ARTICLE pubs.acs.org/JPCA
Emission and Excitation Spectra of Ce3+ and Pr3+ Ions in Hexafluoroelpasolite Lattices Chang-Kui Duan,†,‡ Peter A. Tanner,*,‡ Vladimir Makhov,§ and Nicholas Khaidukov|| †
)
Department of Physics, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, Anhui. People's Republic of China ‡ Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong S.A.R., People's Republic of China § Lebedev Physical Institute, Leninsky Prospect 53, Moscow 119991. Russia Kurnakov Institute of General and Inorganic Chemistry, Moscow 117907, Russia ABSTRACT: The emission and excitation spectra of Ce3+ and Pr3+ doped into the cubic host Cs2NaYF6 have been recorded at room temperature and ∼10 K using synchrotron radiation. The two 5d1 T2g states of Ce3+ have been located from the excitation spectra, whereas the Eg state is placed above the host band gap. Decay measurements of the 5d1 f 4f1 Ce3+ emission, and spectra collected using selective excitation, indicate the occupation of more than one type of site by Ce3+ in this host lattice. By contrast, the location of features in the 4f15d1 f 4f2 emission of Pr3+ is independent of the excitation wavelength. Assignments are presented for some of the 4f15d1 levels and for the Pr3+-F charge transfer band. The 5d emission lifetimes for Ce3+ and Pr3+ in the Cs2NaYF6 host are 42 and 29 ( 1 ns, respectively, and are not temperature-dependent.
’ INTRODUCTION With the current availability of synchrotron radiation sources, and the recent theoretical developments for 4fN15d energy level calculations and 4fN4fN15d transition intensity simulations, new experimental studies and interpretations have been forthcoming, especially for lanthanide ions (Ln3+) doped into wide band gap hosts.14 The vacuum ultraviolet (VUV) spectra of Ln3+ ions doped at the S4 site of LiYF4 have received particular attention.59 Other interconfigurational studies of the 4f1 Ce3+ and 4f2 Pr3+ ions have employed the CaF21012 and KMgF313,14 hosts. The 4f15d1 absorption transition of Ce3+ is electric dipole allowed and comprises up to five bands in the ultraviolet spectral region, corresponding to transitions to the Kramers doublet levels derived from Eg and T2g orbitals of the 2D multiplet. On the other hand, the spectra of Pr3+ are more complex since the 4f15d1 configuration comprises the 10 multiplets 1,3P, 1,3D, 1,3F, 1,3G, and 1,3H. The present study concerns the 4f5d VUV excitation spectra and ultraviolet emission spectra of Ce3+ and Pr3+ at octahedral (Oh) symmetry sites, for which the energy level degeneracies are high: up to four (for Ce3+) and three (for Pr3+). Potential applications of these systems concern tunable ultraviolet upconversion lasers15 and X-ray storage phosphors.16 On the theoretical side, recent attempts have signaled an improvement of ab initio calculations for these ions in crystals.17,18 Some previous studies of Ce3+ and Pr3+ at octahedral (Oh) symmetry sites have been carried out but provided fairly limited information content since the spectral features were broad. This r 2011 American Chemical Society
prompted our investigations because the spectra of the corresponding chloride systems were well-resolved.19,20 Aull and Jenssen21,22 investigated the excitation and emission spectra of Rb2NaYF6 doped with Ce3+. Two different crystal sites were distinguished for Ce3+. Excitation into the “blue site” bands, with excitation maxima at 240, 295 nm, gave a broad emission band peaking at 340 nm. On the other hand, excitation into the broad longer-wavelength band at 310 nm gave a broad emission band with maximum at 400 nm. Kodama et al.23 found that the center of gravity of the 5d1 2D multiplet is at ∼250 nm (40000 cm1) for Ce3+ doped into several oxides and fluorides. The energy levels of the lowest 5d states of Ce3+ are generally lower in cubic symmetry than in octahedral symmetry because the shift of the Eg state from the center of gravity is 1.5 times larger than that of the T2g state. Schiffbauer et al.24 considered that Pr3+ doped into Cs2KYF6 occupies three different types of sites. The substitution at the Td Cs+ site afforded the possibility of quantum cutting because the 4f5d onset (at 208 nm) lies above the Pr3+ 4f2 1S0 energy level. Excitation into the other two sites (assigned to substitution of the K+ and Y3+ octahedral sites by Pr3+) gave broad 4f5d f 4f2 emission, with the band maximum at ∼300 nm. This study concludes our experimental and theoretical investigations of the VUV spectra of Ln3+ doped into the Cs2NaYF6 lattice.2527 Attempts have been made to interpret and Received: April 28, 2011 Revised: June 7, 2011 Published: July 22, 2011 8870
