Optical Spectra and Energy Levels Analysis of the 4f - American

Article pubs.acs.org/JPCA

Optical Spectra and Energy Levels Analysis of the 4fN Ions Doped into Ba2YCl7 M. Karbowiak,†,* J. Cichos,† and C. Rudowicz‡ †

Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland Institute of Physics, West Pomeranian University of Technology, Al. Piastów 17, 70-310 Szczecin, Poland

‡

S Supporting Information *

ABSTRACT: Absorption emission and excitation spectra are measured and analyzed to achieve assignments of energy levels for 4fN ions in Ba2YCl7:RE3+ (RE = Pr, Nd, Tb, Dy, Ho, Er, and Tm) crystals. The experimental energy levels were analyzed in terms of the usual free-ion parameters and the crystal field (CF) ones, Bkq, in the Wybourne notation. The orthorhombic C2v symmetry is shown to be a good approximation of the actual triclinic C1 site symmetry at the metal center. The starting values of the CF parameters Bkq used for fittings for the studied crystals were obtained from superposition model analysis. A good agreement between the calculated and experimental energy levels was obtained with rms deviations in the range from 6.8 cm−1 (for Ho3+) to 14.1 cm−1 (for Pr3+). The fitted sets of Bkq parameters are, in general, consistent across the 4fN series. This study has enabled determination and discussion of the trends in variation of the free-ion parameters and CF ones across the 4fN series. The CF parameter set and energy level structure obtained for Nd3+ ion in Ba2YCl7 crystal are consistent with those for Nd3+ in structurally related RbY2Cl7 crystal. This systematic analysis of CF parameters is one of only few such studies encompassing nearly whole series of RE3+ ions.

1. INTRODUCTION Ba2RECl7 type of compounds exists for the trivalent rare-earth (RE) ions from Eu to Lu, and Y.1,2 Ba2ErCl7 or Er3+-doped Ba2YCl7 were shown to be highly efficient infrared to visible upconversion materials.1,3 The phonon energies are lower in heavier halide crystals than those in fluorides or oxides, and the multiphonon relaxation rates are suppressed. A general disadvantage of chloride matrices is their high moisture sensitivity. In this regard the unique virtue of Ba2LnCl7 compounds is that they are relatively air-stable materials. The up-conversion mechanism in Er3+-doped Ba2YCl7 was thoroughly studied.3,4 Also a comprehensive spectroscopic study of the possible room-temperature green laser transition 2H9/2 → 4 I13/2 in Ba2YCl7:3% Er3+ was reported.5 In spite of possible technological relevance of this material, the energy levels of RE3+ ions in Ba2RECl7 crystals have not been hitherto studied in detail. In a recent series of papers we have reported the results of the crystal field (CF) analysis for RE3+ ions in various laser materials, including oxides of the type ABCO4,6 ABC3O7,7 and ABO3,8 and fluorides BaY2F8, LiKYF5, and K2YF5.9 In the present paper we extend the analysis to chloride hosts, choosing only Ba2RECl7 due to its possible technological importance.3,5 Ba2RECl7 crystals are also suitable objects of basic scientific interest. Because these crystals exhibit the actual triclinic C1 site symmetry of the metal centers, they offer studies of low symmetry CF aspects,10,11 which are not-well-understood as yet. The ascent/descent in symmetry (ADS) method may be used to justify the approximation of the C1 site symmetry to the orthorhombic C2v symmetry. Moreover, Ba2RECl7 crystals form © 2012 American Chemical Society

isostructural group of compounds, whereas Ba2YCl7 can be activated with different RE3+ ions. This enables a systematic analysis of the optical spectra of the 4fN-ions and identification of possible trends in CF parameters (CFPs) across the RE3+ series. Up to now such systematic analysis for the series of RE3+ ions has been carried out only for a few systems that exhibit h i g h e r s y m m e t r y , e. g . , L a C l 3 : R E 3 + , L i Y F 4 : R E 3 + , Cs2NaRCl6:RE3+, Na3[RE(C4H4O5)3]·2NaClO4·6H2O, and RE(C2H5SO4)3·9H2O; for a review and references, see ref 12. The only low symmetry system investigated so far is LaF3, for which the actual C2 site symmetry of La 3+ ion was approximated as C2v.13 Energy levels analysis was also presented for Ce3+, Nd3+, Sm3+, Dy3+, Er3+, and Yb3+ ions in Y2O3.14 For this host, however, some complications arise from random distribution of RE3+ ions throughout the cation sites with C2 and C3i symmetry in the lattice. This paper reports analysis of low temperature absorption and emission spectra for Ba2YCl7 crystals doped with Pr3+, Nd 3+ , Tb 3+ , Dy 3+ , Ho 3+ , Er 3+ , and Tm 3+ ions. The experimentally resolved energy levels are analyzed utilizing simultaneous diagonalization of the free-ion and CF Hamiltonians. We aim at obtaining consistent sets of CFPs for all studied ions, because such sets can provide basis for prediction of the energies of yet unobserved transitions, which may be important, e.g., for elucidation of energy transfer processes. Received: August 28, 2012 Revised: October 3, 2012 Published: October 11, 2012 10574

dx.doi.org/10.1021/jp308539y | J. Phys. Chem. A 2012, 116, 10574−10588

The Journal of Physical Chemistry A

Article

Table 1. Spherical Polar Coordinates of Cl− Ligands in Ba2ErCl7 Crystals

2. EXPERIMENTAL ASPECTS The starting materials for the synthesis of Ba2YCl7:RE3+(1%) crystals were BaCl2, YCl3, and RECl3 (RE = Pr, Nd, Tb, Dy, Ho, Er, and Tm). Barium dichloride was obtained from BaCO3 by dissolution in concentrated HCl and evaporating the solution to dryness. The product was initially dried in a desiccator under P2O5 and finally by heating in vacuum at 400 °C. YCl3 and RECl3 were prepared from appropriate rare-earth oxides (99.99%) following the ammonium halide route.15 So obtained trichlorides were sublimed in a vacuum and passed through a Bridgman furnace for purification. Due to incongruent melting16 the nonstoichiometric mixture of BaCl2 and 5% excess of YCl3 was sealed under vacuum in a silica ampule and passed through a vertical Bridgman furnace. The Ba2YCl7 crystals thus obtained were mixed with RECl3 plus a 5% excess of YCl3, and the mixture was again passed through the furnace. The nominal doping concentration of RE3+ ions was 1 mol %. The structure and composition of synthesized crystals was checked by X-ray powder diffraction. Absorption spectra were recorded in the 3500−50 000 cm−1 range at 4.2 K on a Cary-5000 UV−vis−NIR spectrophotometer, equipped with an Oxford Instrument model CF1204 cryostat. Corrected emission and excitation spectra were recorded at 77 K on an Edinburgh Instruments FLSP 920 spectrofluorimeter using the Optistat DN liquid nitrogen cryostat (Oxford Instrument).

ligand

R (nm)

θ (deg)

φ (deg)

Cl1 Cl2 Cl3 Cl4 Cl5 Cl6 Cl7

0.2651 0.2651 0.2698 0.2701 0.2751 0.2769 0.2699

78.91 78.78 79.56 78.97 143.32 143.68 2.89

−134.10 132.64 40.86 −42.06 88.09 −86.25 5.18

Figure 1. ErCl7 polyhedra represented in the Cartesian (x, y, z) axis system suitable for the approximated C2v symmetry. The orientation of the CAS is also shown.

