- Email: [email protected]
Optical, infrared and EPR spectroscopy of CaF2:Ce3+ crystals co-doped with Li+ or Na+
ARTICLE IN PRESS
Journal of Luminescence 108 (2004) 307–311
Optical, infrared and EPR spectroscopy of CaF2:Ce3+ crystals co-doped with Li+ or Na+ M. Yamagaa,*, S. Yabashia, Y. Masuia, M. Hondab, H. Takahashic, M. Sakaic, N. Sarukurac, J.-P.R. Wellsd, G.D. Jonese a
Department of Mathematical and Design Engineering, Gifu University, Gifu 501-1193, Japan b Faculty of Science, Naruto University of Education, Naruto 772-8502, Japan c Institute for Molecular Science, Okazaki 444-8585, Japan d Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK e Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8020, New Zealand
Abstract Interconfigurational 4f25d VUV absorption and luminescence, intra-4f1 IR absorption and X-band EPR measurements have been carried out on CaF2:Ce3+ crystals co-doped with Na+ and Li+ ions. For both Li+ and Na+ co-doping, cubic, new tetragonal and rhombic-symmetry centres are observed. Cubic centres, which are readily observable by their infrared transitions, could not be identified and remain elusive in the EPR spectra. r 2004 Elsevier B.V. All rights reserved. PACS: 76.30.Kg; 78.30.j; 78.40.q; 78.55.Hx Keywords: VUV spectroscopy; Ce3+; Fluorite crystals; Crystal-field analyses
1. Introduction Cerium-doped materials have been the topic of many investigations for their application as tunable-gain media in the ultraviolet (UV) and near UV [1]. Successful laser operation based on the 5d-4f transitions of Ce3+ has been reported for LiYF4 (LYF) [2], LiLuF4 (LLF) [3,4] and LiCaAlF6 (LiCAF) [4,5], but has not been achieved for Ce3+-doped alkaline-earth fluorides despite their favourably wide energy gaps. In this paper, we report on the Ce3+ centre distribution of CaF2:Ce3+ crystals co-doped with Li+ or Na+ *Corresponding author. Tel./fax: +81-58-293-3052. E-mail address: [email protected] (M. Yamaga).
using electron paramagnetic resonance (EPR), infrared and optical absorption and luminescence.
2. Experimental procedure CaF2 crystals doped with 0.09%Ce3+ or with 0.01–0.04%Ce3+ together with 1%Li+, 0.3%Na+ or 1%Na+ were grown in graphite crucibles under an argon atmosphere by the Bridgman–Stockbarger method in an RF-induction furnace. As LiF and NaF melt around 400 C lower than CaF2, overdoping with LiF or NaF together with growth under argon is needed to wholly remove existing F sites.
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.065
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M. Yamaga et al. / Journal of Luminescence 108 (2004) 307–311
EPR measurements were made over the temperature range of 5–300 K using a Bruker EMX10/ 12 X-band spectrometer with microwave frequencies of 9.686–9.694 GHz, microwave powers of 0.001–1 mW and 100 kHz field modulation. The angular variations of the EPR spectra were measured by rotating samples in the cavity. 10 K infrared-absorption spectra were measured with a BioRad FTS-40 Fourier Transform infrared spectrometer equipped with a closed-cycle helium cryostat at the University of Canterbury. 15 K VUV absorption, VUV excitation and UV luminescence spectra were measured with a 1-m Seya-Namioka spectrometer at the UVSOR facility of the Institute for Molecular Science. Emission lifetimes were measured using the Horiba NAES700F time-resolved photoluminescence spectrometer at the Instrumental Analysis Centre of Gifu University.
