The luminescence properties of rare-earth ions in natural fluorite
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For the first time, the luminescence properties of Pr3+, Nd3+ and Tm3+ and Yb3+ ions in fluorite crystal have been obtained by steady-state measurements. In addition, the luminescence spectra of Ce3+, Sm2+, Sm3+, Dy3+, Er3+ and Yb3+ were measured. It was pointed out that λexc. = 415 nm is most suitable for measuring the Ho3+ emission beside the Er3+. The emission of trivalent holmium and erbium ions was measured independently using time-resolved measurements and tentative assignment of luminescence lines to C3v and C4v symmetry sites was proposed. Besides for natural fluorite crystal, the transitions between Stark energy levels of lanthanide ions were presented.
KeywordsPhotoluminescence Fluorite Rare-earth ions Stark energy levels
Fluorite is one of the best-known fluorescent minerals, and its specific properties led G. Stokes to name this phenomenon “fluorescence”. The luminescence of natural fluorite crystals was measured by various methods, among which the photoluminescence (PL) was most often used and significant. The luminescence of fluorite is connected mainly to the presence of rare-earth ions in it. The electrostatic stability of CaF2 lattice in presence of RE3+ ions demands the additional negative charge, mainly as an F− ion in interstitial sites. The effective symmetry around rare-earth ions could be Oh (for Pr3+, Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Er3+ and Yb3+ ions), C3v (for Gd3+, Tb3+, Dy3+, Ho3+, Er3+ and Yb3+ ions) or C4v (for Ce3+, Nd3+, Gd3+, Tb3+, Dy3+, Ho3+, Er3+ and Yb3+ ions), what was confirmed by EPR measurements (Weber and Bierig 1964). There has been a large number of research papers on the luminescence of natural fluorite measured by the steady-state method (Illiev et al. 1988; Aierken et al. 2000, 2003; Bodył 2006; Petit et al. 2007; Czaja et al. 2008; Bodył 2009) or the time-resolved method (Gaft et al. 1998, 2001a, b, 2005, 2008). Moreover, the synthetic CaF2, BaF2 and SrF2 crystals doped with RE ions were investigated intensively (Wood and Kaiser 1962; Kirilyuk 1973; Fenn et al. 1973; Tallant and Wright 1975; Seelbinder and Wright 1979; Chrysochoos et al. 1982, 1983; Illiev et al. 1988; Caldino et al. 1989; Oskam et al. 2002). The splitting of ground and excited levels of Gd3+, Er3+ and Ho3+ ions were determined, and luminescence lines were assigned to the C4v and C3v symmetry sites on the basis of EPR and electronic optical measurements (Rector et al. 1966; Tallant and Wright 1975; Seelbinder and Wright 1979; Mujaji et al. 1992). In the case of heavy-doped CaF2 synthetic crystals and their analogues, other luminescence centers, such as pairs or clusters, have been discussed by researchers, too (Fenn et al. 1973; Seelbinder and Wright 1979).
In this paper, we have demonstrated the possibility of steady-state photoluminescence measurement for the purpose of identification of 4f ions using the mutual relationship between excitation and emission spectra. For the first time ever, the emission and excitation spectra of Pr3+, Nd3+, Tm3+ and Yb3+ in natural fluorite crystal were investigated by this method. Moreover, we have proposed the most effective excitation for holmium ion emission. Furthermore, the tentative assignment emission line to the respective transition in C3v and C4v symmetry sites was made from the time-resolved spectra of Er3+ and Ho3+ ions.
Sample properties and measurement conditions
To find the most convenient conditions to measure Ho3+ luminescence in the presence of Er3+ ions, the luminescence spectra of phosphate glass doped by holmium and erbium: 44 % P2O5 + 25 % CaO + 15 % BaO + 15 % SrO + 1 % Ho2O3, 44 % P2O5 + 25 % CaO + 15 % BaO + 15 % SrO + 1 % Er2O3 were synthesized.