4. ANALYSIS OF ABSORPTION, EMISSION, AND EXCITATION SPECTRA Below we present the 4.2 and 77 K absorption, emission and excitation spectra of Ba2YCl7 crystals doped with respective 4fN ions. Because the BaCl2/RECl3 type systems contain Ba2RECl7 type compounds only with rare-earth cations equal to and smaller than Eu3+, one may consider that Pr3+ or Nd3+ can substitute also the Ba site symmetry in the doped Ba2YC7 crystals.19 However, we have not found any experimental indication for such assumption. The RE3+ ions occupy one type of crystallographic sites only, as evidenced by the observed single line in the absorption transitions to the nondegenerate 3 P0 level of Pr3+ or unsplit 2P1/2 level of Nd3+. In the following subsections, sample spectra are presented, briefly characterized, and thoroughly analyzed to enable determination of energy levels of Ln3+ ions in Ba2YCl7 crystals. Energy levels derived from this analysis will be subsequently included in the fittings carried out in section 5. 4.1. Ba2YCl7:Pr3+ (4f2). The emission and excitation spectra of Pr3+:Ba2YCl7 crystals were recorded at 77 K. The emission is observed from the 3P0 level. The next higher energy level, i.e., the lowest Stark component of the multiplet 3P1, is located about 500 cm−1 above 3P0, and is practically unpopulated at 77 K, whereas the hot 3P1−2S+1LJ bands do not interfere with the 3 P0−2S+1LJ transitions. This enables straightforward determination of the energy level structure of the multiplets 3H4, 3H5, 3 H6, 3F2, 3F3, and 3F4. The 77 K emission spectra recorded under 447.5 nm excitation are shown in Figure 2. In the figures below, if applicable, arrows indicate the experimentally identified electronic transitions, whereas the insets show the energy level structure of a given multiplet derived from emission spectra and the dotted lines represent the calculated levels in the cases when experimental levels are missing. The knowledge of the CF components of the ground 3H4 multiplet (Figure 2) has allowed resolving excitation spectra (discussed next; see below), which are due to the transitions to

3. STRUCTURAL DATA AND THE AXIS SYSTEMS USED All Ba2RECl7 compounds have structure described by the monoclinic space group P21/c. The lattice constants for Ba2ErCl7 are a = 0.6794(2) nm, b = 1.5525(2) nm, c = 1.0496(2) nm, and β = 90.54(2)°.1 The ErCl7 polyhedra are in the form of trigonal prisms capped on one rectangular face. All Er−Cl distances within the ErCl7 polyhedra are slightly different, ranging from 0.265 to 0.277 nm, thus resulting in slightly distorted polyhedra with C1 symmetry of the Er site. Due to the monoclinic crystal symmetry, instead of the nonCartesian CAS (a, b, c), a modified crystallographic axis system CAS* (a*, b, c) is chosen with the a*-axis perpendicular to the b−c plane and forming the angle 0.54° with the a-axis. For CF analysis we have chosen the Cartesian axis system CAS* defined as x = a*, y = b, and z = c and subsequently transformed it by the Euler angles: α = 22.8°, β = 247.2°, and γ = 47.5°, determined using the program SYMMOL17 included in the computer package WinGX.18 Such transformation ensures that the binary symmetry axis reveals along the z-axis together with approximated symmetry planes parallel to the x−z and y− z planes. Therefore, in so defined axis system the lanthanide ions in Ba2MCl7 crystals possess an approximated C2v site symmetry. The spherical polar coordinates of the chloride ligands in Ba2ErCl7, expressed in the axis system chosen for CF analysis, are listed in Table 1, whereas the ErCl7 polyhedron is depicted in Figure 1. Table 1 indicates that C2v symmetry is a good approximation of the actual C1 symmetry. The average structural changes required to obtain an idealized ErCl7 polyhedron with C2v symmetry amount only to 0.0019 nm (0.7%): for the interionic distances Ri and 0.2° and 2.0° for the angles θ and φ, respectively. 10575



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Figure 2. 77 K emission spectra recorded for Ba2YCl7:Pr3+ under 447.5 nm excitation, which show the transitions from 3P0 at 20 536 cm−1 to the multiplets: (a) 3H4, (b) 3H5, (c) 3H6, (d) 3F2, (e) 3F3, and (f) 3F4.

monitoring the 3P0−3F2 emission line at 650 nm. The low energy transition from the lowest CF component of the ground 3 H4 multiplet to the nondegenerate 3P0 excited multiplet is identified in the spectra at 20 536 cm−1 (marked with arrow). The hot bands observed at lower energies are shifted by 53, 113, 181, 258, and 369 cm−1, which conforms well to the energy level structure of the ground 3H4 multiplet. On the higher energy side of the 3H4−3P0 transitions some vibronic sidebands can be clearly observed. The consecutive (i = 1−4) sidebands are coupled to the lines at 20 536 cm−1 (νi) and 20 483 cm−1 (νi′) of electronic origin and are shifted with respect to these lines by 57 (i = 1), 99 (i = 2), 203 (i = 3), and 256 (i = 4). The energies of ν3 and ν4 vibronic sidebands correspond exactly to those observed also in Ba2YCl7 crystals doped with U3+ and U4+ ions.20 The combined analysis of the 77 K emission and excitation spectra (Figures 2 and 3) has enabled identification of 66 energy levels for Ba2YCl7:Pr3+, which were subsequently included in the fittings (section 5). Comparison of the experimental and calculated energy levels reveals that only the multiplets 1G4 and 1S0 have not been experimentally observed, probably due to the equipment limitations. 4.2. Ba2YCl7:Nd3+ (4f3). The energy levels of Nd3+ ions in Ba2YCl7 were derived from analysis of the 4 K absorption spectrum and the 77 K emission and excitation spectra. Figure 4 presents a sample 4.2 K absorption spectrum in the nearinfrared and visible (VIS) ranges. The observed absorption lines are due to the transitions from the lowest component of the ground 4I9/2 multiplet to the CF components of the excited SLJ multiplets labeled in Figure 4.

the multiplets 3P0, 1I6, 3P1, and 3P2, for which some hot bands appear. Figure 3 presents the excitation spectrum recorded in the 3H4−3P0,1,2 and 3H4−1I6 absorption transition range, while

Figure 3. 77 K excitation spectrum recorded for Ba2YCl7:Pr3+ while monitoring the 3P0−3F2 emission line at 650 nm (15 385 cm−1), which shows the transitions from the ground 3H4 multiplet to the excited multiplets 3P0, 1I6, 3P1, and 3P2. The numbers Hi indicate hot bands accompanying the 3H4−3P0 transition, whereas νi and νi′ are the vibronic sidebands coupled to the emission lines of electronic origin indicated by the arrow (i.e., the transitions from the lowest Stark component of 3H4 to 3P0 at 20 536 cm−1) and H1 (i.e., the transitions from the second lowest Stark component of 3H4 to 3P0 at 20 483 cm−1), respectively. 10576



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Figure 4. Sample absorption spectrum recorded for Ba2YCl7:Nd3+ at 4.2 K.

Figure 6. Sample emission spectrum recorded for Ba2YCl7:Nd3+ at 77 K under 582 nm excitation, which shows the transitions from the lowest level of the 4G5/2 multiplet to the CF levels of the ground 4I9/2 multiplet.

Analysis of the excitation spectrum presented in Figure 5 enables assignment of the energies of CF components arising

above the lowest one. Moreover, the former level is only weakly populated, thus yielding practically no interference with the hot emission bands also observed in the spectra. Figure 7 presents

Figure 5. Excitation spectra recorded for Ba2YCl7:Nd3+ at 77 K while monitoring the emission line at (a) 669 nm (4G5/2−4I11/2) and (b) 387 nm (4D5/2−4I11/2). The inset shows the enlarged part of the spectrum in the region of the 4I9/2−2P1/2 absorption transition. The hot bands are observed at 34, 116, and 166 cm−1 from the line at 23 121 cm−1.