3. Results and discussion 3.1. EPR and FTIR spectra EPR is a powerful tool for revealing local structure of charge-compensated Ce3+ complexes in CaF2 crystals through the angular dependence of their g values in the (0 1 0) and (1 1 0) planes. For the parent CaF2:0.09%Ce3+ crystal, the measured g values of g||=3.026 and g>=1.386 are those of the well-established C4v(F) centre [6]. Annealing this CaF2:Ce3+ sample in an O2 atmosphere at 900 C, yields a trigonal Ce3+ centre [C3v(O2)], with charge compensation by O2 along the [1 1 1] direction, whose observed g values are g||=3.67 and g>=0.31 [7]. Co-doping of CaF2:0.01 to 0.04%Ce3+ crystals with sufficient LiF or NaF results in complete removal of the EPR transitions of the C4v(F) centre, with a corresponding appearance of EPR transitions of a modified tetragonal (C4v) and a rhombic (C2v) symmetry centre induced by the presence of Li+ or Na+ co-dopant ions [8,9]. Table 1 summarises the g values of these Li+/Na+ co-doped Ce3+ centres obtained from EPR measurements on two representative crystals. The new C4v symmetry centres have the same g values
Table 1 The spin-Hamiltonian parameters of Li+/Na+ co-doped Ce3+ centres in CaF2 CaF2 crystals codopant
Tetragonal C4v(Li+/Na+)
Orthorhombic C2v(Li+/Na+)
Ce(0.01%) Li(1%)
g||=0.730 g>=2.403
gx=0.15 gy=1.320 gz=2.650 g* =1.373
g* =1.845 Ce(0.04%) Na(0.3%)
g||=0.729 g>=2.398 g* =1.842
gx=0.345 gy=1.291 gz=2.460 g* =1.365
of g||=0.73 and g>=2.4 irrespective of whether Na+ or Li+ co-doping is used. These C4v(Li+/ Na+) centres are assigned as having on-axis Na+ or Li+ ions (the C4v(2 0 0) configuration of Pack et al. [8]) with these next-nearest neighbour cations producing C4v-symmetry centres not far removed from cubic symmetry. The C2v(Li+/Na+) codopant centres are well established [8,9] and have their monovalent alkali ions located in the nearest cation site along the [1 1 0] direction from the Ce3+ ion. The principal x-, y- and z-axis of the C2v centres are defined as the [1 1 0], ½1% 1 0 and [0 0 1] axes, respectively. Their g values are included in Table 1. Fig. 1 shows the 10-K infrared-absorption spectra of Li+ and Na+-doped CaF2:Ce3+. The infrared spectra show cubic-centres, whose lines are identical for Li+ and Na+ doping, and the C2v(Li+/Na+) rhombic centres identified by EPR whose lines differ between Li+ and Na+ doping. Their transitions to crystal-field levels of the 2F7/2 multiplet are listed in Table 2. Crystal-field analyses for 4f1 configuration levels of the cubic Ce3+ centre were performed and include the spin–orbit interaction. A [1 1 0] basis cubic-field Hamiltonian was used of the form [10] "
# rffiffiffiffiffi pffiffiffiffiffi ð4Þ 3 35 ð4Þ ð4Þ ð4Þ 10ðC2 þ C2 Þ ðC4 þ C4 Þ 7 2 " pffiffiffiffiffiffiffiffi 105 ð6Þ 5 ð6Þ ð6Þ 6 ðC2 þ C2 þ Bc C0 þ Þ 13 25
B4c
C0ð4Þ
ARTICLE IN PRESS M. Yamaga et al. / Journal of Luminescence 108 (2004) 307–311 +
0.8
Li 3+
CaF2:Ce :Li
+
Absorbance (arb. units)
0.7
Oh
0.6 0.5 2140 0.65
2160 3+
CaF2:Ce :Na
+
Na
2180
2200
+
Oh
0.60 0.55 2140
2160 2180 Wavenumbers (cm-1)
2200
Fig. 1. 10-K infrared absorption spectra of CaF2 :0.01%Ce3+:1%Li+ and CaF2 :0.015%Ce3+:1%Na+ codoped crystals.
Table 2 The cubic-centre experimental and calculated energy levels (in cm1) of the 2F5/2 and 2F7/2 multiplets of Ce3+ in CaF2:Ce3+. The spin–orbit parameter z is fixed at 620 cm1, while the exactfit [1 1 0]-basis crystal-field parameters are B4c [1 1 0]=817 cm1 and B6c [1 1 0]=2030 cm1. From B4c [1 1 0]=14B4c [1 0 0] and B6c [1 1 0]=14B6c [1 0 0] [10], these correspond to the more familiar [1 0 0]-basis crystal-field parameter values of B4c [1 0 0]=3268 cm1 and B6c [1 0 0]=1249 cm1 State 2
F5/2
2
F7/2
Z1G8 Z2G7 Y1G8 Y2G6 Y3G7
Calc.
Expt.
