Electronic Spectra and Crystal Field Analysis of Tb3+ in Cs2NaTb

Article pubs.acs.org/JPCC

Electronic Spectra and Crystal Field Analysis of Tb3+ in Cs2NaTb(NO2)6: Tb3+ Situated at a Site of Th Symmetry Peter A. Tanner,*,† Li Wenyu,‡ and Lixin Ning§ †

Department of Science and Environmental Studies, Faculty of Arts and Sciences, The Hong Kong Institute of Education, Tai Po, Hong Kong S.A.R., 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 § Department of Physics, Anhui Normal University, Wuhu, Anhui 241000, People’s Republic of China

ABSTRACT: The assignment of the low-temperature 5D4 electronic emission spectra of Cs2NaTb(14NO2)6 and Cs2NaTb(15NO2)6 enabled the locations of 7FJ (J = 0−6) and 5D4 crystal field levels to be identified for Tb3+ at the Th site symmetry in these hexanitrito complexes. The complex vibronic spectra comprised metal−ligand, lattice mode, and two center NO2− transitions, and the 14,15N vibrational energy shifts of the latter were most helpful in spectral assignments. Variabletemperature studies enabled symmetry representations to be assigned to some crystal field levels because thermally populated upper 5D4 levels also produced emission. The crystal field analysis of the energy level data set by five adjustable parameters yielded a mean deviation of 16.6 cm−1. All of the three independent crystal field parameters were negative, with B62 dominant. The comparison of energy parameters with those from previous studies of Cs2NaLn(NO2)6 (Ln = Pr, Eu) systems showed (i) a reasonably linear increase with lanthanide atomic number Z (Z2) for the Slater (spin orbit coupling) parameter; and (ii) irregular trends for the crystal field parameters for the three systems, because these are biased by the inclusion of a small number of crystal field levels from several multiplet terms in the data sets.

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INTRODUCTION There is a striking difference between the spectra of lanthanide ions (Ln3+) situated at centrosymmetric and noncentrosymmetric sites in crystals. In the luminescence of the latter systems, sharp electronic transitions are observed from the initial state(s) to the crystal field levels of terminal multiplet terms. The transition mechanism mainly derives from the Judd “forced electric dipole” mechanism,1 where the crystal field mixes opposite-parity 4fN−15d states into 4fN states to satisfy the transition parity selection rule. The emission is generally intense, and the excitedstate lifetimes are often in the microsecond range. By contrast, the Judd mechanism is forbidden for pure electronic transitions in Ln3+ centrosymmetric systems so that the emission spectra comprise sharp, generally weak magnetic dipole allowed zero phonon lines, and associated broad vibronic sidebands. Lifetimes of milliseconds, or even seconds, may occur for such systems, such as in the Cs2NaLnCl6 series, where Ln3+ ions are situated at Oh symmetry sites.2 The presence of vibronic structure tends to © 2012 American Chemical Society

make the electronic spectra very complex, particularly at low temperatures where bands are more clearly resolved. Studies of the oneand two-photon spectra of the Cs2NaLnCl6 systems have enabled the 4fN energy level schemes to be sufficiently well-located to provide a test for computational methods. At present, the only calculations that can accurately simulate the 4fN crystal field energy levels are those employing a semiempirical Hamiltonian, with “atomic” and “crystal field” components.3 The comparison of derived crystal field parameters, Bkq, for Ln3+ in Cs2NaLnCl6 systems has enabled a basic interpretation of the physical meaning of the parameters.4 We have extended the study of the spectra of Ln3+ at centrosymmetric sites by investigating the luminescence and absortption spectra of the hexanitrito complexes, Cs2NaLn(NO2)6, (Ln = Pr,5 Eu6) where Ln3+ ions occupy Th symmetry sites with Received: February 13, 2012 Revised: May 15, 2012 Published: May 16, 2012 12764