Figure 7. 77 K emission spectra recorded for Ba2YCl7:Nd3+ in the 4 G5/2−4I11/2 transitions range. Arrows indicate transitions from the lowest level of the 4G5/2. Weak hot bands, originating from the level located at 124 cm−1 above the lowest one, were observed.

from the multiplets for which the absorption transitions are observed in the VIS and UV (ultraviolet) ranges, i.e., up to 34 100 cm−1. The inset in Figure 5 shows the enlarged part of the spectrum in the region of the 4I9/2−2P1/2 absorption transitions. The hot bands are observed at 34, 116, and 166 cm−1 from the line at 23 121 cm−1, which was assigned as due to the transition originating from the lowest component of the 4I9/2 multiplet. These energies correspond well to the energy level structure of the ground multiplet obtained from analysis of the emission spectrum presented in Figure 6. The spectrum in Figure 6 shows the 4G5/2−4I9/2 emission transitions excited at 582 nm, which enables identification of the fifth component of the 4I9/2 multiplet as at 311 cm−1. The emission originating from the 4G5/2 multiplet is particularly suitable for analysis of the arising transitions, because the next level of this multiplet is located 124 cm−1

the emission transitions from the 4G5/2 multiplet to the first excited 4I11/2 one, enabling determination of energies of its CF components. For the Ba2YCl7:Nd3+ crystal a total of 137 energy levels were determined in the range 0−35 000 cm−1, which were subsequently included in the fittings (section 5). 4.3. Ba2YCl7:Tb3+ (4f8). The emission spectrum of Tb3+ ions in Ba2YCl7 is very rich, showing a number of well resolved lines. Figure 8 presents the emission spectra recorded under 486 and 274 nm excitation. The multiple emission lines originating from the transition between the excited multiplets 5 D4, 5D3, and 5H6 and the CF components of the multiplets 7FJ (J = 0−6) are observed in the vis and near UV ranges. 10577



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Figure 10 presents the emission transitions from the 5D4 multiplet to the 7FJ multiplets (J = 0, 1, 2). Although some hot

Figure 8. Emission spectra recorded for Ba2YCl7:Tb3+ at 77 K. The emission below and above 20 000 cm−1 was measured under 468 and 274 nm excitation, respectively.

Figure 9 presents the 5D4−7F6 emission transitions. As discussed in section 5, out of 13 energy levels predicted by the Figure 10. Emission spectrum recorded for Ba2YCl7:Tb3+ in the 5 D4−7F0,1,2 transition range at 77 K under 486 nm excitation. The weak hot bands are shifted by about 34 cm−1 (H1) and 71 cm−1 (H2) toward higher energies from the electronic transition lines indicated by arrows.

bands are observed in the spectrum, all electronic transitions can be unambiguously assigned and thus the energies of all CF components of the 7F0,1,2 were obtained. The CF components of the 5D4 multiplet and other multiplets located at higher energy region were determined from analysis of the excitation spectra obtained while monitoring the 5D4−7F5 emission line (Figure 11). For Ba2YCl7:Tb3+, a total of 155 energy levels in the spectral range 0−33 826 cm−1 were experimentally resolved and included in the CF analysis (section 5). 4.4. Ba2YCl7:Dy3+ (4f9). Energy levels of Dy3+ ions were determined from the 4.2 K absorption spectra and the 77 K emission and excitation spectra. Figure 12 presents the absorption spectra in the region comprising transitions from the ground 6H15/2 multiplet to the excited SLJ multiplets. In all

Figure 9. Emission spectrum recorded for Ba2YCl7:Tb3+ in the 5 D4−7F6 transition range. Arrows indicate the identified electronic transitions, whereas asterisks identify the weak hot bands.

theory, 7 levels were experimentally identified from Figure 9. CF calculations (section 5) reveal, however, that some levels are predicted at very similar energies, with differences smaller than 2 cm−1. It is not feasible to resolve such closely lying levels in the spectrum. For example, the four lowest energy levels are calculated at −6, −5, +99, and +99 cm−1. Taking into consideration the predicted occurrence of such levels, no more than 9 separated lines are expected to be distinguishable in the 5D4−7F6 emission spectrum. Such quasi-degenerated levels are also characteristic for other multiplets of Tb3+ in Ba2YCl7; e.g., the three lowest components of the 5D4 are calculated at 20 562, 20 564, and 20 568 cm−1. Hence, in some cases, the number of levels that can be experimentally determined is smaller than the number (2J + 1) expected for a given SLJ multiplet.

Figure 11. Sample excitation spectrum recorded for Ba2YCl7:Tb3+ at 77 K while monitoring the emission line at 542.5 nm. 10578



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Figure 12. Absorption spectra recorded for Ba2YCl7:Dy3+ at 4.2 K, which show the transitions from the lowest level of the ground 6H15/2 multiplet to the CF components of the excited multiplets: (a) 6H11/2, (b) 6H9/2 + 6F11/2, (c) 6F9/2 and 6H7/2, (d) 6F7/2, (e) 6F5/2, (f) 6F3/2, and (g) 4F9/2, 4I15/2, and 4G11/2.

spectra well-separated lines are observed and their number corresponds to the theoretically predicted value (J + 1/2). Figure 13 presents the emission spectrum recorded for Dy3+ in Ba2YCl7 crystals under 352 nm excitation. The spectrum is dominated by transitions from the 6P5/2 multiplet to the CF components of the multiplets 6FJ (J = 1/2, 3/2, 5/2, 7/2, 9/2, 11/2) and 6HJ (J = 5/2, 7/2, 9/2, 11/2, 13/2). In addition, the transitions from the 4F3/2 multiplet to the CF components of the multiplets 6H11/2, 6H13/2, and 6H15/2 may be also observed. From analysis of the emission spectra, the energies of Stark levels of the two lowest multiplets 6H15/2 and 6H13/2, unobserved in the absorption spectrum, can be determined. For Ba2YCl7:Dy3+, a total of 104 energy levels in the spectral range 0−29 600 cm−1 were experimentally resolved and included in the CF analysis (section 5). 4.5. Ba2YCl7:Ho3+ (4f10). Figure 14 presents the absorption spectra corresponding to the transitions from the ground 5I8 multiplet to the excited multiplets located in the energy range 15 000−24 000 cm−1. The lines are relatively well separated, thus enabling determination of most experimental energies expected in this region. Transitions to the multiplets 5I7 and 5I6 can be observed in the IR range of absorption spectrum. However, those to the multiplets 5I5 and 5I4 are very weak in absorption. Fortunately, the multiplets 5IJ are terminal ones for

Figure 13. Emission spectrum recorded for Ba2YCl7:Dy3+ at 77 K under 352 nm excitation.

emission originating from the multiplet 5D13, thus enabling determination of energies of these multiplets. 10579



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Figure 14. 4.2 K absorption spectrum recorded for Ba2YCl7:Ho3+, which shows the transitions from the ground 5I8 multiplet to the excited SLJ multiplets as labeled.

Figure 16. Emission spectrum recorded for Ba2YCl7:Ho3+ in the 5 F5−5I8 transition range at 77 K under 451 nm excitation. The hot bands are observed at energies above 15 330 cm−1.

Figure 15 presents the emission spectrum obtained at 77 K under 279 nm excitation. The spectrum at higher energy region

4.6. Ba2YCl7:Er3+ (4f11). For Er3+ ions well-separated lines are observed in the 4.2 K absorption spectrum. Figure 17 shows transitions from the ground 4I15/2 multiplet to the multiplets 4 I13/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, and 4F7/2. In all cases, except for 4 F9/2, the number of absorption lines corresponds to the expected value (J + 1/2). For transitions to the multiplet 4F9/2 at the lower energy part of the spectrum, three lines are observed at 15 178, 15 187, and 15 202 cm−1. The emission originates from the level at 15 202 cm−1; therefore, this line is assigned as the lowest component of the 4F9/2 multiplet. The origin of the two remaining lines is not clear at present. Figure 18 shows emission spectra recorded at 77 K under 378.6 nm excitation with the observed lines corresponding to the transitions as labeled. In addition, the weak emission due to the 2H9/2−4I15/2 transitions (not shown) is observed in the region 24 100−24 500 cm−1. Analysis of the transitions terminated on the ground 4I15/2 multiplet enables determinations of all respective CF components, which are schematically shown in Figure 18. The excited state absorption energies of the 4I9/2−2H9/2 transitions reported in ref 3 enable calculation of the energies of the CF components of the 2H9/2 multiplet. They correspond exactly to the energies determined in the present paper, but the CF calculations (section 5) show that appropriate eigenvectors contain the largest fraction of the |4F 9/2⟩ states. For Ba2YCl7:Er3+, a total of 71 energy levels in the spectral range 0−28 015 cm−1 were experimentally resolved and included in the CF analysis (section 5). 4.7. Ba2YCl7:Tm3+ (4f12). Most of energy levels of Tm3+ were determined from the 4.2 K absorption spectra presented in Figure 19. The absorption lines are well separated and their number for a given SLJ multiplet does not exceed the 2J + 1 value. Analysis of the 1G4−3H6 emission spectrum enabled assignment of 10 from among 13 energy levels of the ground 3 H6 multiplet (Figure 20). For Ba2YCl7:Tm3+, a total of 60 energy levels in the spectral range 0−27 728 cm−1 were experimentally resolved and included in the CF analysis (section 5).