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Soviet Union. It was envisaged that the synthesis of crystals containing about 1 atom % optically active ions would ensue, by taking into account that the partition coefficient (i.e., ions in crystals/ions in blends) could be less than 50%. However, the actual dopant ion concentrations in the crystals are unknown. The measurements were performed using synchrotron radiation of the DORIS storage ring at the SUPERLUMI station of HASYLAB at DESY. The spectra and decay kinetics were measured at ∼10 K and room temperature from freshly cleaved surfaces. All measurements were performed under ultrahighvacuum conditions (better than 109 Torr). A 0.3-m CzernyTurner monochromator-spectrograph SpectraPro-308i (Acton Research Inc.) with a R6358P (Hamamatsu) photomultiplier tube was applied for selecting the monitored wavelength when measuring excitation spectra and decay curves. Emission spectra were recorded at the same spectrograph with a liquid nitrogen cooled CCD detector (Princeton Instruments Inc.). Excitation spectra were measured with spectral resolution ∼0.3 nm and emission spectra ∼0.5 nm. Emission spectra were not corrected for the spectral response of the detection system. The WG320 filter employed is a cutoff filter which is transparent for wavelengths longer than 320 nm.
Figure 1. Low temperature emission (a) and excitation (b),(c) spectra of Cs2NaYF6:Ce3+. Panel c shows the scale expansion of (b) in the range from 278 to 303 nm.
rationalize the observed spectral features and comment upon the emission lifetimes.
’ EXPERIMENTAL SECTION The crystals of Cs2NaYF6 doped with Ce3+ (or Pr3+) were synthesized by a hydrothermal method using Y2O3 containing 5.0 atom % Ce (or Pr) from CeO2 or Pr6O11 starting materials in a copper container. The purities of the RE oxides were 99.999% Y2O3, 99.9% CeO2, and 99.9% Pr6O11. All the oxides were manufactured by enterprises of the defense industries of the
’ RESULTS AND DISCUSSION The measured emission spectra under various VUV-excitation wavelengths, the luminescence decay curves, and the excitation spectra are presented for Ce3+ and Pr3+ in the hexafluoroelpasolite lattice in this section. The Cs2NaYF6 host excitation starts at about 125 nm, and this is not discussed further. Excitation and Emission Spectra of Ce3+ in Cs2NaYF6. The 1 4f and 5d1 energy level schemes of Ce3+ doped into Cs2NaYCl6 have been illustrated and are described in detail in ref 19. The reader is referred to this work. Basically, the 4f1 multiplets are in the order of increasing energy: 2F5/2 and 2F7/2; whereas the 5d1 2 D term splits into T2g and Eg. In order to make analogous assignments for the fluoride host, the low temperature emission and excitation spectra are shown in Figure 1. Excitation into the major Ce3+ 5d1 absorption band, using 315 nm radiation gives the emission spectrum, Figure 1a, which corresponds to electronic and vibronic structure of transitions from the lowest 5d1 level, T2g Γ8, to the 4f1 2F5/2,2F7/2 multiplets. Altogether, there are five unresolved electronic transitions within the band extending from 325 to 450 nm. An overview of the corresponding low temperature excitation spectrum, when monitoring 351.7 nm emission, is shown in Figure 1b. The peak maximum is situated between 302 and 314 nm and due to saturation the features cannot be clearly identified in the excitation spectrum. An estimate of the zero phonon 4f1 2 F5/2 Γ7u f 5d1 T2g Γ8g can be made from the crossing overlap of the low temperature excitation and emission spectra, which is at ∼319.1 ( 0.5 nm (i.e., 31333 ( 50 cm1). The reported 1.5 K absorption spectrum of Cs2NaYF6:Ce3+16 exhibits two bands between 350 and 230 nm with maxima estimated from the small Figure 2 therein at 317 and 292 nm (with errors of up to several nanometers). The separation between these two maxima is ∼2660 cm1 and this roughly corresponds to the separations of the 2F5/2 Γ7u f 5d1 T2g Γ8g, Γ7g transitions resulting from the spinorbit splitting of the T2g state. The corresponding separation of the two transitions in Cs2NaYCl6:Ce3+ is much smaller (1239 cm1).19 From the estimated Γ7u f 5d1 T2g Γ8g (319.1 nm) energy of 31333 cm1, 8871
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Figure 2. Emission spectra of Cs2NaYF6:Ce3+ under various excitation wavelengths at room temperature (a) and low temperature (b).