0.0 519.8 2163.4 2178.3 3832.9
0.0 — 2163.4 2178.3 3832.9
# pffiffiffiffiffiffiffiffi rffiffiffiffi 231 ð6Þ 7 ð6Þ ð6Þ ð6Þ ðC6 þ C6 Þ : ðC þ C4 Þ þ 2 4 26 Fitting to these two crystal-field parameters only and holding all free-ion electrostatic and spin– orbit parameters to the values of [11] yields [1 1 0]-basis crystal-field parameters of B4c [1 1 0]= 817 cm1 and B6c [1 1 0] =2030 cm1, which give exact agreement between the measured and calculated energy levels (Table 2). However, a
309
slight degree of distortion from exact cubic symmetry might shift the ground-state g values from those expected for a pure cubic G8 ðJ ¼ 52Þ quartet wavefunction [12] sufficiently to explain the absence of any EPR signals from these cubicsymmetry centres. Fits to the energy levels of the C2v(Li+/Na+) centres could be attempted using this [1 1 0]-basis cubic crystal-field Hamiltonian together with the introduction of a single B20 C0ð2Þ term to account for the presence of the nearby Li+ or Na+. However, only three infrared transitions are observed for the C2v(Li+) centre at 2159.2, 2195.9 and 3812.2 cm1 and just two unambiguous transitions for the C2v(Na+) centre at 2165.4 and 3821.6 cm1, with a third transition underlying the cubic line at 2178.3 cm1. As these are the same number of transitions as observed for the cubic centre itself, inclusion of even a single B20 C0ð2Þ term overparameterises any unconstrained crystal-field fit. In the absence of a definitive crystal-field fit, it is just pointed out that calculated g-values of (gx,gy,gz)=(0.35,1.37,2.46) are given by the following Kramers-doublet wavefunction 0:70 Jz ¼ 752 þ 0:70 Jz ¼ 712 0:15 Jz ¼ 732 ; close to the g values of (0.34, 1.29, 2.46) measured for the C2v(Na+) centres (Table 1). This wavefunction is similar to one of the Kramers-doublet wavefunctions (with coefficients 0.52, 0.76, 0.39) of the cubic G8 quartet ground state obtained from the cubic-centre energy-level fit. A small negative B20 C0ð2Þ term would put this particular Kramersdoublet wavefunction of the cubic G8 quartet ground-state lowest in energy, with a small admixture with the other Kramers-doublet wavefunction of this G8 quartet needed to reach the observed g values. For fits to the g values of the EPR C4v(Li+/ Na+) centre, a [1 0 0]-basis cubic crystal-field Hamiltonian [10] was adopted, together with a small B20 C0ð2Þ term needed to account for the presence of a next-nearest neighbour Li+ or Na+ ion located along the [1 0 0] direction. With this Hamiltonian, the g values g||=0.85 and g>=2.57 of the cubic G8g6 wavefunction ðJz ¼ 712Þ are close to the measured C4v(Li/Na) g values of g||=0.73 and g>=2.40. The adoption of a small
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negative B20 C0ð2Þ term value places this G8g6 wavefunction of the cubic G8 ground-state quartet lowest in energy and suggests that the C4v(Li+/ Na+) centre has a configuration not far removed from cubic symmetry, as its g values are not far from the cubic G8 quartet values discussed in Ref. [12]. For comparison, the observed g values of g||=3.026 and g>=1.386 for the C4v(F) centre are close to those (g||=3.01 and g>=1.43) for the g7 wavefunction of the cubic G8 ground-state quartet for which a positive B20 C0ð2Þ term makes this g7 wavefunction lowest in energy. 3.2. VUV absorption and UV luminescence Fig. 2 shows the vacuum ultraviolet (VUV) absorption spectra of Ce3+ for several CaF2 crystals. Two groups of broad bands, with peaks at 243 and 305 nm, and 194, 203 and 216 nm, are observed for the as-grown CaF2:0.09%Ce3+ crystal and these correspond to the transitions from the 2F5/2 ground state of the 4f1 configuration to two 2E and three 2T2 excited states, respectively, of the 5d1 configuration of Ce3+.
40 0.035%Ce : 0.29%Na 0.02%Ce : 1.1%Na 0.012%Ce : 0.97%Li 0.09%Ce (O2 anneal) 0.09%Ce (as-grown)
-1
30
0.012%Ce:0.97%Li : CaF2 6 Excitation spectra
20
A↓ ↓ ↓
↓↓ ↓
4 B↓ ↓
10 2
15 K
↓
8 A ↓ λem(nm) 320
Emission spectra ↓ B ↓ 15 K A ↓ ↓
250 340
B ↓
↓
400
Wavelength (nm) Fig. 2. 15 K VUV absorption spectra of Ce3+ in various CaF2 crystals.