dx.doi.org/10.1021/jp3014628 | J. Phys. Chem. C 2012, 116, 12764−12771

The Journal of Physical Chemistry C

Article

bidentate O-coordination to six NO2− ions.7 Whereas the crystal field of Cs2NaLnCl6 systems can be modeled by just two adjustable crystal field parameters of fourth and sixth rank, B40 and B60, three independent parameters (B40, B60, B62) are required for the calculations involving hexanitrito systems. Thus far, it has been observed that the dominant parameter is the sixth rank B62, which represents a very different scenario from the dominance of the parameter B40 for Cs2NaLnCl6 systems. In this final study of Cs2NaLn(NO2)6 systems, we present and interpret the electronic spectra of Ln = Tb. An understanding of the complex emission spectra was only achieved through the thorough knowledge of the vibrational behavior of this system. It was necessary to employ 14N, 15N substitution in the NO2− group, and to perform experiments at a low temperature to resolve individual components in the vibronic sidebands. From the derived partial 4f8 energy level scheme of Tb3+, a comparison of the fitting parameters has then been made with those of Ln = Pr, Eu, and some conclusions are presented.

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EXPERIMENTAL SECTION The hexanitrito complexes Cs2NaTb(14NO2)6, Cs2NaTb(15NO2)6, and Cs2NaY(14NO2)6:Tb3+ (1 at. %) were prepared from solutions of the respective hexachloroelpasolite systems2 by mixing with sodium nitrite solution. The starting materials were Tb4O7, CsCl, and NaCl (Strem Chemicals, 99.999%) and concentrated HCl (Aristar, Reidel de Hahn). The aqueous solutions were kept in the refrigerator for crystallization at 4 °C. After a few days, transparent crystals were obtained, which were removed from the mother liquor and dried. The 15N crystals were prepared by using sodium nitrite-15N (99%: Shanghai Research Institute of Chemical Industry). Infrared (IR) Nujol mull spectra were recorded at room temperature in the range from 400 to 4000 cm−1 using a Nicolet FT-IR instrument with resolution 4 cm−1. FT-Raman spectra were recorded at room temperature by a Perkin-Elmer Spectrum 2000 spectrometer using a resolution of 4 cm−1. Electronic emission spectra were recorded at resolution of between 2 and 4 cm−1 by an Acton 0.5 m monochromator having a 1200 groove mm−1 grating blazed at 500 nm, and a back-illuminated SpectruMM CCD detector, using an Optical Parametric oscillator (Panther) pumped by the third harmonic of a Surelite Nd:YAG pulsed laser. The sample was housed in an Oxford Instruments closed cycle cryostat or an Oxford Instruments Optistat CF continuous flow top loading static cryostat. A xenon lamp was employed to record the absorption spectrum at 10 K.

Figure 1. 300 K FT-Raman spectra of (a) Cs2NaTb(NO2)6 and (b) Cs2NaY(NO2)6:Tb3+ (1 at. %).

point group: A, E, and T, with repeating irrep levels being prefixed by a, b, etc. The calculations employed the full basis set of multiplet terms of Tb3+. The details of such calculations can be found in the publication of Prof. Liu.10 The values of the interaction parameters, including Fk (k = 2, 4, 6) for Coulomb interaction between the 4f electrons, ζ(4f) for the spin−orbit interaction, and Bkq for the crystal-field interaction,11 were varied simultaneously within certain allowed ranges, and optimized until the best agreement was obtained between the calculated and observed energy levels. The values of the configuration interaction parameters α, β, and γ, as well as the k = 0 magnetic interaction parameters Mk (M2 = 0.56 M0; M4 = 0.38 M0), were held constant and equal to those of the elpasolite compound Cs2NaTbCl6.4 It was discovered that the free variation of Slater parameters in the energy level fittings produced Fk parameters with unusual ratios. The F2:F4:F6 ratios were therefore fixed at the same values as in Cs2NaTbCl6,4 so that the number of freely varying parameters was then reduced to 5. In view of the extensive computer time involved when using the full basis of SLJ multiplets of Tb3+, the number of multiplet terms utilized in the magnetic dipole intensity calculations was restricted to the lowest 150. Such calculations gave rather poor agreement with experiment, except for the prediction of greatest