the zero phonon line of Γ7u f 5d1 T2g Γ7g is then located at roughly 31333 + 2660 = 33990 cm1 (i.e., at 294 nm). The scale enlargement of Figure 1b, between 276 and 304 nm, is shown in Figure 1c so that the detail is clearly observed. The major progressions in allowed fd transitions result from totally symmetric vibrational modes. The ν1 R1g YF vibration is identified in the Raman spectrum of Cs2NaYF6 at 467 cm1,28 and this provides a guide for the energy of this mode in the Ce3+-doped system. Following the extensive 4f1 f 5d1 T2g vibrational progressions observed in Cs2NaYCl6:Ce3+, the separation (471 cm1) of the features 1,2 in Figure 1c identifies them as the v = 4 and v = 5 members of the totally symmetric CeF ν1 vibrational progression upon the Γ7u f 5d1 T2g Γ8g zero phonon line. The bands marked 3 in Figure 1c are sharper than features to lower energy, and they mark the start of a new transition, with the similar progression interval of 474 cm1. This transition commences at 295 nm (i.e., at 33903 cm1, which is at 2570 cm1 above the lowest 4f f 5d transition), so that from the above estimation, it is assigned to Γ7u f 5d1 T2g Γ7g. Following the assignments for the 4f5d transitions of Cs2NaYCl6:Ln3+,19,20 the two shoulders with vibrational energy displacements of 67 and 134 cm1 to high energy of the first (00) band (with these two features and the 00 line marked in Figure 1c, line 3, by vertical strokes) are assigned to members of the totally symmetric progression in the Ce3+Cs+ stretch, where Cs+ is the second-nearest neighbor. Band 4 is at 299 cm1 above 3 and presumably corresponds to a totally symmetric mode involving the relative motion of the third-nearest neighbor, Na+, rather than to two quanta of a bending mode.