0 100
215 190 B
2
↓ 380
150
↓C ↓
300
↓A ↓
4
B ↓ 360
↓ ↓
200
330 300
6
↓
↓
0 100
λ ex(nm)
↓ ↓
Absorption coefficient (cm )
15 K
All these transitions have been assigned as being C4v(F) centre transitions [13,14]. After annealing this Ce3+:CaF2 sample in an oxygen atmosphere, there appear several new broad bands at 143, 179, 216, 272 and 329 nm ascribed to C3v(O2) centres. The VUV absorption spectrum for the CaF2:0.01%Ce3+:1%Li+ sample (Fig. 2) comprises two groups labeled by A and B (see Fig. 3). The band peaks of the A group are at 179, 186, 193 and 303 nm, while those of the group B are at 135, 150, 215 and 330 nm, close to those of the oxygenated sample. For the two Na+ co-doped samples, it is found that increasing the Na+ concentration leads to a reduction of the line intensities for the B group together with a decrease of the EPR signal of the C4v(Li+/Na+) centre. Hence the group A lines are assigned to C2v(Li+/ Na+) centres. Fig. 3 shows 15 K excitation and luminescence spectra of the CaF2:0.01%Ce3+:1%Li+ sample. Excitation at 100 nm produces broad luminescence with a peak at 280 nm, labeled C. The excitation spectrum of this luminescence has peaks at 106, 116 and 124 nm, which are close to the band-gap energy of CaF2 [13] and supports its assignment to a self-trapped exciton. There are two luminescence bands A and B corresponding to the A and B features of the absorption spectra (Fig. 2). The excitation spectra of the A luminescence peaks at 315 and 335 nm are composed of 250 and 300 nm and 180, 188 and 192 nm sub-bands. Likewise, the
200
↓ 400
300
Wavelength (nm)
400
0 200
120 100
300
400
500
Wavelength (nm)
Fig. 3. 15 K VUV excitation and UV luminescence spectra of Ce3+ in CaF2:0.01%Ce3+:1%Li+.
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excitation spectra for the B luminescence peaks at 353 and 378 nm are composed of sub-bands at 330 nm and at 142, 155 and 218 nm. These excitation peaks match those of the absorption bands in Fig. 2. Both the excitation and emission spectra of the CaF2:0.04%Ce3+:0.3%Na+ sample are almost identical to that of the CaF2:0.01%Li+:1%Ce3+ sample, while the B luminescence for the more strongly NaF-doped CaF2:0.015%Ce3+:1%Na+ sample is negligibly small. Taking account of the EPR and FTIR results, the stronger A excitation and luminescence bands observed for the three Li+/Na+ codoped Ce3+:CaF2 samples are consistently assigned to C2v(Li+/Na+) centres. For these centres, the three close-lying transitions at 179, 186 and 193 nm are assigned to the 2T upper state and two at 250 and 303 nm to the 2E lower state of the 5d1 configuration. It is more difficult to assign conclusively the B luminescence and excitation bands. Because their line profiles are so similar to those of C3v(O2) centres, it is likely that the B luminescence and excitation is from trace amounts of C3v(O2) centres in the co-doped crystals. Room-temperature fluorescence-decay times for the A and B emissions are purely radiative, being measured as 39 and 46 ns, respectively.
4. Conclusions The EPR and FTIR spectra of Li+ or Na+ codoped Ce3+:CaF2 samples show that Li+ or Na+ co-doping removes lines of C4v(F) centres and creates cubic, C4v(Li+/Na+) and C2v(Li+/Na+) centres. UV excitation bands at 179, 186, 193 and 303 nm, which give rise to luminescence bands at 315 and 335 nm, have been assigned to optical transitions of the C2v(Li+/Na+) centres, which
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have charge compensating Li+/Na+ ions located in the [1 1 0] direction from the Ce3+ ions.
Acknowledgements This work was in part supported by a Grant-inAid for Science Research (C) from Japan Society for the Promotion of Science (No. 14550037). One of the authors (M. Yamaga) is indebted to the Iwatani Naoji Foundation for a Research Grant.