Phosphate glasses doped with lanthanide ion or ions can be easily synthesized–easier than CaF2 crystals. However, the optical properties of 4f ions in these glasses, especially Ωi parameters, are different from those in other host materials, although the positions of emission and excitation bands can be treated as a standard (with accuracy measured in ± nm) in order to identify each particular 4f ion. We have observed that it is especially useful for Pr3+–Sm3+ ions in apatite crystals (Bodył et al. 2009) and for Er3+–Ho3+ ions in fluorite and scheelite crystals (Czaja et al. 2008). To enhance the emission of Ho3+ ions in the presence of Er3+ ions in the spectral range of 550 nm, a 415 nm excitation was chosen. For this excitation, the transition 5I8 → 5G5 of Ho3+ ions can be observed besides the transition 4I15/2 → 2H9/2 of Er3+ ions. However, the emission from the 2H9/2 level was not measured, in contrast to the emission from 4S3/2.
The steady-state fluorescence measurements for Ce3+, Pr3+, Sm3+, Eu2+, Tb3+, Dy3+, Ho3+, Er3+ and Tm3+ were performed using a Jobin–Yvon (SPEX) spectrofluorimeter FLUORLOG 3-12 at room and low temperatures using a 450 W xenon lamp, a double-grating monochromator, and a Hamamatsu 928 photomultiplier. Other steady-time measurements were done for Nd3+, Sm2+, Er3+ and Yb3+ using an Edinburgh Instruments FLS920 spectrofluorimeter with a xenon lamp and Hamamatsu 928 or Hamamatsu R5509-72 photomultipliers or Physik LPD3000 laser (pumped by a Lambda Physik LPX100 excimer laser). The time-resolved emission spectra and decay times were measured using a GDM–1000 double-grating monochromator, equipped with a Hamamatsu 928 photomultiplier. The resulting luminescence signal was stored in a Stanford model SRS 250 Boxcar Integrator coupled with a PC computer. The emission-line accuracy was 0.1 nm. Because the sensitivity of the Hamamatsu 928 or Hamamatsu R5509-72 photomultipliers is almost constant in the range 400–700 nm, no correction curve was needed.
The luminescence decay curves were excited by applying a short impulsive light from an OPO Optical Parametrical Oscillator pumped by the third-harmonic of a YAG:Nd laser. The decay kinetics of excited states were recorded utilizing a Tektronix model TDS 3052 digital oscilloscope. The decay time and gate width were chosen according to the decay time of each expected luminescence center.
Results and discussion
Steady-state measurements for 4f-5d transitions
Steady-state measurements for 4f-4f transitions
Moreover, additional emission peaks of Er3+ at 1,529 nm (4I13/2 → 4I15/2) and Er3+ (4I11/2 → 4I15/2) together with Yb3 (4F5/2 → 2F7/2) at 978 nm were observed. The luminescence lifetime for 4F5/2 state of Yb3+ ion in this fluorite was measured and was equal to τ = 9.3 ms, which was coherent with the results obtained earlier. For the Yb3+ ion in C4v site symmetry in synthetic CaF2, Petit et al. (2007) have observed that the luminescence lifetime was τ = 8 ms.
