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J . Phys. Chem. 1986, 90, 5605-5608
Assignment of the 51, Crystal-Field Levels of HoCI2Peter A. Tanner Department of Analytical and Biological Chemistry, Kingston Polytechnic, Kingston-upon- Thames, KTI 2EE. United Kingdom (Received: March 12, 1986)
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From the analysis of the 'F3, 'FS 'Is, and 'I5 'I8 infrared luminescence of HoC16> between 300 and 15 K, the 'Is crystal-field components of Ho3+in cubic symmetry have been located at 11 198 1 (ar4), 11 229 2 (F5), 11 258 2 (r3),and 11 273 cm-' (br4). The decay lifetime of '1, is slightly longer than 0.1 s.
Introduction Rare earth ions are situated at octahedral sites of symmetry in many hexachloroelpasoZitesl~Z so that these lattices provide alternative systems to the extensively studied lanthanum trihalides3-' for the investigation of the energy levels of tripositive lanthanide ions. The electronic spectra of centrosymmetric Cs2NaLC16 (L = lanthanide) are largely vibronic in character6,' although magnetic-dipole (MD)-allowed electronic origins may dominate some transitions.8 The study of the energy levels is necessary in order to elucidate the mechanisms of energy transfer9-" and upconversionI2 that occur in these materials, besides the comparison with the most recent crystal-field calculation^.^^^^^ The crystal-field components of the 51s term of Ho3+ in H o c & have been ~alculated'~ to lie between 11 177 and 11 280 cm-'. We are unable to locate these levels by absorption spectroscopy so that in this study luminescence transitions originating from and terminating upon 'I5 have been investigated. Further checks on the assignments of the energy levels were possible from the analyses of several transitions in which the intensities are controlled by different mechanisms. In one case, the location of electronic origins may be inferred from electric-dipole (ED)-allowed characteristic vibronic structure, whereas in the other instance MDallowed zero-phonon lines are directly observed. Experimental Section The preparation of CS2NaHOC16 and of other elpasolites doped ~ and with Ho3+ has previously been d e ~ c r i b e d .Luminescence absorption spectra were recorded with the apparatus at Birkbeck Co1Iege.l'
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Results and Discussion 'F5 'I8 Absorption Spectrum. A detailed account of the 'F5 'I5 luminescence spectrum is given in the next section. At (1) Morss, L. R.; Seigal, M.; Stinger, L.; Edelstein, N. Inorg. Chem. 1970,
9, 1771. (2) Meyer, G. Prog. Solid State Chem. 1982, 14, 141. (3) Dieke, G. H. In Spectra and Energy Levels of Rare Earth Ions in
Crystals; Crosswhite, H. M., Crosswhite, H., Eds.; Interscience: New York, 1968. (4) Crosswhite, H. M.; Crosswhite, H.; Edelstein, N.; Rajnak, K. J . Chem. Phys. 1977, 67,3002. (5) Leavitt, R. P.; Morrison, C. A. J . Chem. Phys. 1980, 73,749.
(6) Morley, J. P.; Faulkner, T. R.; Richardson, F. S.; Schwartz, R. W. J . Chem. Phys. 1981, 75, 539. (7) Tanner, P. A. Mol. Phys. 1984, 53, 835. (8) Tanner, P. A. Chem. Phys. Lett. 1985, 119, 213. (9) Tanner, P. A. Mol. Phys. 1984, 53, 813. (10) Tanner, P. A. Mol. Phys. 1986, 58, 317. (1 1) Banerjee, A. K.; Stewart-Darling, F. S.; Flint, C. D.; Schwartz, R. W. J . Phys. Chem. 1981, 85, 146. (12) Tanner, P. A. Mol. Phys. 1986, 57,137. (13) Richardson, F. S.; Reid, M. F.; Dallara, J. J.; Smith, R. D. J . Chem. Phys. 1985, 83, 3813. (14) Reid, M. F.; Richardson, F. S . J . Chem. Phys. 1985, 83, 3831. (15) Flint, C. D.; Tanner, P. A. Mol. Phys. 1984, 53, 801.