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The magnitude of 10 Dq = B40(d)/2.1 for Cs2NaYCl6:Ce3+ has been given as 18471 cm1,19 so that the Γ7u f 5d1 Eg Γ8g transition in Cs2NaYF6:Ce3+ is expected at higher energy than [10Dq + T2g(barycenter)] = 18471 + [(31333 4) + (33903 2)]/6 = 50660 cm1 (197 nm). Very weak bands are present in Figure 1b at 188.5 and 178.6 nm, but no conclusive assignment can be given for the location of 5d1 Eg. Excitation into this upper state therefore does not populate the 5d luminescent level. Aull and Jenssen22 have reported a weak, broad absorption band at 197 nm in the vacuum ultraviolet transmission spectrum of Rb2NaYCl6:Ce3+, which was not present in the spectrum of the undoped crystal and this can be assigned to the absorption transition from the 4f1 ground state to 5d1 Eg. Notice also that emission is not observed from 5d1 Eg although it is situated at an energy level spanned by more than 30 phonons above the next lowest 5d1 state. The reason is clearly because this state lies within the host conduction band, so that the 4f1 Ce3+ ground state is then estimated to be at least ∼(83300 50660) ∼ 32640 cm1 above the top of the valence band (where 83300 cm1 (120 nm) is an estimation of the band gap energy). This value is reasonable in comparison with the values for Ce3+ doped into YPO429 and NaLaF4.30 Figure 1b also displays excitation bands between 275 and 200 nm. Bands due to various color centers in Cs2NaYF6 have been assigned at much lower energies.31 Also, these bands are not due to the CeF charge transfer (CT) band, which is expected to be located at much shorter wavelengths. The 4f f 6s transitions are also calculated to be at shorter wavelengths and have been assigned at 146 nm in CeF3.32 Another type of transition is from a localized state of Ce3+ to a delocalized state in the conduction band. From the estimated energies of 4f1 ground state and the host band gap, this would require energy of less than ∼50600 cm1 (i.e., longer wavelength than ∼200 nm). This type of transition is not well-documented and is expected to be very broad. Marshall et al. have assigned a broad band at 290 nm in the excited state absorption spectrum of LiCaAlF6:Ce3+ to the excitation of a 5d electron into the conduction band.33 Schweizer et al.34 have shown that transitions can be observed between localized 4fN states even though the terminal state is situated above the band gap. Aull and Jenssen22 identified two types of Ce3+ centers from the excitation spectra of Rb2NaYF6:Ce3+ when monitoring emissions at 340 and 400 nm. Some degree of energy transfer exists between these sites because the strong features at 296, 238, 218 nm observed for the “minority” site were also present (although much weaker) in the excitation spectrum of the regular Oh-type Ce3+ site. Following these authors, the presence of other Ce3+ sites in the crystal of Cs2NaYF6:Ce3+ was investigated by spectrally selective excitation in this work. Figure 2a presents the room temperature emission spectra measured using different excitation wavelengths. The emission spectrum under 296.5 nm excitation into the “normal” Ce3+ site gives a peak at 351 nm, but under 252.5, 238.5, and 222 nm wavelength excitations, the emission band shifts with respect to the 296.5 nm excited spectrum by ∼6 nm to high energy. The 127.5 and 93 nm wavelength excitations produce emission bands, which almost completely overlap with those under 296.5 nm excitation. Since the structures in the spectra corresponding to 4f5d transitions are usually much more clearly resolved at low temperature, the emission spectra were also measured at 8.8 K using selective excitation and the results are presented in 8872
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Table 1. Summary of Lifetime (τ) Data for Cs2NaYF6 Doped with Ce3+ and Pr3+ and Some Other Fluorides Doped with Pr3+ system Cs2NaYF6:Ce3+
λexc (nm)
ref
λem (nm)
T (K)
τ (ns) 41.9 ( 0.3
this work
316
350, 400
300
this work
222, 238.5, 252.5, 296.5, 317.5
350
300
43
300
42
16 this work
296.5
350, 400
8.7
42
this work
317.5, 296.5
350
8.6
41
this work
252.5, 238.5, 222
350
8.6
36
Cs2NaYF6:Pr3+
this work
148, 163, 209.5, 229
252
300, 8.8
2830
LiKYF5:Pr3+ K2YF5:Pr3+
42 42
157 157
∼240 ∼240
300 300
20.3 25.5
CsY2F7:Pr3+
42
215
∼230
300
18.8
KYF4:Pr3+
42
157
∼230
300
26.0