Figure 15. Emission spectra recorded for Ba2YCl7:Ho3+ at 77 K under 279 nm excitation.

is dominated by transitions from the multiplet 3D13 (the lowest component is calculated at 32 955 cm−1) to the multiplets 5IJ (J = 4−7). At lower energy regions also the emission transitions originating from the multiplets 5G5 (23 792 cm−1), 5F3 (20 488 cm−1), 5S2 (18 366 cm−1), and 5F5 (15 330 cm−1) may be observed. Figure 16 presents the emission spectrum obtained under 451 nm excitation in the region of transitions from the multiplet 5F5 to the ground 4I8 one. The arrows indicate transitions assigned as originating from the lowest level of the excited multiplet. Above 15 330 cm−1 some hot bands are observed at energies matching well the energy level structure of the 5F5 multiplet. Analysis of this spectrum enables assignment of 14 CF levels of the ground multiplet, shown schematically in the inset in Figure 16. For Ba2YCl7:Ho3+, a total of 103 energy levels in the spectral range 0−23 926 cm−1 were experimentally resolved and included in the CF analysis (section 5). 10580



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Figure 17. 4.2 K absorption spectra recorded for Ba2YCl7:Er3+, which show the transitions from the lowest component of the ground 4I15/2 multiplet to the CF levels of the multiplets: (a) 4I13/2, (b) 4I9/2, (c) 4F9/2, (d) 4S13/2 and 2H11/2, and (e) 4F7/2.

and the crystal field Hamiltonian (Ĥ CF) expressed in the compact form25 in the Wybourne notation21 Ĥ CF =

(k)

∑ BkqCq̂ k ,q

5. ENERGY LEVEL CALCULATIONS The observed energy levels are fitted to the phenomenological Hamiltonian, Ĥ = Ĥ FI + Ĥ CF, suitable for 4fN ions in crystals,12,21−24 by simultaneous diagonalization of the two parts: the free-ion (Ĥ FI) Hamiltonian

∑

F k(nf ,nf )fk̂ + ζ4f Â SO + αL̂(L̂ + 1)

k = 2,4,6

+ βĜ(G2) + γĜ(R 7) +

∑

T itî

i = 2,3,4,6,7,8

+

∑ j = 0,2,4

j

M m̂ j +

∑ k = 2,4,6

P kpk̂

(2)

Because the basic notation used in the present CF calculations and superposition model (SPM) analysis has been well-defined in our recent papers,10,26 for brevity we refrain from repeating the notation in details here. The twobody electrostatic correction parameters α, β, and γ in Ĥ FI, eq 1, are not to be confused with the Euler angles (α, β, γ) used above. The symbolic CFPs27,28 in the Wybourne notation, Bkq, of rank k and component q, are defined according to the prevailing conventions12,21−23 and satisfy the relations23,29 ReBk−q = (−1)qReBkq, ImBk−q = (−1)q+1ImBkq. The CF strength parameters,14 i.e., the rotational invariants,30 Sk (k = 2, 4, 6), and the global quantities S are also used. The general SPM31,32 expressions in the Wybourne notation10,26 involve the following quantities: the intrinsic parameters B̅ k, the coordination factors gk,q, the power law exponents tk, and the average metal−ligand distance R0 (calculated using the individual ionic distances Ri).10,26 The combined coordination factors Sgk,q defined in the Wybourne notation:13 Sgk,q = ∑L(R0/RL)tkgk,q(θL,φL), are also used. Due to the lack of symmetry at the Ln3+ site in the Ba2YCl7:Ln3+ crystal as many as 27 independent (real and imaginary) CFPs must be included in the CF Hamiltonian. As discussed in section 3, using an appropriate choice of the axis system, the actual triclinic C1 site symmetry may fortunately be approximated by orthorhombic C2v symmetry. The quality of such an approximation may be judged by the relative values of the combined coordination factors Sgk,q calculated using SPM. The SPM results are presented below and then utilized for the energy level calculations.

Figure 18. Emission spectrum recorded for Ba2YCl7:Er3+ under 378.6 nm excitation at 77 K. The lines are due to transitions between the multiplets as labeled.

ĤFI = Eave +

(x ,y ,z)

(1) 10581



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Figure 19. Absorption spectra recorded for Ba2YCl7:Tm3+ at 4.2 K, which show the transitions from the lowest level of the ground 3H6 multiplet to the CF components of the excited multiplets: (a) 3F4, (b) 3H5, (c) 3H4, (d) 3F3, (e) 3F2, and (f) 1G4.

Table 2. Combined Coordination Factors Sgk,q Calculated Using the Ligands’ Positions from Table 1 for the Actual C1 Symmetry Assuming tk = k + 1 and R0 = 0.27 nm

Figure 20. Emission spectrum recorded for Ba2YCl7:Tm3+ under 360 nm excitation, which shows the 1G4−3H6 transitions. The hot bands marked by asterisks are observed at higher energies.

k,q

Sgk,q

k,q

Sgk,q

2,0 2,1 2,−1 2,2 2,−2 4,0 4,1 4,−1 4,2 4,−2 4,3 4,−3 4,4 4,−4

0.024 −0.003 0.007 −0.186 −0.005 0.631 −0.035 −0.004 −0.354 −0.015 −0.087 0.000 −0.734 0.039

6,0 6,1 6,−1 6,2 6,−2 6,3 6,−3 6,4 6,−4 6,5 6,−5 6,6 6,−6

−0.036 −0.039 −0.008 −0.154 0.002 0.021 −0.010 0.340 −0.004 0.039 0.002 −0.048 −0.001

The energy level calculations were carried out to confirm the assignment of some experimental bands presented in section 4 and determine reliable sets of CFPs. The fittings were carried out for a given ion using at first a preliminary set of levels determined experimentally, which consisted of levels that could be most unambiguously assigned. The results of these preliminary fittings typically show that some observed bands should be reassigned and additional bands can be included in the fittings. Subsequent fittings based on the refined sets of levels yield improved agreement between the calculated and observed energies. Due to the low site symmetry of the Ln3+ ions in Ba2YCl7, all energy levels are labeled by the same irreps and thus no specific selection rules exist for the electronic transitions for the parallel and perpendicular light polarization. Hence, the experimental

Table 2 lists the factors Sgk,q calculated using the ligands’ positions from Table 1 for the actual C1 symmetry, and assuming the point charge model values for tk,33 i.e., tk = k + 1, and the average distance R0 = 0.2703 nm. Analysis of the relative values of the coordination factors listed in Table 2 reveals that the orthorhombic Sgk,q (indicated in bold) are markedly larger as compared with the triclinic ones (indicated in italics). This proves that C2v symmetry is a good approximation of symmetry of Ln3+ site in Ba2YCl7:Ln3+ and justifies the usage of orthorhombic C2v CF Hamiltonian in energy levels calculations. 10582