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THEORETICAL SECTION DFT Calculations on Vibrational Frequencies. The firstprinciples calculations were carried out using the DFT planewave code VASP8 with the GGA-PBE exchange-correlation functional.9 These calculations have previously been described.5,6 Because of the complexity associated with the 4f8 electrons of Tb3+, the calculations were restricted to the closed-shell systems Cs2NaLa(14NO2)6 and Cs2NaLa(15NO2)6. In the following text, the Cs2NaLn(14NO2)6 systems are abbreviated to Cs2NaLn(NO2)6. Energy Level and Magnetic Dipole Transition Intensity Calculations. The calculation of electronic energy levels for 4f8 configuration employed the f-shell programs of Prof. M. F. Reid, in which the energy levels were derived by simultaneous diagonalization of interaction Hamiltonians. The energy levels are labeled by the irreducible representations (irreps) of the Th 12765



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Table 1. (a) Calculated Vibrational Frequencies (cm−1) for Cs2NaLa(xNO2)6 (x = 14, 15) and Experimental Raman, Infrared, and Vibronic Data for Cs2NaTb(NO2)6; and (b) Assignment of Low Frequency Vibrations (cm−1) from Experiment in Cs2NaLn(NO2)6 (Ln = Pr, Eu, Tb) Systemsa (a) Cs2NaTb (NO2)6 experiment

calculation symm

type (major contribution)

Cs2NaLa (14NO2)6

Ag Tu Eg Tg Tu Ag Eg Tu Tu Tg Ag Tu Tg Tu Au Eg Tu Tg Tu Tg Eu Tu Tg Tu Tg Tu

N−O sym str N−O antisym str N−O str N−O str N−O str NO2 sciss NO2 sciss NO2 sciss NO2 wag NO2 wag Ln−O sym str Ln−O antisym str NO2 rock NO2 rock NO2 twist Ln−NO2 str Ln−NO2 str NO2 bend Ln−NO2 bend Ln−NO2 bend NO2 bend Ln−NO2 bend Ln−NO2 rot Cs−LnNa(NO2) trans Cs trans acoustic

1325 1318 1316 1244 1228 814 804 804 292 288 221 210 192 183 175 169 157 133 122 112 100 92 65 55 46 2

symm

type

Tu Tg Ag Tu Tg Tu Au Eg Tu Tg Tu Tg Eu Tu Tg Tu Tg Tu

NO2 wag NO2 wag Ln−O sym str Ln−O antisym str NO2 rock NO2 rock NO2 twist Ln−NO2 str Ln−NO2 str NO2 asym bend Ln−NO2 bend Ln−NO2 bend NO2 bend Ln−NO2 bend Ln−NO2 rot Cs−LnNa(NO2) trans Cs trans acoustic

IR/R Ln = Pr

Cs2NaLa (15NO2)6

vibronic Ln = Pr

70

1338

1256 1246

841

162 162 138 140 116

134

109, 123 77 56 43

77 69

1335 1279 1259 1245

843 338

334 240 209, 230, 256 176, 188 164 144 138

122 75 57 45

326 325 230 212, 234 209 180, 190

186

3 K vibronic

848 845

vibronic Ln = Eu

219, 231, 240

300 K infrared 1333

321 323 239 222, 248 192 179

158

100

23 24 24 25 25 4 4 5 4 5 3 2 0 0 0 3 2 0 1 −1 0 1 −1 0 0 0 IR/R Ln = Eu

153vs 130ms 140

300 K Raman

1302 1294 1292 1219 1203 810 800 799 288 283 218 208 192 183 175 166 155 133 121 113 100 91 66 55 46 2 (b) 305

302 m 239w 230s 198m 182

Δ(14,15N)

IR/R Ln = Tb

vibronic Ln = Tb 338

334 240 209, 230, 256 176, 188 164 144

138

138

117 77 57 46

122 75 57 45

a symm, symmetry; sym, symmetric; str, stretch; antisym, antisymmetric; sciss, scissor; rot, rotational; trans, translational; IR, infrared (data in italics); R, Raman.

intensity for 5D4 → 7F3,7F5 transitions, so the results are not reported here.