References [1] P. Moulton, in: M. Bass, M.H. Stitch (Eds.), Laser Handbook, Vol. 5, North-Holland, Amsterdam, 1985, p. 282. [2] T.D.J. Ehrlich, P.F. Moulton, R.M. Osgood, Opt. Lett. 4 (1978) 184. [3] I.M. Raieri, K. Shimamura, K. Nakano, T. Fujita, Z. Liu, N. Sarukura, T. Fukuda, J. Cryst. Growth 217 (2000) 151. [4] N. Sarukura, M.A. Dubinskii, Z. Liu, V.V. Semashko, A.N. Naumov, S.L. Korableva, R.Y. Abdulsabirov, K. Edamatsu, Y. Suzuki, T. Itoh, Y. Segawa, IEEE J. Selected Topics Quantum Electron. 1 (1995) 792. [5] C.D. Marshall, J.A. Spaeth, S.A. Payne, W.F. Krupke, G.J. Quarles, V. Castillo, B.H.T. Chai, J. Opt. Soc. Am. B 11 (1994) 2054. [6] A. Kiel, W.B. Mims, Phys. Rev. B 6 (1972) 34. [7] S.D. McLaughlan, D.A. Forrester, Phys. Rev. 151 (1966) 311. [8] D.W. Pack, W.J. Manthey, D.S. McClure, Phys. Rev. B 40 (1989) 9930. [9] G.D. Jones, R.J. Reeves, J. Lumin. 87–89 (2000) 1108. [10] M.T. Hutchings, in: F. Seitz, D. Turnbull (Eds.), Solid State Physics, Vol. 16, Academic Press, New York, London, 1964, p. 227. [11] C.A. Freeth, G.D. Jones, J. Phys. C: Sol. Stat. Phys. 15 (1982) 6833. [12] A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Dover, New York, 1986, p. 145. [13] E. Loh, Phys. Rev. 154 (1967) 270. [14] W.J. Manthey, Phys. Rev. B 8 (1973) 4086.
Journal of Luminescence 108 (2004) 307–311
Optical, infrared and EPR spectroscopy of CaF2:Ce3+ crystals co-doped with Li+ or Na+ M. Yamagaa,*, S. Yabashia, Y. Masuia, M. Hondab, H. Takahashic, M. Sakaic, N. Sarukurac, J.-P.R. Wellsd, G.D. Jonese a
Department of Mathematical and Design Engineering, Gifu University, Gifu 501-1193, Japan b Faculty of Science, Naruto University of Education, Naruto 772-8502, Japan c Institute for Molecular Science, Okazaki 444-8585, Japan d Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK e Department of Physics and Astronomy, University of Canterbury, PB 4800, Christchurch 8020, New Zealand
Abstract Interconfigurational 4f25d VUV absorption and luminescence, intra-4f1 IR absorption and X-band EPR measurements have been carried out on CaF2:Ce3+ crystals co-doped with Na+ and Li+ ions. For both Li+ and Na+ co-doping, cubic, new tetragonal and rhombic-symmetry centres are observed. Cubic centres, which are readily observable by their infrared transitions, could not be identified and remain elusive in the EPR spectra. r 2004 Elsevier B.V. All rights reserved. PACS: 76.30.Kg; 78.30.j; 78.40.q; 78.55.Hx Keywords: VUV spectroscopy; Ce3+; Fluorite crystals; Crystal-field analyses
1. Introduction Cerium-doped materials have been the topic of many investigations for their application as tunable-gain media in the ultraviolet (UV) and near UV [1]. Successful laser operation based on the 5d-4f transitions of Ce3+ has been reported for LiYF4 (LYF) [2], LiLuF4 (LLF) [3,4] and LiCaAlF6 (LiCAF) [4,5], but has not been achieved for Ce3+-doped alkaline-earth fluorides despite their favourably wide energy gaps. In this paper, we report on the Ce3+ centre distribution of CaF2:Ce3+ crystals co-doped with Li+ or Na+ *Corresponding author. Tel./fax: +81-58-293-3052. E-mail address: [email protected] (M. Yamaga).
using electron paramagnetic resonance (EPR), infrared and optical absorption and luminescence.
2. Experimental procedure CaF2 crystals doped with 0.09%Ce3+ or with 0.01–0.04%Ce3+ together with 1%Li+, 0.3%Na+ or 1%Na+ were grown in graphite crucibles under an argon atmosphere by the Bridgman–Stockbarger method in an RF-induction furnace. As LiF and NaF melt around 400 C lower than CaF2, overdoping with LiF or NaF together with growth under argon is needed to wholly remove existing F sites.
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.065
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EPR measurements were made over the temperature range of 5–300 K using a Bruker EMX10/ 12 X-band spectrometer with microwave frequencies of 9.686–9.694 GHz, microwave powers of 0.001–1 mW and 100 kHz field modulation. The angular variations of the EPR spectra were measured by rotating samples in the cavity. 10 K infrared-absorption spectra were measured with a BioRad FTS-40 Fourier Transform infrared spectrometer equipped with a closed-cycle helium cryostat at the University of Canterbury. 15 K VUV absorption, VUV excitation and UV luminescence spectra were measured with a 1-m Seya-Namioka spectrometer at the UVSOR facility of the Institute for Molecular Science. Emission lifetimes were measured using the Horiba NAES700F time-resolved photoluminescence spectrometer at the Instrumental Analysis Centre of Gifu University.
