transitions and the narrow peaks detected are due to pure 0±0 magnetic-dipole lines between crystal-®eld levels; nevertheless the considerable wide structures observed in the 5 D4 M7 F6 and 5 D4 ® 7 F4 transitions are assigned to vibronically induced electric-dipole transitions. Furthermore the 5 D4 ® 7 F5 transition shows the greatest total emission intensity, in good agreement with some theoretical studies in chloride elpasolites [8±10]. Lifetimes of 5 D4 and 5 D3 levels (18 and 8 ms, respectively) have been found long enough to allow a thermalization process before emission. The assignments of transitions between crystal-®eld levels have been made following this e€ect and comparing with the literature data in some chloride [8±11] and ¯uoride [12,13] elpasolites. Table 1 shows the assignments of the di€erent features observed in the continuous line emission and excitation spectra measured at 16 K.

Fig. 12. Emission spectra of the 5 D3 ®7 F6 transition measured at 16 K and excited at 27 700 cmÿ1 . Labels are explained in the text and the assignments are shown in Table 1.

4.1. Luminescence of 5 D4 level

ÿ1

cm and detecting the ¯uorescence at 16 K at every peak of the 5 D3 emission were exponential with a decay time of 8 ms.

4. Discussion Continuous line emission and excitation spectra at 16 K can be explained as due to the Tb3‡ ions placed in a Oh symmetry with 6 Fÿ ions as ®rst neighbours. This octahedral site can be due to the Tb3‡ ions which occupy the place of In3‡ in the elpasolite matrix because both ions have a very similar ionic radius. An energy level diagram ap-

At very low temperatures (5 K) the only emitting crystal-®eld level is the lowest energy one, A1g (5 D4 ). In the 5 D4 ® 7 F5 transition the only transitions allowed by the magnetic-dipole operator, which transforms as T1g in Oh , are A1g (5 D4 ) ® T1ga (7 F5 ) and A1g (5 D4 ) ® T1gb (7 F5 ); nevertheless the second one presents a signal much weaker than the ®rst one due to the very small magnetic-dipole transition integral calculated for it [8]. Consequently at 16 K the most intense feature observed (3) is assigned to A1g (5 D4 ) ® T1ga (7 F5 ) transition. The following less intense structures with almost the same intensity (bottom of Fig. 4) are assigned to the magnetic-dipole allowed transitions from the upper level T1g (5 D4 ); nevertheless the T1g (5 D4 ) ® T1gb (7 F5 ) transition is

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much weaker than the other ones due to its very small magnetic-dipole transition integral [8]. Finally the least intense features (bottom of Fig. 4) are assigned to the allowed magnetic-dipole transitions from the third level, Eg (5 D4 ) to the crystal®eld levels of 7 F5 manifolds as shown in Table 1. The emissions from the T2g (5 D4 ) crystal-®eld level are not currently observed at 16 K because this level is weakly populated at 16 K due to the large energy gap between T2g and the rest of the crystal-®eld levels of 5 D4 . Thermalization e€ect is also present in chloride elpasolites [8±10], but at low temperature the intensities of the transitions from one emitting crystal-®eld level are not so di€erent from the intensities of the transitions from other emitting crystal-®eld levels like in our ¯uoride elpasolites; this can be due to the bigger size of the chloride ions which produce a less intense crystal-®eld. At RT 5 D4 manifold is thermalised and the emission spectra can be explained by assuming complete equilibration among the four crystal-®eld sublevels for the 5 D4 emitting state. The presence of three peaks in the A1g (5 D4 ) ® T1g (7 F5 ) transition shows a C2v distortion of the octahedral Tb3‡ site which removes all the symmetry imposed degeneracy of T1g (7 F5 ) level. Table 1 shows the assignments of these peaks to the magnetic-dipole allowed transitions from A1 (5 D4 ) level to A1 , B1 and B2 sublevels in which C2v distortion splits T1g (7 F5 ) crystal-®eld level. This C2v distortion accounts for some peaks observed in the bottom of Fig. 4 in the transitions from the T1g (5 D4 ) crystal-®eld level; these structures could be assigned taking into account a C2v distortion in the initial (T1g (5 D4 )) and ®nal levels (T1ga , T2g and Eg (7 F5 )) of the di€erent transitions but the weak emission signal does not allow us to make these assignments. Although this C2v symmetry is well known for Tb3‡ sites in LaF3 : Tb3‡ crystals [17], to our knowledge it is the ®rst time that such a distortion is observed in the octahedral Tb3‡ sites of a Tb3‡ -doped elpasolite and it could be due to the phase transitions from cubic to quadratic and to monoclinic structure demonstrated in Rb2 KInF6 between RT and 269 K [14]. The same arguments are used to make the assignments in the other continuous line emission and excitation spectra obtained at 16 K. The emission spectrum of the 5 D4 ® 7 F3 transition is