modes are doubly or triply degenerate. The Ln(NO2)63− moiety modes transform as 3Ag + Au + 3Eg + Eu + 5Tg + 8Tu (Th). The vibrational spectra of several lanthanide hexanitrito systems have previously been reported by Barnes and Peacock12 and Bünzli et al.,13 but no data are available for Ln = Tb. The Raman spectra of Cs2NaTb(NO2)6 and Cs2NaY(NO2)6:Tb3+ are shown in

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RESULTS AND DISCUSSION Vibrational Spectra. There are 66 vibrational degrees of freedom in the Cs2NaLn(NO2)6 crystal, and some vibrational 12766



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Figure 2. The 10 K electronic absorption spectrum of Cs2NaTb(NO2)6 between 20 700 and 28 200 cm−1. The zero phonon line (0−0) and members of the Ag NO2− scissor mode progression are indicated.

Figure 4. Survey 10 K emission spectrum of Cs2NaTb(NO2)6 between 21 347 and 14 280 cm−1. The luminescent state is 5D4, and terminal multiplets are marked.

wavenumbers for Cs2NaLa(NO2)6 with the experimental IR and Raman frequencies of Cs2NaTb(NO2)6. The description of the vibrational motions is included in the table (column 2), and these are categorized as internal NO2− modes, Ln−ligand modes, and external (lattice) modes, although many vibrations are of mixed character. The N−O stretching vibrations in Cs2NaTb(NO2)6 (columns 6−8: 1246−1338 cm−1) are located at highest energy, and there is a gap of ∼500 cm−1 below the NO2− scissoring (841−848 cm−1) vibrations to the lower energy modes. Column 5, Table 1a, lists the calculated 14N−15N frequency shifts for the vibrational modes of Cs2NaLa(NO2)6, and these are ∼25 cm−1 for the N−O stretching modes, ∼5 cm−1 for NO2− scissoring, and rather less for lower energy vibrations. The detailed assignment of low energy vibrations in the hexanitrito systems requires the use of polarized Raman studies and far-infrared data. However, with the assumption that the prominent vibronic structures in the luminescence spectra of these compounds correspond to the Tu νas(Ln−O) and the two Tu δ(Ln−NO2) modes, a consistent interpretation of the present data sets for Ln = Pr, Eu, Tb is presented in Table 1b. This assumption is based upon the assignment of the corresponding strongest vibronic structure in the spectra of Cs2NaTbCl6, because the above Tu (Th) vibrations closely correspond to the T1u (Oh) stretch, bend, and T2u (Oh) bend vibrations of the corresponding hexachloroelpasolite. Electronic Absorption Spectrum. The 10 K ultraviolet absorption spectrum of Cs2NaTb(NO2)6 is shown in Figure 2. The bands are due to the 1A1 → 3B1 transition of the NO2− anion, which exhibits a strong progression in the frequency of ∼650 cm−1, corresponding to the Ag NO2− scissor mode in the triplet state. The quanta of this vibration are labeled in the spectrum. The spectrum is considerably more complex than that of NaNO2,14 and the interpretation is out of the scope of the present work. The major point of interest is that the lowest energy zero phonon line in Figure 2 is at 20 992 cm−1 so that the study of 4f8−4f8 transitions of Tb3+ above this energy is precluded. Some very weak bands (not shown) are observed between 20 557 and 20 596 cm−1, and these correspond to vibronic structure of the 7F6 → 5D4 transition of Tb3+. Identification of Symmetry Representations of Tb3+ Crystal Field Levels from Luminescence Hot Bands. The

Figure 3. Emission spectra of Cs2NaTb(15NO2)6 at 7, 20, and 40 K: (a) between 16 200 and 16 050 cm−1; and (b) between 18 630 and 18 250 cm−1.