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Table 3. Fitted Free-Ion Parameters and Orthorhombic CF Ones for Ln3+ Ions in Ba2YCl7:Ln3+ Crystalsa Eavg F2 F4 F6 ζ4f α β γ T2 T3 T4 T6 T7 T8 M0 P2 B20 B22 κ (Si) B40 B42 B44 B60 B62 B64 B66 n rms S2 S4 S6 S a

Pr (4f2)

Nd (4f3)

Tb (4f8)

Dy (4f9)

Ho (4f10)

Er (4f11)

Tm (4f12)

10060 (2) 68286 (30) 50239 (109) 33074 (56) 751 (1) 24.4 (0.2) −741 (15) [1413]

24000 (16) 70793 (194) 50480 (530) 33605 (583) 876 (1) 22.2 (0.1) −657 (7) 2142 (181) 420 (21) 41 (2) 48 (3) −311 (7) 393 (13) 411 (16) 1.8 (0.1) 281 (33) 157 (11) −325 (8) −2.07 (S6) 951 (40) −293 (43) −779 (27) −236 (45) −140 (39) 516 (24) −4 (38) 137 13.4 217 504 220 341

68031 (58) 87347 (218) 63978 (696) 47845 (293) 1698 (2) 22 (1) −960 (66) 1570 (167) [330] [40] [45] [−365] [320] [349] [2.7] [482] 65 (13) −259 (11) −3.94 (S6) 763 (23) −393 (15) −774 (13) −386 (25) 6 (19) 364 (19) −13 (19) 155 9.4 166 482 179 312

55955 (7) 90830 (41) 64186 (157) 49194 (78) 1909 (1) 18.5 (0.3) −652 (8) 1818 (41) [329] [36] [127] [−314] [404] [315] 3.7 (0.1) 717 (32) 72 (14) −261 (7) −3.63 (S6) 881 (26) −423 (18) −780 (17) −178 (25) −126 (19) 243 (18) −21 (19) 104 7.2 168 511 118 318

48304 (8) 93666 (39) 66579 (240) 50506 (146) 2149.0 (0.5) 25.7 (0.7) −857 (15) 2077 (40) [400] [37] [107] [−264] [316] [336] [2.54] [605] 70 (23) −308 (14) −4.40 (S6) 736 (26) −350 (26) −785 (18) −147 (24) 13 (16) 193 (16) 20 (20) 103 6.8 197 474 86 300

35393 (8) 95535 (67) 68745 (108) 53843 (138) 2364 (2) 15.6 (0.4) −605 (13) [1570] [426] [53] [60] [−346] [186] [674] 5.4 (0.3) 764 (92) −102 (36) −287 (18) 2.81 (S5) 535 (99) −321 (58) −836 (31) −94 (57) −131 (32) 84 (48) 39 (35) 71 11.8 187 458 68 288

17801 (4) 100766 (85) 69627 (79) 50834 (219) 2634.2 (0.7) 15.4 (0.5) [−665] [1936]

[1.88] [244] 24 (13) −341 (9) −14.2 (S6) 1094 (34) −406 (27) −835 (24) −422 (63) −45 (47) 391 (40) 5 (37) 66 14.1 216 570 194 369

[4.93] [730] −47 (21) −191 (14) 4.06 (S5) 520 (37) −445 (25) −734 (20) −39 (37) −244 (23) −192 (25) −111 (26) 60 10.9 123 440 130 274

All values are in cm−1, except for n and κ (dimensionless); for explanations, see text.

strongly on the position of the missing 2FJ levels, this value should be treated with caution. A relatively complete set of energy levels was determined also for the Pr3+ ion. Only the CF components of the multiplets 1G4 and 1S0 are missing. Due to the lack of the latter multiplet, γ cannot be determined. Because the values of the free-ion parameter α strongly depend on the positions of the 1I6 levels and most components of the 1I6 multiplet were experimentally assigned, the fitted values of α may be considered as well determined. Similarly, the relatively large number of 134 energy levels was also determined for the Tb3+(4f8) ion. However, unlike in the case of, e.g., the Nd3+(4f3) ion, only a small fraction of the levels arising from the 4f8 configuration is observable at energies below 30 000 cm−1. This may cause some problems in determination of the parameters Fk, although the obtained values seem to be consistent with the systematic trends in the parameter values (Table 3). Analogously as for other ions, some free-ion parameters could be freely varied in the fittings, whereas those parameters that could not be well determined from available data were kept at their constant mean values listed in ref 12. The latter quantities are given in square brackets in Table 3, whereas the values in parentheses indicate the experimental uncertainty. The procedure described above has been used in the energy level calculations. The fitted free-ion parameters and CF ones for the Ln3+ ions considered are listed in Table 3 together with the number (n) of the experimental energy levels

measurements do not provide any additional support for assignment of the experimental energy levels to the calculated ones. Therefore, the experimental levels of a particular multiplet were assigned to the calculated ones according to the increasing energies. In the case when the number of levels was smaller than that predicted by theory, the experimental levels were assigned to the nearest calculated value. The procedure to estimate the starting CFP values was as follows. First, the starting CFPs for Er3+ were evaluated using the SPM model for Ba2ErCl7, for which the crystal structure has been reported.1 Due to the very similar ionic radii of Er3+ (103.0 pm) and Y3+ (104.1 pm), the starting CFPs for fittings evaluated for the pure Ba2ErCl7 crystal may also be reliably employed for the doped Ba2YCl7:Er3+ crystal. Then, in turn, so obtained fitted CFPs for Er3+ may serve as a reasonable approximation for the starting CFPs for Tm3+ and Ho3+ doped into Ba2ErCl7. Subsequently, keeping in mind the sequence across the 4fN series, the CFPs for Ho3+ were used for Dy3+ and the latter for Tb3+. The CFPs obtained for these ions were then extrapolated to evaluate the starting values for Pr3+ and Nd3+. The most complete set of CF levels was obtained for Nd3+ ions in Ba2YCl7. Out of the total 182 levels existing in the f3 configuration, 137 levels were assigned, thus enabling free variation of all free-ion parameters in eq 1 and CF ones in eq 2. It should be noticed that although a reasonable value of the free-ion parameter γ (eq 1) was obtained, because γ depends 10583



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Figure 21. Variation of the free-ion parameters (a) Fk and ζ4f and (b) α and β for Ba2YCl7:Ln3+ as a function of the number of 4f electrons N.

Table 4. CFPs Obtained from the Fitted Ones in Table 3 by the Orthorhombic Standardization Transformations S5 and S6 Defined in Ref 34a Pr (4f2) B20 B22 κ B40 B42 B44 B60 B62 B64 B66 a

Nd (4f3)

Tb (4f8)

Dy (4f9)

Ho (4f10)

Er (4f11)

Tm (4f12)

S5

S6

S5

S6

S5

S6

S5

S6

S5

S6

S5

S6

S5

S6

406 185 0.46 −784 −782 736 −118 −96 472 −70

−429 −156 0.36 −142 −1188 199 −166 −41 459 −132

320 259 0.81 −690 −745 594 −195 −59 527 116

−477 −66 0.14 −226 −1038 206 −382 84 477 −79

285 169 0.59 −834 −617 562 −126 −49 433 −95

−350 −90 0.26 −213 −1010 42 −143 −74 429 −87

284 175 0.62 −820 −653 643 −14 −68 287 65

−356 −86 0.24 −151 −1076 83 −216 37 233 −111

342 197 0.58 −822 −635 518 −117 −21 201 −30

−412 −111 0.27 −268 −985 55 −62 −7 216 −11

403 81 0.20 −927 −604 388 17 −112 114 66

−300 −206 0.68 −420 −925 −37 −76 81 89 −113

257 67 0.26 −924 −469 475 409 −98 −72 106

−210 −124 0.59 −221 −914 −114 −115 7 −212 −240

The standard values of κ are indicated in bold.