also only magnetic-dipole. The very intense group of three main peaks is assigned to A1g (5 D4 ) ® T1g (7 F3 ) transition. The three peaks are assigned to the transitions from A1 (5 D4 ) to A1 , B1 and B2 sublevels in which C2v distortion splits the T1g (7 F3 ) level. Following the literature [12], the T1g (5 D4 ) ® T1g (7 F3 ) and Eg (5 D4 ) ® T1g (7 F3 ) emissions (less intense than the emission from A1g (5 D4 ) due to the smaller populations at 16 K), should be in the middle of the three main peaks mentioned above and could be the assignments for the little peak observed at 15 968 cmÿ1 . T1g (5 D4 ) ® T1g , T2g (7 F3 ) and Eg (5 D4 ) ® T1g , T2g (7 F3 ) transitions have been assigned in the high energy side of the A1g (5 D4 ) ® T1g (7 F3 ) transition as shown in Table 1 [8,12]. As we have said before in the excitation spectrum corresponding to 7 F6 ® 5 D4 and in the emission spectra corresponding to 5 D4 ® 7 F6 and 7 F4 , transitions both magnetic-dipole lines and vibronically induced electric-dipole lines contribute signi®cantly to the excitation and emission intensities, respectively. In the excitation spectrum the most intense peak without structure is assigned to the A1g (7 F6 ) ® T1g (5 D4 ) magnetic-dipole transition. In the emission spectra corresponding to 5 D4 ® 7 F6 and 7 F4 transitions the intense group of three peaks (11 and 12) is assigned to the A1g (5 D4 ) ® T1g (7 F6 ) and T1g (7 F4 ) transition respectively; the three peaks correspond to the transitions from A1 (5 D4 ) to A1 , B1 and B2 sublevels of T1g (7 F6 ) and T1g (7 F4 ), respectively, under a C2v distortion. The other magnetic-dipole transitions from T1g and Eg levels of 5 D4 are masked by the more intense vibronically induced features that occur at similar energy. Nevertheless the literature data [12] and the presence of a narrow doublet at 16 502 cmÿ1 in Fig. 8 could indicate the position of T1g (5 D4 ) ® T2g (7 F4 ) transition. The RT spectra of all transitions are partially explained by the thermal equilibrium among the crystal-®eld sublevels and the growing of the vibronically induced structures of the magnetic and/ or electric-dipole transitions when temperature increases. Nevertheless, as we have said before, a complete explanation of RT measurements must have taken into account the short dotted line spectra.

M.A. Bu~ nuel et al. / Optical Materials 13 (1999) 211±223

In order to explain the short dotted line emission and excitation spectra at 16 K, two solutions can be invoked. In the short dotted line spectrum of 5 D4 ® 7 F5 , 7 F4 and 7 F3 transitions the positions of the features not present in the continuous line spectrum are close to those predicted for the emissions from T2g (5 D4 ) level (and in the 5 D4 ® 7 F4 , 7 F3 transitions also from Eg (5 D4 ) level) in Cs2 NaTbF6 elpasolite [12]. In this way the two short dotted line peaks of Fig. 6 are assigned to the excitation from T1g and T2ga (7 F6 ) to T2g (5 D4 ). Dotted line emission spectra are due to the excitation at 20 603 cmÿ1 directly into the T2g (5 D4 ) level before the emissions from T2g (5 D4 ) and the other crystal-®eld sublevels of 5 D4 occur. These assignments are indicated in Table 2. The energy gap between T2g (5 D4 ) and the other crystal®eld levels of 5 D4 could be large enough to explain a lifetime of 7 ms, less than a half of the lifetime of A1g (5 D4 ) (18 ms) and T1g (5 D4 ) (17 ms). The short risetime of the luminescence decays excited at 20 603 cmÿ1 and detected at the structures 1±9 with a time constant shorter than 7 ms, indicates that the emissions from Eg , T1g and A1g (5 D4 ) crystal-®eld levels in the short dashed line spectra are due to both a population of these via the T2g (5 D4 ) level and a direct excitation into these levels at 20 603 cmÿ1 through a vibronically allowed electric-dipole transition. At 16 K the intensity of the short dashed line excitation and emission spectra are much less than that of the continuous line spectra due to the small population of the ®rst excited crystal-®eld levels T1g and T2ga (7 F6 ) from which the excitation takes place to T2g (5 D4 ) level (A1g (7 F6 ) ® T2g (5 D4 ) are magneticdipole forbidden); but at RT the emission and excitation spectra are the convolution of both continuous and shout dashed line spectra due to the thermal equilibrium reached among the crystal-®eld sublevels of the di€erent manifolds and the growing of vibrational structures of magnetic and/or electric-dipole transitions with temperature. In this way Fig. 5 can be explained by means of a thermalization process. Nevertheless there are some peaks of the short dotted line spectra (not labelled in ®gures) which are not explained in the previously described way; moreover the assignment of the 30 and 40 excitation