Figure 1. The vibrational wavenumbers are several cm−1 higher in the latter case. Table 1a compares the calculated vibrational 12767



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Figure 5. Emission spectra of Cs2NaTb(14NO2)6 at 3 K (lower panel) and Cs2NaTb(15NO2)6 at 8 K (upper panel) between 20 450 and 13 200 cm−1. The initial luminescent state is 5D4 aT. The terminal crystal field states and derived vibrational energies are indicated.

highest luminescent multiplet of Tb3+ in Cs2NaTb(NO2)6 is therefore restricted to be below 20 992 cm−1 and is 5D4, for which calculation predicts the lowest level to be aT, at 20 414 cm−1 (Table 3) with three higher levels up to 20 463 cm−1. Emission transitions from the aT crystal field level are potentially magnetic dipole allowed to all lower states by the Th site transition selection rules, but the intensities are greatly modified by the free ion ΔS = 0; ΔL = 0; |ΔJ| ≤ 1 selection rules. If the lowest 5D4 level corresponded to the E or A (Th) irrep, then magnetic dipole transitions would only be allowed to terminal T (Th) levels. Inspection of the low-temperature emission spectra indicates that the zero phonon lines are in fact observed for nearly all emission transitions so that the assignment of the lowest 5D4 aT level is reasonable. Note that the next highest levels are calculated to be A, E ≈ bT so that only the bT level has magnetic dipole intensity in emission transitions to lower A and E crystal field levels. Thus, the examination of luminescence spectra when the 5 D4 levels are thermally populated can provide information about the assignment of lower levels with T irreps. This is illustrated in Figure 3 for several cases. Figure 3a shows the 5D4 → 7F3 transition of Cs2NaTb(15NO2)6 between 16 050 and 16 200 cm−1 at three temperatures. At the lowest temperature of 7 K, the 5D4 aT → aT 7F3 transition is observed. At more elevated temperatures, the magnetic dipole

transitions from the excited A, E, and bT (5D4) levels are also observed because the terminal level is aT. The transitions from 5D4 E, bT are the strongest at 20 and 40 K, as shown in the figure. The energy separations of the excited 5D4 levels are marked. The scenario differs for the 5D4 → E 7F5 transitions (Figure 3b) where only the aT, bT → E transitions are magnetic dipole allowed. By contrast, all three hot bands are observed for 5D4 → bT 7F5, where the weak transition from 5D4 aT is subsequently more clearly displayed in Figure 5c. Assignment of Emission Spectra. The room-temperature emission spectrum of Cs2NaTb(NO2)6 has maximum intensity at ∼545 nm, but because the features are not well-resolved it is not further discussed. A 10 K survey emission spectrum is shown in Figure 4. The most intense bands, in the region of 18 500− 18 000 cm−1, are due to the 5D4 → 7F5 transition. The transitions to 7FJ (J = 0−2) are too weak to be identified in the figure, but these are clearly shown in Figure 5. The low-temperature 5D4 emission spectra of Cs2NaTbx ( NO2)6 (x = 14, 15) are displayed in Figure 5a−i, and except for a few hot bands (two of which are starred in Figure 5a,b for Cs2NaTb(15NO2)6), all of the transitions originate from 5D4 aT. The terminal crystal field states and vibrational displacements from zero phonon lines are indicated in the figures. The 15N 12768