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elevated temperatures many transitions originate from thermally populated excited crystal-field components of 'F5 so that these latter levels need to be determined prior to analysis. The 15 K absorption spectrum of C ~ ~ N a H obetween c 1 ~ 15 300 and 15 950 cm-', shown in Figure 1, is more clearly resolved than that of Morley and co-workers6 At this temperature the transitions to 5FSare observed from the a r 3 , a r 4 components of the electronic ground state. The locations of the electronic origins of the transitions from the lowest level, ('18)ar3, are marked 1-1V in the figure. The derived wavenumbers of the ('FS)ar4, rs,r3, and b r 4 levels are then 15 353, 15 396, 15 527, and 15 546 cm-', respectively. Since these energies are similar to those previously given6 we do not include a retabulation of the data. 5F5 '1' Luminescence Spectrum. Three intense luminescence transitions of Ho3+ in the region 7100-4000 cm-I are observed ', 'I8, has previously been for neat Cs2NaHoC&. One of these, 1 reported.8 The other two transitions exhibit the decay lifetime of 'F5 and their intensities decrease with decreasing temperature, characteristic of the transitions originating from this level. The lowest energy structure, between 4300 and 4080 cm-', is shown in Figure 2 and corresponds to transitions terminating on the crystal-field components of 'I5. The features are tabulated in Table I. The 'F5 515luminescence transition is unusual in that 15 (of the possible 16) transitions between the crystal-field components of these terms are MD-allowed in octahedral symmetry. All of the bands in Figure 2 are associated with MD transitions, the most intense being ('FS)ar4 aI'4(515)at 4155 cm-I. This places the lowest 515 level, a r 4 , at 15 353 - 4155 = 11 198 cm-'. The calculated intensity ratios for other transitions between the 5F5and 'I5 terms, relative to a r 4 a r 4 , are given16 in Table 11. Transitions from ('F5)ar4 to the ('15)r5, r3levels are assigned (Figure 2b) on the basis of the agreement between the calculated and observed intensity ratios. That terminating upon the '15(br4) level is not observed, being calculated only " ' / 6 0 as intense as ar,. At -43 cm-' to high energy of each transition ar4 originating from a r 4 , features are observed that correspond to emission from (5FS)I's (Table I). More strongly temperaturedependent bands are associated with the higher energy 'F5 levels, r3and br4. In particular, the strongest transitions from the latter b r 4 levels, thereby enabling (Table 11) are to the terminal ('15)r3, the location of all four crystal-field components of '1'. The assignments are summarized in Table I, and the derived energy level scheme for 51s is included in Table 11. 5F3 Luminescence Spectrum. The lowest energy crystal-field component of the 'F3 term of Ho3+,F2,is located at 20 427 cm-' in C S , N ~ G ~ C ~ ~ - H OThe C ~20 ~ -K. luminescence spectrum of this material between 9800 and 9000 cm-' is shown in Figure 3. Several transitions are observed in this region and the weakest, lowest energy bands correspond to ('F3)r2 515.Only one M D transition is expected at this temperature, r2 r5,and this is assigned to the strongest band of 'F, 'I, (line 2, at 9196 cm-I).
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(16) Morrison, C. A.; Leavitt, R. P.; Wortman, D. E. J . Chem. Phys. 1980,
73, 2580.
0 1986 American Chemical Society
5606 The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 TABLE I: 'Fq
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Tanner
51q Luminescence Spectrum of Cs2NaHoC16 wavenumber/cm-' line, Figure 2a 85 K
300 K 4286 m 4271 m 4252 m (4199) b
1 2 3 4
4155 s
5
4290 4274 4253 4194 4167 4155 4140
w
line, Figure 2b
assignt and derived vib wavenumber upper level, 5F5 lower level, 515 bra
1
w w mw bsh
s w
4122 s
6
4122 s
4093 bw
7
4094 mw 4073 bvw 4042 vw
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TABLE 11: Calculated Magnetic Dipole Intensity Ratios for the IF5 'I5 Luminescence Transition in Cs2NaHoCb
a r4
r5 r3 br4
4155 1 .o 1.o 4198 0.22 0.13 4329 0.076 0.042 4348 0.015 0.0014
4126 0.85 0.85 4169 0.60 0.36 4300 0.27 0.15 4319 0.082 0.0080
4093 0.50 0.50 4136 0.45 0.27 4267 f f 4286 0.72 0.070
4080 0.017 0.017 4123 0.27 0.16 4254 0.90 0.50 4273 0.37 0.036
9'2
vavenumber/lO'
9.6
cm-'
Figure 3. 488-nm excited 20 K luminescence spectrum of C S ~ N ~ G ~ C ~ ~ -between H O C ~9800 ~ - and 9000 cm-I.