3. Results and discussion 3.1. EPR and FTIR spectra EPR is a powerful tool for revealing local structure of charge-compensated Ce3+ complexes in CaF2 crystals through the angular dependence of their g values in the (0 1 0) and (1 1 0) planes. For the parent CaF2:0.09%Ce3+ crystal, the measured g values of g||=3.026 and g>=1.386 are those of the well-established C4v(F) centre [6]. Annealing this CaF2:Ce3+ sample in an O2 atmosphere at 900 C, yields a trigonal Ce3+ centre [C3v(O2)], with charge compensation by O2 along the [1 1 1] direction, whose observed g values are g||=3.67 and g>=0.31 [7]. Co-doping of CaF2:0.01 to 0.04%Ce3+ crystals with sufficient LiF or NaF results in complete removal of the EPR transitions of the C4v(F) centre, with a corresponding appearance of EPR transitions of a modified tetragonal (C4v) and a rhombic (C2v) symmetry centre induced by the presence of Li+ or Na+ co-dopant ions [8,9]. Table 1 summarises the g values of these Li+/Na+ co-doped Ce3+ centres obtained from EPR measurements on two representative crystals. The new C4v symmetry centres have the same g values
Table 1 The spin-Hamiltonian parameters of Li+/Na+ co-doped Ce3+ centres in CaF2 CaF2 crystals codopant
Tetragonal C4v(Li+/Na+)
Orthorhombic C2v(Li+/Na+)
Ce(0.01%) Li(1%)
g||=0.730 g>=2.403
gx=0.15 gy=1.320 gz=2.650 g* =1.373
g* =1.845 Ce(0.04%) Na(0.3%)
g||=0.729 g>=2.398 g* =1.842
gx=0.345 gy=1.291 gz=2.460 g* =1.365
of g||=0.73 and g>=2.4 irrespective of whether Na+ or Li+ co-doping is used. These C4v(Li+/ Na+) centres are assigned as having on-axis Na+ or Li+ ions (the C4v(2 0 0) configuration of Pack et al. [8]) with these next-nearest neighbour cations producing C4v-symmetry centres not far removed from cubic symmetry. The C2v(Li+/Na+) codopant centres are well established [8,9] and have their monovalent alkali ions located in the nearest cation site along the [1 1 0] direction from the Ce3+ ion. The principal x-, y- and z-axis of the C2v centres are defined as the [1 1 0], ½1% 1 0 and [0 0 1] axes, respectively. Their g values are included in Table 1. Fig. 1 shows the 10-K infrared-absorption spectra of Li+ and Na+-doped CaF2:Ce3+. The infrared spectra show cubic-centres, whose lines are identical for Li+ and Na+ doping, and the C2v(Li+/Na+) rhombic centres identified by EPR whose lines differ between Li+ and Na+ doping. Their transitions to crystal-field levels of the 2F7/2 multiplet are listed in Table 2. Crystal-field analyses for 4f1 configuration levels of the cubic Ce3+ centre were performed and include the spin–orbit interaction. A [1 1 0] basis cubic-field Hamiltonian was used of the form [10] "
# rffiffiffiffiffi pffiffiffiffiffi ð4Þ 3 35 ð4Þ ð4Þ ð4Þ 10ðC2 þ C2 Þ ðC4 þ C4 Þ 7 2 " pffiffiffiffiffiffiffiffi 105 ð6Þ 5 ð6Þ ð6Þ 6 ðC2 þ C2 þ Bc C0 þ Þ 13 25
B4c
C0ð4Þ
ARTICLE IN PRESS M. Yamaga et al. / Journal of Luminescence 108 (2004) 307–311 +
0.8
Li 3+
CaF2:Ce :Li
+
Absorbance (arb. units)
0.7
Oh
0.6 0.5 2140 0.65
2160 3+
CaF2:Ce :Na
+
Na
2180
2200
+
Oh
0.60 0.55 2140
2160 2180 Wavenumbers (cm-1)
2200
Fig. 1. 10-K infrared absorption spectra of CaF2 :0.01%Ce3+:1%Li+ and CaF2 :0.015%Ce3+:1%Na+ codoped crystals.
Table 2 The cubic-centre experimental and calculated energy levels (in cm1) of the 2F5/2 and 2F7/2 multiplets of Ce3+ in CaF2:Ce3+. The spin–orbit parameter z is fixed at 620 cm1, while the exactfit [1 1 0]-basis crystal-field parameters are B4c [1 1 0]=817 cm1 and B6c [1 1 0]=2030 cm1. From B4c [1 1 0]=14B4c [1 0 0] and B6c [1 1 0]=14B6c [1 0 0] [10], these correspond to the more familiar [1 0 0]-basis crystal-field parameter values of B4c [1 0 0]=3268 cm1 and B6c [1 0 0]=1249 cm1 State 2
F5/2
2
F7/2
Z1G8 Z2G7 Y1G8 Y2G6 Y3G7
Calc.