221

peaks is not in good agreement with the literature, T1g (7 F6 ) ® Eg , T1g (5 D4 ) and T2ga (7 F6 ) ® Eg , T1g (5 D4 ) transitions appear as more convenient assignments to be in good agreement with the spectroscopic data in Cs2 NaTbF6 elpasolite [12]. Finally the short lifetime of 7 ms of the luminescence decays excited at 20 603 cmÿ1 and detected at the emission features associated with the short dotted line spectra is hardly explained by the previous solution. The other explanation for the short dotted line spectra is the presence of a very small concentration of a Tb3‡ site with a symmetry very di€erent from Oh . The lifetime of this site would be 7 ms. The low symmetry accounts for a lifetime shorter than the lifetime of Oh site. The short risetime of the luminescence decays excited at 20 603 cmÿ1 and detected at the structures 1±9 indicated that the emissions from the Tb3‡ Oh site in the short dashed line spectra are due to both an energy transfer between the two sites and a direct excitation into the crystal-®eld levels of the octahedral site at 20 603 cmÿ1 through a vibronically induced structure of electric or magnetic-dipole transitions. At 16 K both e€ects are weak and the intensity of the excitation and emission spectra of the non-octahedral site is much weaker than that of the spectra of the octahedral site. But when temperature increases both e€ects increase and an e€ective equilibrium is achieved between the two sites at RT, that is also the explanation for the lifetime behaviour shown in Fig. 5. 4.2. Luminescence of 5 D3 level In order to do the assignments of the continuous line anti-Stokes spectra of Figs. 10 and 12, the same arguments use for the assignments of the luminescence features of 5 D4 level has been taken. These assignments have been presented in Table 1. Following the literature [6,7,12] 5 D3 ® 7 F5 transition should have an important electric-dipole contribution, nevertheless, no wide vibrational structure has been observed in the corresponding spectral range and at 16 K the most important features are the transitions, allowed by the magnetic-dipole operator, from A2g (5 D3 ) crystal-®eld level to T2ga and T2gb (7 F6 ), T2g (7 F5 ) and T2g (7 F4 ). The C2v distortion mentioned above accounts for

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the two peaks observed in the A2g (5 D3 ) ® T2gb (7 F6 ) transition in Fig. 12; the third peak is not observed due to the low resolution in this spectral range. On the other hand, three explanations would be possible to account for the antiStokes emission. The ®rst one is a process of twophoton absorption before emission occurs; but this is not a realistic explanation because this process is normally very less ecient and an excitation spectrum accounting for the selection rules for such a process clearly di€erent from the excitation spectrum of the 5 D4 level would be expected. The second one is an upconversion process through an energy transfer among the excited ions in the 5 D4 level; although the expected excitation spectrum would be as shown in Fig. 11, the luminescence decay detected anywhere on the anti-Stokes emission spectrum would show a risetime with a time constant of the order of one half of the 5 D4 lifetime; but in our case such a luminescence decay is perfectly exponential and no risetime is shown. The third explanation indicates that an up-conversion process through a sequential two-photon absorption (excited state absorption) takes place before emission occurs; this explanation is the most correct watching the excitation spectrum shown in Fig. 12 and the luminescence decay of the anti-Stokes emission where no risetime has been found. The excitation spectrum shown in Fig. 11 indicates clearly the ®rst absorption 7 F6 ® 5 D4 but no excitation feature can be related with the excited state absorption; this indicates that the second photon absorption is a non-resonant process exciting ions from the 5 D4 level to a spectral position at about 41 430 cmÿ1 between the A1g (7 F6 )(4f8 ) ® t2g (4f7 5d) and A1g (7 F6 )(4f8 ) ® eg (4f7 5d) (see bands B and A, respectively, in Fig. 1) intercon®gurational transitions; this process is followed by non-radiative decays to 5 D3 level before emission occurs. 5. Conclusions This work has led to the synthesis, analysis of Tb3‡ sites and study of some optical properties in crystalline Rb2 KInF6 :Tb3‡ . An Oh site of Tb3‡ ions has been found (this site could indicate that