difficult to identify any definite trends in the values of β for Ba 2 YCl 7 :Ln 3+ . However, the slope of β for LaCl 3 :Ln3+ (decreasing) is found opposite to that for LaF 3 :Ln 3+ (increasing). It is worth noting, however, that a kind of mirror symmetry is revealed between the α- and β-curves presented in Figure 21b. An overall inspection of the fitted CFPs Bkq listed in Table 3 reveals that the sets of Bkq values are, in general, consistent across the 4fN series. The rms deviations are about 7−14 cm−1, which may be considered as very satisfactory. It is worth to discuss first the dependence of the fitted CFP sets on the starting ones revealed by Table 3. The starting CFPs for Er3+ were evaluated by SPM using the intrinsic parameters B̅ k (in cm−1) 950, 370, and 150, and the power law exponents tk 7, 10, and 9, for k = 2, 4, and 6, respectively,31 as (in cm−1) B20 = −97, B22 = −287, B40 = 641, B42 = −304, B44 = −747, B60 = −17, B62 = −84, B64 = 193 and B66 = −26 (yielding after the standardization transformation S5:34 B20 = 400, B22 = 84, B40 = −781, B42 = −595, B44 = 443, B60 = −52, B62 = 1, B64 = 184 and B66 = 100). The former CFPs match well with the fitted CFPs obtained for Er3+ in Table 3. For each ion, except for Ho3+, the fitted CFP set (obtained in the way described above) turns out to correspond to the same region in the CF parameter space34,27,28 as the respective starting CFP set; i.e., the same standardization transformation Si is required to obtain the standardized CFP set. It is known that the region in which the fitted CFP sets are obtained may strongly depend on the

used for fittings, the rms values, and the rotational invariants Sk.30 To enable discussion of the orthorhombic standardization,34,27,28 the rhombicity ratios κ and the respective transformations Si required to obtain the standardized CFP sets from the originally nonstandard ones are also included in Table 3. The experimental energy levels and the calculated ones are listed in Tables S1−S7 in the Supporting Information. In Figure 21a we present the variation of the free-ion parameters Fk and ζ4f across the 4fN series, i.e., as a function of the number of f electrons N. As expected, as the 4f orbitals across the series contract, the parameters Fk and ζ4f increase. In general, this increase is smooth; the only larger deviation is observed for the value of F6 for Tm3+(f12). This value depends strongly on the positions of the multiplets 3F3 and 3F2, which seem, however, to be accurately determined from absorption spectrum. On the other hand, the value of F6 for Tm3+ depends strongly on the parameter γ. An increase of F6 for Tm3+ to a value larger than that for Er3+ would require, however, a decrease in the value of γ below 1000 cm−1, which seems to be a too small value. Moreover, using this small γ the resulting fitted value of F4 would be larger than that expected from the trend presented in Figure 21a. The trends are less pronounced for the two-body electrostatic corrections α and β. Nevertheless α appears to decrease with increasing N. The largest deviation from the emerging trend is observed for the 4f10 ion. A decrease in α across the 4fN series was also observed for LaCl3:Ln3+ and LaF3:Ln3+.13 It is 10584



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Figure 22. Variation of the CF parameters Bkq (in cm−1) for Ba2YCl7:Ln3+ as a function of the number N of f-electrons. The solid black squares (■) represent the fitted CFPs from Table 3, whereas the red empty squares (□) represent CFPs transformed by S5 (Table 4). Bars indicate the errors in determination of the parameter values. Dashed lines illustrate trends obtained by a linear approximation of the data points.

region of the starting CFP sets; various alternative fitted CFP sets may be obtained by selecting other alternative starting CFP sets (see examples cited in ref 28). For illustration we have made a test. Using the standardized fitted CFP set for Er3+ (set S5 in Table 4 presented below) as the starting set for Ho3+, we obtain an alternative fitted set, which is identical with the nonstandard set S5 for Ho3+ from Table 4 obtained from the original fitted Ho3+ set in Table 3 using the standardization transformation S5. Note that this set requires the standardization transformations S2 and not S6 as in the previous case. The existence of five additional alternative CFP sets34,27,28 for any specific orthorhombic CFP set should be kept in mind when CFP sets are compared. The values of κ listed in Table 3 show that the fitted CFPs, which are expressed in the undefined nominal axis systems,27 are highly nonstandard.34,28 For discussion of the standardization properties of CFPs,34,27,28 in Table 4 we list the CFP sets obtained by the standardization transformations S6 and S5 from the fitted CFPs in Table 3. Importantly, the transformations S6 and S5 are complementary for Nd to Ho ions and Er and Tm ions, because they correspond to the regions −III and +III, respectively, yielding comparable CFP magnitudes but differing signs of some CFPs. From the point of view of the standardization idea,34,27,28,35,36 for a meaningful comparison of CFP values for different fN ions, any two compared CFP sets should be expressed in the same region out of the six possible regions ±I to ±III in the CF parameter space. The six possible regions ±I to ±III are determined by the specific values of the rhombicity ratios κ = B22/B20 defined originally in ref 34, with the standard region +I corresponding to κ between (0, 1/6 ≈ 0.408). The most natural, so not unique, choice for CFP presentation seems to be the standard region +I.34,28,35,36

Note that alternative attempts to deal with the confusion created by the nonuniqueness and incompatibility of available CFP sets exist in literature.37,38 The six choices of CFP sets proposed in ref 37 and recently modified by Burdick et al.38 are limited only for rare-earth ions in garnets. The attempts37,38 try to avoid utilizing the rhombicity ratio, because it is based on the second-rank CFPs, which are, in general, least well-determined. As discussed in ref 28, the usage of six arbitrarily numbered CFP sets37,38 without clear physical criteria for selection of a particular CFP set and, moreover, limited to garnets only, seems not a viable option. Our recent study39 indicates that the orthorhombic standardization idea, based on the simplest value of the rhombicity ratio, needs to be complemented by consideration of nonuniqueness of the signs of CFPs, i.e., that several physically identical sets may be obtained by changing the signs of some CFPs. Note that this problem is more acute for monoclinic CFP sets as revealed by the algebraic symmetry considerations.40 Comprehensive considerations of the nonuniqueness of CFP signs are beyond the scope of this paper. Such considerations are to be carried out in a separate paper to enable working out consistent conventions for presentation of orthorhombic and lower symmetry CFP sets. Subsequent adherence to such conventions would facilitate intercomparison of CFP sets available in literature. In our case, the standardized fitted CFP sets are obtained using the standardization transformation S6 for Nd to Ho ions, but S5 for Er and Tm ions. Hence, the two groups of CFP sets, so formally intrinsically incompatible27,28 as belonging to two different regions, are complementary concerning the signs. Therefore, for presentation of the variation of the CFP values Bkq across the 4fN series we have chosen the fitted CFP sets and also the CFP sets obtained using S5. This transformation yields the standardized CFPs for Er3+ ion and for this ion the starting 10585



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Finally, we present alternative approaches to visualization of the CFP trends across the 4fN series and their analysis. In general, the trends in the CFP sets transformed by S5 apparently match closely those observed for the fitted CFP values. The lines obtained by a linear approximation for a given CFP in both cases are either convergent (when the fitted and the S5-transformed Bkq’s possess the same signs) or divergent (when the fitted and the S5-transformed Bkq’s possess the opposite signs), thus yielding the same trends in the absolute CFP values. One possible exception is B40, for which the trend in the absolute CFP values is clearly decreasing for the fitted CFPs, but slightly increasing for the S5-transformed CFPs. This observation calls for some caution. Therefore, for comparison in Figure 23a we present, with a better resolution than in Figure