The most convenient conditions for steady-state measurements of some RE ions; excitation and emission lines and electronic transitions
Ion: emission line (s) [nm]: transition (s)
Excitation line (s)
336: Ce3+: 5d(Eg) → 4f(2F7/2)
Sm2+: 683, 699, 730, 761, 793:
5D0 → 7FJ (J = 0, 1, 2, 3, 4)
365, 452, 467, 485, 494
Nd3+: 864: 4F3/2 → 4I9/2, 1065: 4F3/2 → 4I11/2
Yb3+: 978: 4F5/2 → 2F7/2,
Er3+: 1529: 4I13/2 → 4I15/2
Sm3+: 598, 607, 614: 4G5/2 → 6H7/2, Pr3+: 641: 3P0 → 3F2
442, 466, 479, 490,
442, 466, 479, 490, 398
Sm3+: 560 4G5/2 → 6H5/2, 596, 606: 4G5/2 → 6H7/2, 644, 653: 4G5/2 → 6H9/2;
Er3+: 538: 4S3/2 → 4I15/2, 653: 4F9/2 → 4I15/2
Dy3+ : 574: 4F9/2 → 6H13/2
363, 378, 399, 442,
451, 466, 473, 480, 363, 377
399, 440, 449, 461, 467, 473, 486
Dy3+: 478: 4F9/2 → 6H15/2 574: 4F9/2 → 6H13/2
323, 348, 362, 387, 449
Tm3+: 453: 1D2 → 3H4,
Dy3+: 478: 4F9/2 → 6H15/2,
574: 4F9/2 → 6H13/2
Er3+: 538: 4S3/2 → 4I15/2
Er3+: 523: 2H11/2 → 4I15/2 539, 546, 549, 554:
4S3/2 → 4I15/2
331, 343, 362, 377, 403, 414, 448, 486
Er3+: 520: 2H11/2 → 4I15/2 535, 542, 547, 550:
4S3/2 → 4I15/2,
666: 4F9/2 → 4I15/2
Sm3+: 594, 604: 4G5/2 → 6H7/2, 650: 4G5/2 → 6H9/2
Nd3+: 850: 4F3/2 → 4I9/2, 1047, 1065: 4F3/2 → 4I11/2,
Yb3+: 978: 4F5/2 → 2F7/2,
Er3+: 974: 4I11/2 → 4I15/2, 1529: 4I13/2 → 4I15/2
In order to properly identify the origin of emission lines in the 539–554 nm part of the spectrum, to assign them to Er3+ or Ho3+ ions, and also to determine the crystal sites occupied by these ions in fluorite lattice, time-resolved measurements were performed. The lifetimes for Ho3+ 540 nm and Er3+ 545 nm emissions measured by Gaft et al. (2001a, b) are similar and equal 5 μs and 23 μs, respectively. When the delay time is 10 μs and the gate width 10 μs, the emission of Ho3+ will be observed as well, while the Er3+ center is still in the excited state during the time of measurement and does not participate in the emission. By contrast, for delay time 10 μs and gate width 30 μs, the emission of Ho3+ will be already quenched, while the emission of Er3+ will remain.
On the time-resolved emission spectra, it is normal for the most intensive Stark multiplets to appear. The crystal field can split the energy levels of the RE ion and remove their degeneracy, as the complex character of absorption and emission spectra have often revealed. The number of multiplets, commonly referred to as Stark levels, depends on the quantum number J of 2S+1LJ terms and on the crystal site symmetry of the ion. The emission lines belong to transitions between Stark’s multiplets of excited and ground levels. The ground level is usually named Z, and the excited levels are designated as “Y, A, B, D, E”, etc. According to these designations, the 5F4,5S2 levels of Ho3+ and 5S3/2 level of Er3+ are designated as E. The degeneracy of the ground level of Ho3+ (5I8) and of excited 5F4,5S2 levels for the tetragonal symmetry site C4v amount to 13 and 11, while for the trigonal symmetry site C3v–11 and 9. The degeneracy of the excited level for the Er3+ ion is double and of ground level it is eightfold, for both symmetry sites.
The energy of transitions between Stark’s levels–excited and ground–and assignment to C4v and C3v symmetry sites
Emission of Ho3+ ion
Emission of Er3+ ion
Number of emission line (Fig. 10a)
Emission line at 10 K [cm−1] (nm)
Site symmetry C4v
Site symmetry C3v
Number of emission line (Fig. 10b)
Emission line at 10 K [cm−1] (nm)
Site symmetry C4v
Site symmetry C3v
5F4–5I8 or J–Y
There were some difficulties with assigning numerous emission lines to luminescence transitions and comparing them to the values of energy, which are known for synthetic CaF2:Ho3+ or CaF2:Er3+crystals. Furthermore, some lines are present on the emission spectrum for holmium ion (Fig. 10a) at the shorter wavelength, whose energy should correspond to the transitions from the higher energy sublevels of 5F4,5S2 excited level (E11-9) to the ground 5I8 level (Z1-4), according to Mujaji et al. (1992). Because the population of (E11-9) states at T = 10 K is very small, this assumption should be rejected. The above discrepancies have led us to accept the assumption that the energies of excited states of Ho3+ and Er3+ ions in natural crystals are different—insensibly but measurably—from the energies of excited levels in synthetic crystals. The reason for these differences is the different value of local crystal field strength, due to the more complicated chemical composition of a natural fluorite crystal as compared to its synthetic counterpart.