Figure 2b. The emission spectra are still poorly resolved and comprise a broad band, similar to the spectra at room temperature, but shifted to short wavelength by 510 nm. Emission Lifetimes of Cs2NaYF6:Ce3+. It is well-known that if the broadening of an emission band is mainly due to inhomogeneous broadening caused by the presence of many sites, then the decay time may change with detection wavelength across the emission band. Moreover, the superposition of emissions from sites with different decay times results in a multiexponential luminescence decay curve. Hence we measured the decay curves by monitoring various emission wavelengths and by using various excitation wavelengths, with the following results. At room temperature, under 316 nm excitation, the decay curves for 350 and 400 nm emissions were both single exponential, with the same lifetime of 41.9 ( 0.3 ns. This shows that the emissions at 350 and 400 nm are from the same set of Ce3+ sites, and therefore represent (unresolved) transitions to 2F5/2,7/2. The 350 nm emission decay curves under other excitation wavelengths (Table 1) were single exponential, with a lifetime of 43 ns, except for excitation of the host at 93 and 127.5 nm when the decay becomes nonexponential with the contribution of the slow component because of the delayed energy transfer to Ce3+ from the host excitations. Pawlik and Spaeth have reported the lifetime of 42 ns for Cs2NaYF6:Ce3+.16 At low temperature (8.7 K), under 296.5 nm excitation, the emission measured at both 350 and 400 nm decay single exponentially with the same lifetime of 42 ns within experimental error, but the decay curve detected at high-energy tail (320 nm emission) is nonexponential and faster (25 ns for the part of decay to 1/10 of the initial intensity). This indicates that even the majority Ce3+ ions are from the same site or set of sites; some Ce3+ ions with an emission of higher energy are of different origin. When the 350 nm emission at 8.6 K is monitored, the decay curves under the excitation wavelengths employed can be fitted reasonably well with single exponential curves, but with the lifetime decrease from 41 to 36 ns (Table 1). Under the host excitation (at 127.5 and 93 nm) the decay curves are nonexponential with faster (∼29 ns) initial stage of decay and a remarkable contribution of the slow emission component. Note that under these host excitation wavelengths the emission of only “majority” Ce3+ sites is excited. Combining the emission spectra and the decay curve analysis, we conclude that there is apparent inhomogeneous broadening on top of homogeneous broadening due to electronphonon coupling typical for 4f5d transitions. At room temperature, the single-exponential decay of the curves and the unique lifetime for
all emission wavelengths and under various excitation wavelengths indicate that the migration of excitation energies among Ce3+ ions at different sites is fast. This can also explain why the shift of emission bands under different excitation wavelengths at room temperature is much smaller than that at low temperature. Yamaga et al.35 found a consistent increase in emission lifetime with increasing detection wavelength from 320 to 540 nm for CaNaYF6:Ce3+. In that case, the changes were attributed to a random distribution of Ce3+ sites. We have found a decrease in emission lifetime at a fixed wavelength when the excitation wavelength is decreased. As mentioned in the Introduction, the coordination polyhedron of Ce3+ plays a key role in determining the energy of the lowest 5d state.23 The bands in the excitation spectrum between 275 and 200 nm could possibly be associated with the occupation of the cubic Cs+ site by Ce3+, where the crystal field strength is very small. However, as subsequently shown, some of the intensity of these bands derives from the presence of other lanthanide ions. Excitation and Emission Spectra of Cs2NaYF6:Pr3+. The energy level scheme of Cs2NaYF6:Pr3+, with Pr3+ situated at an Oh symmetry site expected to be analogous to that of Cs2NaYCl6: Pr3+.20 The 4f2 electronic ground state is 3H4 Γ1g, with a 3H4 Γ4g state some hundreds of cm1 to higher energy. The lowest 4f15d1 state in Cs2NaYCl6:Pr3+ is Γ3u, with predominant triplet character. Note that the electronic transition to this state is forbidden (by point group selection rules) from the electronic ground state. Since the absorption transitions from the electronic ground state are exclusively allowed to terminal Γ4u states, only 18 electronic transitions are expected for 4f2 f 4f15d1 in this host lattice. These are expected to lie in two groups of bands with a gap of 10Dq ∼ 18500 cm1 between their barycenters. The onset of 4f2 f 4f15d1 absorption for Cs2NaYF6:Pr3+ has been predicted to be at ca. 226 nm (44300 cm1).25 The low temperature (10 K) excitation spectrum was measured by monitoring the 249 nm emission, and it is shown in Figure 3a. Apart from the broad band at 200230 nm due absorption to the T2g component of the 5d orbital, there are several other bands at ca. 175150, 145135, 130116, and at 110 nm to less than 70 nm. Figure 3b shows the region between 230 and 200 nm in this excitation spectrum on an expanded scale and the comparison is made with the excitation spectrum of Cs2NaYF6:Ce3+ when monitoring 252 nm emission. Clearly, a trace of Pr3+ is present in the Cs2NaYF6:Ce3+ sample and the lower intensity of absorption 8873
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Figure 3. (a) 10 K excitation spectrum of Cs2NaYF6:Pr3+ between 70 and 240 nm, by monitoring the emission at 249 nm. (b) Top line: expanded detail of spectrum a between 196 and 234 nm; Bottom line: 8.7 K excitation spectrum of Cs2NaYF6:Ce3+ containing a trace of Pr3+ between 196 and 234 nm, by monitoring the emission at 252 nm. The locations of electronic origins IV are marked.