CFPs were evaluated by SPM in a well-defined axis system. Importantly, comparisons of the CFP sets across the 4fN series presented in literature utilize not the theoretical CFP sets but the fitted CFP sets, generally expressed in the Wybourne notation Bkq. However, in most cases, no check of the intrinsic compatibility28 of the respective CFP sets has been performed. The pitfalls of such indiscriminate approach to the CFP trends across the 4fN series have been discussed in refs 28, 35, and 36. Variation of the CFP values Bkq across the 4fN series presented in Figure 22 allows for the following observations. Although no linear trends in the CFP values may be expected, some general trends illustrated in Figure 22 by dashed lines may be noticed. The variation across the series is not quite smooth and some individual points deviate from the trends apparent in the lines representing the remaining parameters. From SPM analysis, a decrease in the CFP magnitudes over the series may be expected due to the increased distances between Ln3+ ion and Cl− ligands resulting from the decreasing ionic radius of Ln3+ ion that substitute for Y3+ ion in Ba2YCl7. In general, considering the absolute CFP values, all fitted CFPs follow this trend, with the possible exception of B40. Only a weak dependence on N is observed for the values of B42 and, with the exclusion of B42 obtained for Tm3+(f12), the trend obtained for other ions would show a slight decrease in accordance with the general expectations. The same conclusions concern the values of B62; the line representing CFPs for other ions, with exception of Tm3+, appears to show a slightly decreasing trend across the series. One may also notice from Figure 22 that the trends for the 4f2 and 4f3 ions seem to differ in some cases from those for the 4f8 to 4f12 ions. However, more firm conclusions are unwarranted, because only two representatives of the light lanthanides have been studied. Nevertheless, it has been observed also for LaF3:Ln3+ (ref 13) that most of the CFPs were best represented by different lines for the light and heavy ions of the series. The changes in the CFPs across the series are not smooth. However, up to now, in any systematic analysis of the lanthanides energy levels, the parameters Bkq did not show a smooth variation along the series.12 Different factors may be responsible for the lack of the expected smooth variation, e.g., possible errors in assignments of experimental energies for particular ions or different numbers of parameters varied in the fittings, while other parameters were constrained at constant values. It is also possible that the available experimental energy level data sets do not allow for accurate determination of all varied parameters, especially those that are relatively insensitive to the changes in the energy levels. Another possible reason may be due to the usage of approximated site symmetry, because the quality of such approximation may be different for particular ions. For example, when Ln3+ ions substitute for Y3+ ions, the network of Cl− ions may, to a certain extent, expand or contract around the impurity ion, depending on whether its ionic radius is larger or smaller than that of Y3+ ion. The extent of such local changes may depend on the relative difference between the ionic radii of Y3+ and substituting ion, whereas such changes may also influence the local site symmetry. Another factor may arise from the difficulties encountered in comparisons of intrinsically incompatible CFP sets, compounded by the nonuniqueness of the signs of CFPs discussed above. Hence, the question of smooth variations in CFPs along the 4f series must await solution of these difficulties, which requires further studies.

Figure 23. Variation of (a) the CFP B40 (in cm−1), fitted and the transformed by S5 and S6, and (b) B40/B40(min) ratios defined in text for Ba2YCl7:Ln3+ as a function of N.

22, the B40 values across the 4fN series also expressed in the region obtained by the transformation S6. Note that the B40 values obtained by the remaining transformations, i.e., S2, S3, and S4,34 exhibit features comparable to those already visible in Figure 23a. The S3-transformed B40 value (as well as the fitted one) is maximal, the S2- and S5-transformed B40 is minimal, whereas B40 is at a saddle point in the regions obtained by S4 and S6 transformation. Hence, only the fitted CFP sets and those obtained by S5 and S6 were chosen as representative. For better visualization of the trends, the relative changes in B40 are additionally presented in Figure 23b as B40/B40(min) ratios, where B40(min) is the smallest absolute value in a given CFP region. An intriguing aspect observed in Figure 22 and 23 is that the trends, especially for the dominant CFP B40, reveal somewhat different behavior when presented in different regions. Comparing CFP values including their signs, we observe clearly a decreasing trend for the fitted values, whereas for CFPs transformed by S5 and S6 the trends are less clear. Considering the absolute CFP values a decreasing trend for the fitted values, whereas an increasing for the S5 and S6 values. Such differences are more pronounced when the ratios B40/ B40(min) are compared (Figure 23b). The fitted and S5 value of B40 for 4f3 is smaller than for 4f2, whereas the S6 value is 1.6 times larger. Similarly, these values are comparable for 4f11 and 4f12, whereas the S6 value for 4f11 is nearly 2 times larger than for 4f12. The relative values for the first and the last ion in the available series of ions, i.e., 4f2 and 4f12, also depend on the 10586



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(for Ho3+) to 14.1 cm−1 (for Pr3+). This indicates a good agreement between the calculated and experimental energy levels for all ions studied. The variation of the free-ion parameters Fk and ζ4f across the 4fN series are found to be well represented by linear trends. Although the variations of the CF parameters Bkq are not smooth, a decrease in the magnitude of the CF parameters across the 4fN series may be observed. The CF parameter set obtained for Nd3+ ion in Ba2YCl7 is consistent with that for Nd3+ ion in structurally related RbY2Cl7 crystal, so far the only ion studied in this crystal. The CF parameter sets obtained by us may be used for calculations of the energies of the levels experimentally not yet observed, which may play important role in the energy transfer processes. The efficient up-conversion observed for Ba2YCl7 makes this material technologically important for laser applications or optical amplifiers.1,3 In this aspect the Ba2YCl7:3%Er3+ crystal was considered as a green upconversion laser excited at 800 nm.5 In a recent series of papers we have reported the results of the crystal field (CF) analysis for RE3+ ions in various fluoride9 and oxide laser materials.6−8 This paper extends this analysis to chloride hosts. It is worth mentioning that the present systematic analysis of CF parameters is one of only few such studies encompassing nearly whole series of RE3+ ions.

region in which the data are presented. The fitted B40 for 4f2 is 2.1 times larger than for 4f12, whereas the S6 value is 1.6 times smaller. As discussed above, better understanding of such intricacies in the CFP trends as well as the factors responsible for them requires working out consistent conventions for presentation of orthorhombic and lower symmetry CFP sets. Analogously to the CFP values (see above), also the CF strength parameter S is expected to decrease across the series.31 An inspection of the respective values in Table 3 confirms, in general, this expectation. For all Ln3+ ions in Ba2YCl7 crystals, the CF effect is dominated by the fourth-rank CFPs and relatively small values of S2 are obtained, which show that the long-range CF interactions (mainly electrostatic) are relatively small. The smallest contribution to the CF effect in Ba2YCl7:Ln3+ arises from the sixth-rank CFPs, unlike in the case of LaF3:Ln3+ where the parameters S6 are the largest (e.g., for LaF3:Nd3+, S2 = 118, S4 = 398, and S6 = 521 cm−1).13 The fourth-rank CFPs dominate also the CF effect for Ln3+ ions in ABCO4 and ABC3O7 oxides (e.g., SrLaAlO4 or SrLaGa3O7), but contributions from the second- and sixth-rank CFPs are also relatively larger (e.g., S2 = 297, S4 = 458, and S6 = 326 cm−1 for SrLaAlO4:Nd3+).6 Some conclusions may be drawn by considering RbY2Cl7 host crystal, which is structurally similar to Ba2YCl7 and also consists of YCl7 polyhedra sharing a triangular face to form Y2Cl11 dimeric units arranged in layers.41 The RE3+ ions substitute the Y3+ ions in the two slightly inequivalent positions of Cs symmetry, which can be approximated by C2v point symmetry for a monocapped trigonal prism. The CF calculations for Nd3+ ions at the two sites in RbY2Cl7 have been reported in ref 42. For the Nd(1)/Nd(2) site the following CFPs were obtained (after transformation S6 to the standard region, in cm−1): B20 = −343/−438, B22 = −84/−64, B40 = −13/−169, B42 = −1115/−1036, B44 = −83/−215, B60 = −712/−534, B62 = −45/30, B64 = 317/207 and B66 = −37/4. We note that the CFPs values for Nd(1)/Nd(2) in RbY2Cl7 correspond well with those obtained for Nd3+ in Ba2YCl7 in the present study. The CF strength parameters for RbY2Cl7:Nd(1)/Nd(2) are S2 = 162/200, S4 = 527/502, S6 = 234/169, and S = 345/326 cm−1 and agree well with those for Ba2YCl7:Nd3+, with S4 being predominant. Also the structures of energy levels within the ground 4I9/2 multiplet are similar for Nd3+ ions in the two hosts Ba2YCl7 [RbY2Cl7:Nd(1)/Nd(2)], namely (in cm−1): 0 [0/0]; 33 [27/39]; 116 [99/98]; 166 [169/163]; 311 [317/278]. The fact that such mutually consistent sets of CF parameters were independently obtained for the two hosts enhances the validity the respective results.