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spectra exhibit the expected vibrational energy shifts, as compared to the 14N spectra, but the positions of electronic origins are very similar to those in the 14N spectra. The derived vibrational displacements are included in Table 1, and, as mentioned above, whereas the assignments are firm for those with energies above 300 cm−1, the assignments for the lower energy modes are tentative. A brief mention is now made of some salient points in the assignments of electronic transitions. The emission spectrum of Cs2NaY(NO2)6:Tb3+ was assigned analogously to that of Cs2NaTb(NO2)6 and served to confirm the interpretation. Because it did not present additional information, it is not discussed herein. The highest energy 5D4 aT → 7F6 transition is congested because there are six terminal levels (Table 3). Five of these levels are readily assigned from the one (Figure 5a) and two-center (Figure 5b) vibronic transitions, and the remaining unassigned bands are assigned to vibronic structure at 121, 138, and 234 cm−1 of the aT → A transition (Figure 5a). The temperature dependence of the emission spectra enables the ground and first excited crystal field states to be assigned to the E and aT irreps, respectively. The 5D4 aT → aT 7F5 zero phonon line is weak (Figure 5c), but a detailed vibronic sideband can be assigned at lower energy, including the N−O scissor and stretching modes (Figure 5d). The 5D4 aT → E 7F5 zero phonon line is intense (Figure 5c). The assignment of the remaining two 7F5 levels is less secure because the two-center NO2− transitions are silent. The region 17 281− 16 411 cm−1, corresponding to the 5D4 aT → 7F4 zero phonon lines and associated vibronic sidebands, is more complex (Figure 5d,e) partly due to overlap with two-center NO2− transitions. The one-phonon sideband of 5D4 aT → 7F3, between 16 082 and 15 609 cm−1 (Figure 5f), enables clear assignments to be made to the aT and A crystal field levels. Every band in the transitions to the highest 7F3 level, bT, is doubled, and assigned to transitions to bT and b′T levels. These two levels are located at 135, 158 cm−1 above the lowest 7F3 level, aT. The splitting is attributed to the resonance of the aT + Tg (Tb-NO2 bend, 144 cm−1) electron−phonon state with the (unperturbed) bT level. The 5D4 aT → 7F2, 7F1, 7F0 transitions (Figure 5g−i) are less cluttered, and all structure is readily assigned. Energy Level Calculations. The spectral analysis permits the assignment of only the 5D4 and 7FJ (J = 0−6) crystal field energy levels. The energy level calculations employed F2, ζ(4f), B40, B60, and B62 as variable parameters, with other parameters fixed at the values for Cs2NaTbCl6,4 except for Fk where the parameter ratios were constrained. The other crystal field parameters are constrained by the ratios B44 = ±(5/14)1/2B40, B64 = ∓(7/2)1/2B60, and B66 = −(5/11)1/2B60. The resulting energy level fit is given in Table 2, and the parameter values are listed in Table 3. The standard error, δ1, for fitting 25 levels with five adjustable parameters is 18.6 cm−1. The 7F6 aT and E crystal field states are calculated to be very close, with aT located as the electronic ground state. The crystal field parameters were varied in the ranges (in cm−1) of −600 < B40 < −200, −800 < B60 < −400, and −1500 < B62 < −1000, and it was found that although the ground state was calculated to be E for some parameter values, the gap between E and aT was always less than 5 cm−1. It is noted that from magnetic susceptibility measurements, Roser and Coruccini15 assigned the A irrep to the electronic ground state. However, in the emission, hot band measurements serve to confirm the order E < aT. The only other discrepancy in the order of irreps for the energy levels occurs for the 5D4 E and bT crystal field levels. These levels are calculated to be very close so

Table 2. Calculated and Experimentally Derived Energy Levels of Tb3+ in Cs2NaTb(NO2)6 Ebarycenter SLJ Γ 7

F6 E T A bT cT A 7 F5 aT E bT cT 7 F4 aT A bT E 7 F3 aT A bT 7 F2 E T 7 F1 T 7 F0 A 5 D4 aT A E bT

Ecal − Eexpt Cs2NaTb(NO2)6 Cs2NaTbCl6

Eexpt

Ecal

0 27 44 90 105 174 1911 2100 2109 2284 3146 3230 3645 3680 4345 4459 4491 5060 5101 5512 5749 20 427 20 434 20 455 20 470

0 −3 38 78 124 175 1894 2111 2136 2265 3152 3196 3622 3652 4361 4463 4487 5056 5116 5533 5750 20 414 20 439 20 467 20 463

0 −30 −6 −12 19 1 −17 11 27 −19 6 −34 −23 −28 16 4 −4 −4 15 21 1 −13 5 12 −7

0

0

2033

2055.5

3372.3

3294

4355.9

4258.7

5016.6

4997.8

5444 5681 20 380

5430.8 5650.3 20 320

Table 3. Parameter Values from the Energy Level Fit of Cs2NaTb(NO2)6a parameter

value

F2 F4 F6 ζ(4f) α β γ M0 P0 B40 B60 B62 T2 T3 T4 T6 T7 T8 δ1 δ2 N np

88 774.46 [63 273.11] [45 199.52] 1706.92 [19] [−568] [1754] [4.29] [854] −421.52 −612.34 −1218.06 [105] [40] [45] [−365] [320] [139] 18.6 16.6 25 5

a Parameter values are in cm−1 except for N, np, which are dimensionless. Parameter values in square brackets were held constant in the calculation. N, number of levels fitted; np, number of parameters employed.