enables some conclusions to be made concerning the energy level structure of 5d1 T2g in Cs2NaYF6:Pr3+. Five electronic transitions (Γ1g f Γ4u) are marked (IV) in the figure. From the preliminary intensity simulation (not shown) and by comparison with the spectrum of Cs2NaYCl6:Pr3+,20 four strong and one weaker 4f2 f 4f15d1 T2g transitions are expected to be observed in this group of bands. By reference to the excitation spectra of Cs2NaYF6:Ce3+, Figure 1c, where features are more clearly resolved, the maxima in Figure 3b consist of unresolved vibronic structures. The locations of these maxima are given in cm1, with the corresponding zero phonon lines of Cs2NaYCl6: Pr3+ in parentheses: I, 43758 (39792); I, 44752 (40343); III, 45779 (41110); IV, 46830 (41940); V, 48560 (43711); and alternatively, in nanometers: I, 228.5 (251.3); II, 223.5 (247.9); III, 218.4 (243.2); IV, 213.5 (238.4); V, 205.9 (228.8). Thus the corresponding features are between 3970 and 4890 cm1 to higher energy in the fluoride than in the chloride. The ν1 progression frequencies (in cm1) for the transitions IV are measured as follows: I, 480 ( 10; III, 500 ( 12; IV, 494 ( 5; V, 490 ( 6. The barycenter of the 4f15d1 T2g group is at ∼215 nm (46500 cm1) so that the barycenter of the transition to the 4f15d1 Eg group is expected to be at ∼18500 cm1 to higher energy. Thus, the feature peaking at 156.8 nm (63776 cm1) in Figure 3a is associated with the unresolved 4f2 f 4f15d1 Eg
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Figure 4. Room and low temperature emission spectra of Cs2NaYF6: Pr3+ at 298 K (200700 nm) and 8.8 K (200600 nm).
absorption bands comprising seven Γ1g f Γ4u transitions and their vibronic structures. A band is also observed at 143.2 nm (69832 cm1) in Figure 3a. For fluoride hosts doped with Pr3+, Brewer36 placed 4f16s1 at shorter wavelengths than 100 nm (100000 cm1), whereas Elias et al.37 assigned the 4f2 f 4f16s1 transitions at 125 nm (80000 cm1) in LaF3:Pr3+. Loh assigned these bands at 132 nm (76000 cm1) in CaF2:Pr3+.38 Two groups of states are expected for the 4f16s1 configuration, with a separation similar to that between the 2F5/2,2F7/2 terms of Ce3+, i.e., 2000 3000 cm1. The transitions to these multiplets gain electric dipole intensity by mixing of the terminal states with 4f15d1. From the above two estimates, it appears that the band at 143.2 nm in Cs2NaYF6:Pr3+ is not associated with the 4f2 f 4f16s1 transition. The band at 143.2 nm in Cs2NaYF6:Pr3+ can be elucidated by comparison with the spectrum of Cs2NaYCl6:Pr3+, where a corresponding band situated between the 4f2 f 4f15d1 T2g,Eg bands at 200210 nm was not conclusively assigned, with two candidates being the CT and 4f2 f 4f16s1 transitions. Subsequently, Srivastava et al.39 assigned the ClPr3+ CT band at 217 nm ( ̅ν(CT) = 46500 cm1) for LuCl3:Pr3+. Their rationale was based upon the optical electronegativity (χopt) concept of Jørgensen40 νðCTÞ ¼ 30000½χopt ðXÞ χuncorr ðMÞ
ð1Þ