Expt.
0.0 519.8 2163.4 2178.3 3832.9
0.0 — 2163.4 2178.3 3832.9
# pffiffiffiffiffiffiffiffi rffiffiffiffi 231 ð6Þ 7 ð6Þ ð6Þ ð6Þ ðC6 þ C6 Þ : ðC þ C4 Þ þ 2 4 26 Fitting to these two crystal-field parameters only and holding all free-ion electrostatic and spin– orbit parameters to the values of [11] yields [1 1 0]-basis crystal-field parameters of B4c [1 1 0]= 817 cm1 and B6c [1 1 0] =2030 cm1, which give exact agreement between the measured and calculated energy levels (Table 2). However, a
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slight degree of distortion from exact cubic symmetry might shift the ground-state g values from those expected for a pure cubic G8 ðJ ¼ 52Þ quartet wavefunction [12] sufficiently to explain the absence of any EPR signals from these cubicsymmetry centres. Fits to the energy levels of the C2v(Li+/Na+) centres could be attempted using this [1 1 0]-basis cubic crystal-field Hamiltonian together with the introduction of a single B20 C0ð2Þ term to account for the presence of the nearby Li+ or Na+. However, only three infrared transitions are observed for the C2v(Li+) centre at 2159.2, 2195.9 and 3812.2 cm1 and just two unambiguous transitions for the C2v(Na+) centre at 2165.4 and 3821.6 cm1, with a third transition underlying the cubic line at 2178.3 cm1. As these are the same number of transitions as observed for the cubic centre itself, inclusion of even a single B20 C0ð2Þ term overparameterises any unconstrained crystal-field fit. In the absence of a definitive crystal-field fit, it is just pointed out that calculated g-values of (gx,gy,gz)=(0.35,1.37,2.46) are given by the following Kramers-doublet wavefunction 0:70 Jz ¼ 752 þ 0:70 Jz ¼ 712 0:15 Jz ¼ 732 ; close to the g values of (0.34, 1.29, 2.46) measured for the C2v(Na+) centres (Table 1). This wavefunction is similar to one of the Kramers-doublet wavefunctions (with coefficients 0.52, 0.76, 0.39) of the cubic G8 quartet ground state obtained from the cubic-centre energy-level fit. A small negative B20 C0ð2Þ term would put this particular Kramersdoublet wavefunction of the cubic G8 quartet ground-state lowest in energy, with a small admixture with the other Kramers-doublet wavefunction of this G8 quartet needed to reach the observed g values. For fits to the g values of the EPR C4v(Li+/ Na+) centre, a [1 0 0]-basis cubic crystal-field Hamiltonian [10] was adopted, together with a small B20 C0ð2Þ term needed to account for the presence of a next-nearest neighbour Li+ or Na+ ion located along the [1 0 0] direction. With this Hamiltonian, the g values g||=0.85 and g>=2.57 of the cubic G8g6 wavefunction ðJz ¼ 712Þ are close to the measured C4v(Li/Na) g values of g||=0.73 and g>=2.40. The adoption of a small
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negative B20 C0ð2Þ term value places this G8g6 wavefunction of the cubic G8 ground-state quartet lowest in energy and suggests that the C4v(Li+/ Na+) centre has a configuration not far removed from cubic symmetry, as its g values are not far from the cubic G8 quartet values discussed in Ref. [12]. For comparison, the observed g values of g||=3.026 and g>=1.386 for the C4v(F) centre are close to those (g||=3.01 and g>=1.43) for the g7 wavefunction of the cubic G8 ground-state quartet for which a positive B20 C0ð2Þ term makes this g7 wavefunction lowest in energy. 3.2. VUV absorption and UV luminescence Fig. 2 shows the vacuum ultraviolet (VUV) absorption spectra of Ce3+ for several CaF2 crystals. Two groups of broad bands, with peaks at 243 and 305 nm, and 194, 203 and 216 nm, are observed for the as-grown CaF2:0.09%Ce3+ crystal and these correspond to the transitions from the 2F5/2 ground state of the 4f1 configuration to two 2E and three 2T2 excited states, respectively, of the 5d1 configuration of Ce3+.