Tb3‡ ions substitute In3‡ ions in the elpasolite matrix) and the identi®cation and assignment of the pure magnetic-dipole transitions split out of the Tb3‡ 5 D4 ® 7 F6 , 7 F5 , 7 F4 , 7 F3 and 5 D3 ® 7 F6 , 7 F5 , 7 F4 , transitions have been achieved assuming a thermal equilibrated set of crystal-®eld sublevels for the states and comparing with the literature data in chloride and ¯uoride elpasolites. One of the most remarkable features is the detection of a C2v distortion of the octahedral site at low temperature which could be due to the well-known transition phase to monoclinic structure of this material. Another feature of the emission spectra obtained is the absence of any vibrational structure in DJ ˆ 1;  3 transitions (5 D4 ® 7 F5 , 7 F3 and 5 D3 ® 7 F6 , 7 F4 ) and the presence of extensive vibrational structure in the DJ ˆ 0;  2 transitions (5 D4 ® 7 F6 , 7 F4 and 7 F6 ® 5 D4 ), unless the 5 D3 ® 7 F5 transition. Some experimental results could indicate the presence of a small concentration of other Tb3‡ sites with a symmetry lower than Oh . The behaviour with temperature of the emission and excitation spectra and luminescence decay measurements indicates also an energy transfer between the Oh site and the lower symmetry one that increases when the temperature does. Finally up-conversion process by means of sequential two photon absorption has been demonstrated. Acknowledgements Support for this work has been received from C.N.R.S. M.A. Bu~ nuel acknowledges post-doctoral grants from the Spanish Government and the `Human Capital and Mobility' program of the EC (contract n ERBFMBICT983309), as well as support of the I.C.M.A. (Universidad de ZaragozaC.S.I.C., Spain). One of us, J.P. Chaminade, acknowledges INTAS for a grant N 97-10177 and the Region Aquitaine. References [1] J.P. Chaminade, A. Garcõa, T. Gaewdang, M. Pouchard, J. Grannec, B. Jacquier, Rad. E€. Def. Sol. 135 (1995) 137.

M.A. Bu~ nuel et al. / Optical Materials 13 (1999) 211±223 [2] K.S. Aleksandrov, S.V. Misyul', Soviet Phys. Cryst. 26 (1981) 612. [3] D. Babel, A. Tressaud, Crystal Chemistry of Fluorides, in: P. Hagenmuller (Ed.), Inorganic Solid Fluorides, Academic Press, New York, 1985, p. 77. [4] F.S. Richardson, M.F. Reid, J.J. Dallara, R.D. Smith, J. Chem. Phys. 83 (1985) 3813. [5] P.A. Tanner, J. Chem. Phys. 85 (1986) 2344. [6] P. Selgert, C. Lingner, B. L uthi, Z. Phys. B 55 (1985) 219. [7] W. B uhrer, H.U. G udel, J. Phys. C 20 (1987) 3809. [8] R.W. Schwartz, H.G. Brittain, J.P. Riehl, W. Yeakel, F.S. Richardson, Mol. Phys. 34 (1977) 361. [9] T.R. Faulkner, F.S. Richardson, Mol. Phys. 36 (1978) 193. [10] L.C. Thompson, O.A. Serra, J.P. Riehl, F.S. Richardson, R.W. Schwartz, Chem. Phys. 26 (1977) 393.

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[11] A.J. Berry, C.S. McCaw, I.D. Morrison, R.G. Denning, J. Lumin 66/67 (1996) 272. [12] A.J. Berry, I.D. Morrison, R.G. Denning, Mol. Phys. 93 (1998) 1. [13] H.D. Amberger, Z. Anorg, Allg. Chem. 467 (1980) 231. [14] I.N. Flerov, M.V. Gorev, S.V. Mel'Nikova, S.V. Misyul, N.N. Voronov, K.S. Aleksandrov, A. Tressaud, J. Grannec, J.P. Chaminade, L. Rabardel, H. Guengard, Sov. Phys. Solid State 34 (1992) 1870. [15] W.T. Carnall, P.R. Fields, B.G. Wybourne, J. Chem. Phys. 42 (1965) 3797. [16] R.C. Ropp, B. Carroll, J. Phys. Chem. 81 (1977) 746. [17] W.T. Carnall, G.L. Goodman, K. Rajnak, R.S. Rana, J. Chem. Phys. 90 (1989) 3443.