since the BrPr3+ CT band is observed at 250 nm (40000 cm1) in LaBr3:Pr3+. This assignment for Cs2NaYCl6: Pr3+ accounts for the rather lower ν1 progression frequency 8874
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The Journal of Physical Chemistry A (276 cm1) observed for this band (corresponding to PrCl64) than in the ground state of PrCl63. Taking the derived χopt(Pr) value (1.47) from the observed XPr3+ (X = Br, Cl) CT bands, and employing χopt(F) = 3.9,41 then substitution into eq 1 predicts the FPr3+ CT band to be at 137 nm (72900 cm1). An alternative justification employs the difference in the CT band energies of ClEu3+ (37453 cm1; 267 nm) and FEu3+ (60200 cm1; 166 nm) = 22747 cm1, together with the above ClPr3+ CT energy of (210 nm, 47619 cm1), to predict the FPr3+ CT band at 22747 + 47619 = 70366 cm1 (142.7 nm). Note that these predictions are rather different from the calculated FPr3+ CT band wavelength of 125 nm (80000 cm1),25 where the difference between the Eu2+ and Pr2+ ground state energies in fluoride hosts was taken as 20970 cm1 (e.g., as in Figure 7 of ref 30). The room temperature emission spectrum measured under 209.5 nm excitation is plotted together with those using other excitation wavelengths in Figure 4a. The emission spectrum features a broad band ranging from ∼225 to 325 nm with two peaks at 252 and 279 nm. The low temperature emission spectrum using 210 nm excitation is very similar to the room temperature spectrum and is plotted in Figure 4b. The two strongest bands in the 80 K emission spectrum of Cs2NaYCl6: Pr3+, at 278 and 302 nm, were assigned to transitions from the lowest 4f5d level to the 4f2 3H5 and 3H6,3F2 multiplets, respectively,20 so the fluoride bands are analogously assigned. Notice that the emission maxima most likely correspond to the first members of the ν1 progression on the zero phonon line and that the lowest energy transition in absorption is 4f2 Γ1g f T2g Γ4u, whereas that in emission is T2g Γ3u f 4f2 Γ4u. The above assignments are consistent with this energy gap between bands in emission and absorption. The emission spectra at both room temperature and low temperature under other excitation wavelengths presented in panels a and b of Figure 4 show almost identical bands in the ∼225325 nm range, but new features between 350 and 425 nm occur for short excitation wavelengths. This 350425 nm band becomes comparable in strength with the ∼225325 nm features under 75 nm excitation. Emission lifetimes in Cs2NaYF6:Pr3+. The emission decay curves were measured for the 252 nm emission of Cs2NaYF6: Pr3+ using various excitation wavelengths. Under “nonhost” excitation the decay of the 252 nm emission can be fitted within experimental error by a single-exponential decay function with the lifetime ∼30 ns for both room temperature and low temperature. This value is comparable with those between 18.8 and 26.0 ns for four other fluoride hosts doped with Pr3+ (Table 1).42 The single-exponential character and lifetime value indicates that the d f f emission at 252 nm is dominated by a single Pr3+ site.