■

ASSOCIATED CONTENT

S Supporting Information *

Tables listing the calculated and experimental energy levels. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was partially supported by the research grant from the Polish Ministry of Science and Tertiary Education in the years 2006-2010. Thanks are due to Dr. P. Gnutek for helpful comments.

■

REFERENCES

(1) Wickleder, M. S.; Egger, P.; Riedener, T.; Furer, N.; Güdel, H. U.; Hulliger, J. Chem. Mater. 1996, 8, 2828−2831. (2) Meyer, G.; Masselmann, S. Chem. Mater. 1998, 10, 2994−3004. (3) Riedener, T.; Egger, P.; Hulliger, J.; Güdel, H. U. Phys. Rev. B 1997, 56, 1800−1808. (4) Egger, P.; Rogin, P.; Riedener, T.; Güdel, H. U.; Wickleder, M. S.; Hulliger, J. Adv. Mater. 1996, 8, 668−672. (5) Burlot-Loison, R.; Pollnau, M.; Krämer, K.; Egger, P.; Hulliger, J.; Güdel, H. U. J. Am. Opt. Soc. B 2000, 17, 2055−2067. (6) Karbowiak, M.; Rudowicz, C. Chem. Phys. 2011, 383, 68−82. (7) Karbowiak, M.; Gnutek, P.; Rudowicz, C.; Ryba-Romanowski, W. Chem. Phys. 2011, 387, 69. (8) Karbowiak, M.; Gnutek, P.; Rudowicz, C. Chem. Phys. 2012, 400, 29. (9) Karbowiak, M.; Gnutek, P.; Rudowicz, C. Spectrochim. Acta A 2012, 87, 46−60. (10) Karbowiak, M.; Gnutek, P.; Rudowicz, C. Physica B 2010, 405, 1927. (11) Rudowicz, C.; Gnutek, P. Spectrochim. Acta A 2011, 79, 60−68.

6. SUMMARY Absorption, emission, and excitation spectra for the series of lanthanide Ln3+ ions doped into Ba2YCl7 have been measured. Nearly all ions in the 4fN series, namely, Pr3+, Nd3+, Tb3+, Dy3+, Ho3+, Er3+, and Tm3+, have been taken into account, making our study most complete for the first time. The experimental energy levels identified in the spectra have been analyzed using a Hamiltonian, which includes the usual free-ion terms and the crystal field (CF) Hamiltonian expressed in the Wybourne notation21 and suitable for C2v symmetry. The superposition model analysis has been carried out to (i) provide the starting values of the CF parameters Bkq used for fittings for the studied crystals and (ii) verify the goodness of the approximation of the actual C1 site symmetry at the metal center by C2v symmetry. The obtained values of the rms deviation range from 6.8 cm−1 10587



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(12) Görller-Walrand, Ch.; Binnemans, K. Rationalization of CrystalField Parametrization. In Handbook on the Physics and Chemistry of Rare-Earths; Gschneidner, K. A., Jr., Eyring, L. R., Eds.; Elsevier: Amsterdam, 1996; Vol. 23, Chapter 155. (13) Carnall, W. T.; Goodman, G. L.; Rajnak, K.; Rana, R. S. J. Chem. Phys. 1989, 90, 3443−3457. (14) Chang, N. C.; Gruber, J. B.; Leavitt, R. P.; Morrison, C. A. J. Chem. Phys. 1982, 76, 3877−3889. (15) Meyer, G. Inorg. Synth. 1989, 25, 146−150. (16) Blachnik, R.; Alberts, J. E. Z. Anorg. Allg. Chem. 1982, 490, 235− 241. (17) Pilati, T.; Forni, A. J. Appl. Crystallogr. 1989, 31, 503−504. (18) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838. (19) Meyer, G. J. Alloys Compd. 2000, 303−304, 409−415. (20) Karbowiak, M.; Mech, A.; Drozdzynski, J.; Edelstein, N. M. Phys. Rev. B 2003, 67 (195108), 1−17. (21) Wybourne, B. G. Spectroscopic Properties of Rare Earths; Interscience: New York, 1965. (22) Morrison, C. A. Angular Momentum Theory Applied to Interactions in Solids; Springer: Berlin, 1988. (23) Liu, G., Jacquier, B., Eds. Spectroscopic Properties of Rare Earths in Optical Materials; Springer Series in Material Science, Vol. 83; Springer, Berlin, 2005. (24) Carnall, W. T.; Crosswhite, H.; Crosswhite, H. M.; Hessler, J. P.; Edelstein, N. M.; Conway, J. G.; Shalimoff, G. V.; Sarup, R. J. Chem. Phys. 1980, 72, 5089−5102. (25) Rudowicz, C. Magn. Reson. Rev. 1987, 13, 1−89. Rudowicz, C. Magn. Reson. Rev. 1988, 13, 335 (erratum). (26) Karbowiak, M.; Rudowicz, C.; Gnutek, P. Opt. Mater. 2011, 33, 1147−1161. (27) Rudowicz, C.; Qin, J. J. Lumin. 2004, 110, 39−64. (28) Rudowicz, C.; Gnutek, P. Physica B 2010, 405, 113−132. (29) Rudowicz, C. Chem. Phys. 1985, 97, 43−50. (30) Rudowicz, C.; Qin, J. Phys. Rev. B 2003, 67 (174420), 1−14. (31) Newman, D. J.; Ng, B. Rep. Prog. Phys. 1989, 52, 699−763. (32) Andrut, M.; Wildner, M.; Rudowicz, C. In Spectroscopic Methods in Mineralogy − EMU Notes in Mineralogy; Beran, A., Libowitzky, E., Eds.; Eötvös University Press: Budapest, 2004; Vol. 6, Chapter 4, p 145. (33) Newman, D. J.; Ng, B. Superposition model. In Newman, D. J., Ng, B., Eds. Crystal Field Handbook; Cambridge University Press: Cambridge, U.K., 2000; Chapter 5. (34) Rudowicz, C.; Bramley, R. J. Chem. Phys. 1985, 83, 5192−5197. (35) Rudowicz, C.; Gnutek, P.; Karbowiak, M. Phys. Rev. B 2007, 76 (125116), 1−11. (36) Rudowicz, C.; Gnutek, P. J. Phys.: Condens. Matter 2010, 22 (045501), 1−11. (37) Morrison, C. A.; Leavitt, R. P. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Eyring, L.. Eds.; North Holland: Amsterdam, 1982; Vol. 5, Chapter 46, pp 461−692. (38) Burdick, G. W.; Gruber, J. B.; Nash, J. B.; Chandra, S.; Sardar, D. K. Spectrosc. Lett. 2010, 43, 406−422. (39) Rudowicz, C.; Lewandowska, M. J. Alloys Compd. 2012, 540, 279−289. (40) Rudowicz, C. J. Chem. Phys. 1986, 84, 5045−5058. (41) Meyer, G. Z. Anorg. Allg. Chem. 1982, 491, 217−224. (42) Karbowiak, M.; Edelstein, N. M.; Drozdzynski, J.; Kossowski, K. Chem. Phys. 2002, 277, 361−372.

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Optical Spectra and Energy Levels Analysis of the 4f - American

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