40 0.035%Ce : 0.29%Na 0.02%Ce : 1.1%Na 0.012%Ce : 0.97%Li 0.09%Ce (O2 anneal) 0.09%Ce (as-grown)
-1
30
0.012%Ce:0.97%Li : CaF2 6 Excitation spectra
20
A↓ ↓ ↓
↓↓ ↓
4 B↓ ↓
10 2
15 K
↓
8 A ↓ λem(nm) 320
Emission spectra ↓ B ↓ 15 K A ↓ ↓
250 340
B ↓
↓
400
Wavelength (nm) Fig. 2. 15 K VUV absorption spectra of Ce3+ in various CaF2 crystals.
0 100
215 190 B
2
↓ 380
150
↓C ↓
300
↓A ↓
4
B ↓ 360
↓ ↓
200
330 300
6
↓
↓
0 100
λ ex(nm)
↓ ↓
Absorption coefficient (cm )
15 K
All these transitions have been assigned as being C4v(F) centre transitions [13,14]. After annealing this Ce3+:CaF2 sample in an oxygen atmosphere, there appear several new broad bands at 143, 179, 216, 272 and 329 nm ascribed to C3v(O2) centres. The VUV absorption spectrum for the CaF2:0.01%Ce3+:1%Li+ sample (Fig. 2) comprises two groups labeled by A and B (see Fig. 3). The band peaks of the A group are at 179, 186, 193 and 303 nm, while those of the group B are at 135, 150, 215 and 330 nm, close to those of the oxygenated sample. For the two Na+ co-doped samples, it is found that increasing the Na+ concentration leads to a reduction of the line intensities for the B group together with a decrease of the EPR signal of the C4v(Li+/Na+) centre. Hence the group A lines are assigned to C2v(Li+/ Na+) centres. Fig. 3 shows 15 K excitation and luminescence spectra of the CaF2:0.01%Ce3+:1%Li+ sample. Excitation at 100 nm produces broad luminescence with a peak at 280 nm, labeled C. The excitation spectrum of this luminescence has peaks at 106, 116 and 124 nm, which are close to the band-gap energy of CaF2 [13] and supports its assignment to a self-trapped exciton. There are two luminescence bands A and B corresponding to the A and B features of the absorption spectra (Fig. 2). The excitation spectra of the A luminescence peaks at 315 and 335 nm are composed of 250 and 300 nm and 180, 188 and 192 nm sub-bands. Likewise, the
200
↓ 400
300
Wavelength (nm)
400
0 200
120 100
300
400
500
Wavelength (nm)
Fig. 3. 15 K VUV excitation and UV luminescence spectra of Ce3+ in CaF2:0.01%Ce3+:1%Li+.
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excitation spectra for the B luminescence peaks at 353 and 378 nm are composed of sub-bands at 330 nm and at 142, 155 and 218 nm. These excitation peaks match those of the absorption bands in Fig. 2. Both the excitation and emission spectra of the CaF2:0.04%Ce3+:0.3%Na+ sample are almost identical to that of the CaF2:0.01%Li+:1%Ce3+ sample, while the B luminescence for the more strongly NaF-doped CaF2:0.015%Ce3+:1%Na+ sample is negligibly small. Taking account of the EPR and FTIR results, the stronger A excitation and luminescence bands observed for the three Li+/Na+ codoped Ce3+:CaF2 samples are consistently assigned to C2v(Li+/Na+) centres. For these centres, the three close-lying transitions at 179, 186 and 193 nm are assigned to the 2T upper state and two at 250 and 303 nm to the 2E lower state of the 5d1 configuration. It is more difficult to assign conclusively the B luminescence and excitation bands. Because their line profiles are so similar to those of C3v(O2) centres, it is likely that the B luminescence and excitation is from trace amounts of C3v(O2) centres in the co-doped crystals. Room-temperature fluorescence-decay times for the A and B emissions are purely radiative, being measured as 39 and 46 ns, respectively.
4. Conclusions The EPR and FTIR spectra of Li+ or Na+ codoped Ce3+:CaF2 samples show that Li+ or Na+ co-doping removes lines of C4v(F) centres and creates cubic, C4v(Li+/Na+) and C2v(Li+/Na+) centres. UV excitation bands at 179, 186, 193 and 303 nm, which give rise to luminescence bands at 315 and 335 nm, have been assigned to optical transitions of the C2v(Li+/Na+) centres, which
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have charge compensating Li+/Na+ ions located in the [1 1 0] direction from the Ce3+ ions.
Acknowledgements This work was in part supported by a Grant-inAid for Science Research (C) from Japan Society for the Promotion of Science (No. 14550037). One of the authors (M. Yamaga) is indebted to the Iwatani Naoji Foundation for a Research Grant.
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