’ CONCLUSIONS This work has presented the vacuum ultraviolet excitation and emission spectra of Ce3+ and Pr3+ in elpasolite fluoride hosts, also with attention to the energy level assignments and emission decay kinetics. The 5d4f emission of Ce3+ in Cs2NaYF6 is inhomogenously broadened due to emission from Ce3+ ions of different local structures, whereas the emission of Pr3+ only exhibits homogeneous broadening. Spectral assignments have been made where possible and in particular these have elucidated the 5d1 Γ8,Γ7 (T2g) levels of
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Ce3+ and the 4f15d1 T1u (T2g) levels of Pr3+. The 5d1 Eg levels of Ce3+ have been estimated to lie within the conduction band. An understanding of the CT transitions of Pr3+ in elpasolite chloride and fluoride hosts has been achieved. The emission spectra under most excitations are purely due to 5d4f and not 4f4f transitions and are analogous to the corresponding spectra of these ions doped into Cs2NaYCl6.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful for the assistance given by Dr. Aleksei Kotlov, Station Scientist, Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronensynchrotron DESY. P.A.T. acknowledges financial support for this work under the Hong Kong Research Grants Council GRF Research Grant CityU 102609. C.K.D. is partially supported by the National Science Foundation of China, under Grant Nos. 11074245 and 11074315. V.M. acknowledges the partial support by DFG Grant 436 RUS 113/437/0-3, RFBR Grants 10-02-91167-NNSF and 10-03-90305, and European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement 226716 is also acknowledged. ’ REFERENCES (1) Piejzel, P. S.; Vergeer, P.; Meijerink, A.; Reid, M. F.; Boatner, L.; Burdick, G. W. Phys. Rev. B 2005, 71, 045116. (2) van Pieterson, L.; Reid, M. F.; Burdick, G. W.; Meijerink, A. Phys. Rev. B 2002, 65, 045113. (3) G^acon, J. C. Opt. Mater. 2003, 24, 209. (4) Malkin, B. Z.; Solovyev, O. V.; Malishev, A.; Saikin, S. K. J. Lumin. 2007, 125, 175. (5) Kirikova, N.; Kirm, M.; Krupa, J. C.; Makhov, V. N.; Negodin, E.; Gesland, J. Y. J. Lumin. 2004, 110, 135. (6) Burdick, G. W.; Reid, M. F. Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., B€unzli, J.-C., Pecharsky, V. K., Eds.; Elsevier Science B.V.: Amsterdam, 2007; Vol. 37 Chapter 232. (7) van Pieterson, L.; Reid, M. F.; Wegh, R. T.; Soverna, S.; Meijerink, A. Phys. Rev. B 2002, 65, 045113. (8) van Pieterson, L.; Wegh, R. T.; Meijerink, A.; Reid, M. F. J. Chem. Phys. 2001, 115, 9382. (9) Reid, M. F.; van Pieterson, L.; Wegh, R. T.; Meijerink, A. Phys. Rev. B 2000, 62, 14744. (10) Oskam, K. D.; Houtepen, A. J.; Meijerink, A. J. Lumin. 2002, 97, 107. (11) Sarantopoulou, E.; Kobe, S.; Kollia, Z.; Podmiljsak, B.; McGuiness, P. J.; Drazic, G.; Cefalas, A. J. Magn. Magn. Mater. 2003, 267, 182. (12) van Pieterson, L.; Dullens, R. P. A.; Piejzel, P. S.; Meijerink, A.; Jones, G. J. Chem. Phys. 2001, 115, 9393. (13) Yamaga, M.; Honda, M.; Kawamata, N.; Fujita, T.; Shimamura, K.; Fukuda, T. J. Phys.: Condens. Matter 2001, 13, 3461. (14) K€uck, S.; Sokolska, I. J. Phys.: Condens. Matter 2006, 18, 5447. (15) Nicolas, S.; Descroix, E.; Joubert, M. F.; Guyot, Y.; Laroche, M.; Moncorge, R.; Abdulsabirov, R. Y.; Naumov., A. K.; Semashko, V. V.; Tkachuk, A. M.; Malinowski, M. Opt. Mater. 2003, 22, 139. (16) Pawlik, Th.; Spaeth, J. -M. J. Appl. Phys. 1997, 82, 4236. (17) Duan, C. K.; Reid, M. F.; Xia, S. D. J. Lumin. 2007, 122123, 939. (18) Ruiperez, F.; Barandiaran, Z.; Seijo, L. J. Chem. Phys. 2005, 123, 244703. 8875
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