The excited levels 5F4 and 5S2 are energetically separated, and besides the emission from the 5F4 level, the emission from the 5S2 level was measured as well, as was shown for a few other crystals: YSGG (Pugh et al. 1997), KGd(WO4)2 (Pujol et al. 2001), GdLiF4, YLiF4 and LuLiF4 (Walsh et al. 2005), YAG (Walsh et al. 2006), YAB (Baraldi et al. 2007) or YGG (Gruber et al. 2009). The 5F4 and 5S4 levels were usually separated in them by an interval of 210–300 cm−1;
The splitting of ground 5I8 level (ΔZ) and excited level 5S2 (ΔE) is different in this case than for a synthetic CaF2:Ho3+ crystal;
Similarly to a synthetic CaF2:Ho3+ crystal, the most intensive transitions for a holmium ion in a tetragonal site should be E1–Z1 and E1–Z4, but for a trigonal site, they could also be E1–Z1 and E1–Z3.
There are differences in the splitting of the ground 4I15/2 level (ΔZ) and the excited level 5S3/2 (ΔE) for our fluorite and synthetic CaF2:Er3+ crystals;
the energy of excited level 5S3/2 is different (lower) in comparison with a synthetic crystal;
Similarly to a synthetic CaF2:Er3+ crystal, the most intensive transition for erbium ion in a tetragonal site should be E2–Z1, E1–Z1, E1–Z2 and E1–Z5,6, but for trigonal site also E1–Z1, E1–Z2 and E1–Z3;
For the Er3+ ion in a CaF2 crystal, the energy of the Z4 level could not be identified.
The above assumptions allow us to make a tentative assignment of luminescence lines to a particular transition of holmium and erbium ions in C3v and C4v symmetry sites (Fig. 10; Table 3). The excited level 5S3/2 of Er3+ has lower energy by about 320 and 240 cm−1 for C4v and C3v sites, respectively, whereas the excited level 5S2 of Ho3+ has higher energy by about 114 and 37 cm−1 for C4v and C3v sites, respectively. The splitting of ground level (4I15/2) of the Er3+ ion occupying C4v and C3v symmetry sites in synthetic CaF2 is equal to 84 and 16 cm−1, respectively, while the splitting of the excited level 5S3/2 equals 452 and 461 cm−1. For the Ho3+ ion in synthetic CaF2, it was found that the splitting of the ground level is equal to 512 and 423 cm−1 for the C4v and C3v symmetry sites, respectively, while the splitting of the excited level equals 289 and 206 cm−1. From our tentative assignment, the splitting of the excited level for both ions seemed to be generally lower than in a synthetic crystal.
This study has shown that steady-state measurements can identify the 4f ions in natural crystals. The luminescence of Pr3+, Nd3+ and Tm3+ ions in fluorite crystal have been received by steady-state measurements; the luminescence of Ce3+, Sm2+, Eu2+, Sm3+, Dy3+, Er3+ and Yb3+ was measured as well. In comparison with earlier studies on this subject (e.g. Aierken et al. 2003), the intensive luminescence was measured for concentrations of RE ions 1.5–4.0 times smaller. The fluorite crystal studied in this paper is another example of a fluorite for which the violet luminescence did not dominate. Transitions between energy levels of the Ho3+ and Er3+ ions for a natural fluorite crystal were measured. The discrepancies between the energy of Stark levels of holmium and erbium ions in synthetic and natural crystals could have been caused by differences in crystal field strength of natural and synthetic crystals. In order to remove any doubt regarding the assignment of the emission transitions and to calculate the energy of Stark levels, it is necessary to measure the energy of transitions from other excited levels of Er3+ and Ho3+ to ground state and will be the subject of a separate study.
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