δ1 = 12769

(Ecal − Eexp)2 N − np

, δ2 =

(Ecal − Eexp)2 N



Article

The 2S+1LJ experimentally derived barycenter energies of the FJ and 5D4 multiplets are presented in column 5 of Table 2, and the comparison is made with the corresponding values for Cs2NaTbCl6 in column 6, where the 7FJ barycenters do not differ from those of the hexanitrito complex by more than 30 cm−1. The Slater parameter F2 (Table 3) is slightly greater for the hexanitrito complex than for the hexachloroelpasolite, but the difference is well within the computational error. Figure 6a compares the calculated Slater parameters for the hexanitrito systems Cs2NaLn(NO2)6 (Ln = Pr, Eu, Tb) by plotting the values of Fk against atomic number, Z, of Ln. An increase of Fk is noted with increasing Z, in an almost linear manner. The plot also includes the corresponding values for the Cs2NaLnCl6 systems, and the differences from the NO2− complexes are fairly small. Figure 6b displays the linear variation of the 4f spin−orbit coupling constant with Z2 across the Cs2NaLn(NO2)6 series. The trends in derived crystal field parameters across the Cs2NaLn(NO2)6 (Ln = Pr, Eu, Tb) series are shown in Figure 6c. Individual Bkq parameter values for different Ln agree to within a few hundred cm−1, but unlike the corresponding plot for the Cs2NaLnCl6 series,4 no clear trends are discerned. The considerable scattering in the Bkq plots results from the sparse numbers of multiplet term data available for energy level fittings of the Cs2NaLn(NO2)6 complexes. It is well-known that crystal field parameters exhibit variation for different multiplets of a given lanthanide ion so that acceptable mean values of the parameters are only achieved by fitting large numbers of energy levels. Another, probably less-important, reason for the scatter of crystal field parameter values lies in the derived data sets themselves. There are many vibrational degrees of freedom so that the electronic energy of a given crystal field state may be perturbed by resonance with an electron−phonon coupled state. In that case, the perturbed energy should be corrected prior to the data parametrization, but this is not a trivial task. 7

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CONCLUSIONS The complex luminescence spectra of Cs2NaTb(NO2)6 have been interpreted and assigned by employing 15N isotopic substitution, recording spectra at variable temperatures, and understanding the vibrational behavior of the complex. The data were restricted to two multiplets, 5D4 and 7FJ, due to the superposition of NO2− electronic absorption transitions with Tb3+ transitions to levels above 20 500 cm−1. This has produced uncertainties in the derived parameter values from the energy level fittings, although the energy levels themselves were reasonably well-replicated by calculation. Just as for the Ln = Pr, Eu systems, the crystal field is dominated by the parameter B62, with other Bkq parameters also negative. The Slater and spin−orbit coupling parameters derived from the energy level fittings of the Ln = Pr, Eu, Tb complexes exhibit regular behavior.

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k

Figure 6. (a) Plot of Slater parameters, F , against atomic number (Z) of Ln in the Cs2NaLn(NO2)6 and Cs2NaLnCl6 complexes. The latter are denoted by Cl in the legend, and two calculations for Ln = Eu are shown for Cs2NaEu(NO2)6.4−6 (b) Variation of spin−orbit coupling constant, ζ(4f), against Z2 for Ln = Pr, Eu, Tb in the Cs2NaLn(NO2)6 series. The linear fit is shown. (c) Plot of crystal field parameter values, Bkq, against atomic number for the Cs2NaLn(NO2)6 series.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation of China (Grant 11174005) is gratefully acknowledged by L.N.

that the reversal of the order is also within the mean deviation of the calculation. 12770



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REFERENCES

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Electronic Spectra and Crystal Field Analysis of Tb3+ in Cs2NaTb

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