wavenumbcr/lQ'
For each transition the first line gives the calculated energy (in cm-') and the second and third lines give the calculated MD intensity ratios at 295 and 85 K, respectively. The following energy level scheme was used: 5F5levels, 15 353 (ar,), 15 396 (I''), 15 527 (r3), and 15 546 (br4); ?I5 levels, 11 198 (ar,,), 11 227 (r5),11 260 (r3),and 11 273 (br4).
10.8
11.2
cm-'
*r"e""mner/.O:
11 2
Em-
Figure 4. 488-nm excited 20 K luminescence spectra between 11 200 and 10620 cm-' of (a) CSzNaHOCI6 and (b) C~~NaGdC1~-HoC163-.
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15.8
%"/,
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15.4
n
Figure 1. 15 K absorption spectrum of C ~ ~ N a H o between c1~ 15 300 and 15 950 cm-'
TABLE 111:. 5F3 '1' Luminescence Spectrum of Cs2NaGdo.do0.01C16" assignt and derived vib wavenumber wavenumber/ line, cm-' at 20 K Figure 3 upper level, 5Fp lower level, 515 (9237) 1 r2.defect site a r , 9196 s 2 r2 rs 9148 m 3 r2 ar4 + SICl(82) 4 r2 aF4 + S,(102) 9128 & 4 bm 9088 ms 5 r2 r3 + S I O W 6 r2 r3+ ~ ~ ( 1 0 2 ) 9069 mw
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'The electronic origins of the r2 ar,, r2 r3transitions are inferred to be at 9230 and 9171 cm-I, respectively. Since (5F3)I'zis at 20427 cm-I in CSzNaGdC16-HOC163- the (515)aI'4,rs,and r3levels are located at 11 197, 11 231, and 11 256 cm-I, respectively, from this analysis.
4.3
4.1 wavenumber/ 10'
Cm-'
Figure 2. 488-nm excited luminescence spectrum of CS2NaHOC16 between 4300 and 4080 cm-' (a) a t 300 K and (b) at 85 K.
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A very weak feature is observed at -7 cm-' to high energy of the EQ-allowed electronic origin r2 a r , and corresponds to emission from Ho3+ at a defect site. The remaining bands (Table 111) correspond to vibronic origins. Since the wavenumbers of vibrational structure in the electronic spectra of Cs2NaH&1, are well characterized6J2and do not vary from one intraconfigurational transition to another, these false origins permit the location of the (?15)ar4and I'3levels (Table 111). The values agree with those
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derived from 5F5 515. 'I5 Luminescence Transition. The lowest '1, level, a r 4 , is at -9 phonons above 516so that we expect luminescence to occur when this level is populated in Cs2NaHocl6. Under 488-nm excitation various radiative and nonradiative pathways are possible in populating 'I5 (see later). The energy levels have previously been located6q8and the calculated MD oscillator strengths for the strongest transitions between (515)aI'4 518are nearly two orders of magnitude weaker than (5F5)aI'4 ar4(515). Most of the intensity of the former AJ = 3 transition is therefore expected to arise from the ED vibronic mechanism. The 488-nm excited 20 K luminescence spectrum of Cs2NaHOC&in the region below 11200 cm-' is shown in Figure 4a. The
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Crystal-Field Levels of TABLE I V
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The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5607
Luminescence Spectrum of HOC&* wavenumber/cm-l CS2NaHOC16 line, 20 K Figure 4a
85 K
CS2NaGdC16-HOC16320 K
line, Figure 4b
11 197 vw
1
11 147 w
2
assignt and derived vib wavenumber upper level,
lower level,
11 331 vw 11 312 bvw (1 1 298) 11 276 vw 11 264 vw
1
11 162 m
11 198 w 11 188 vw
1 2
11 162 bvw 11 152 vw 11 144 w
3 4 5
11 131 vw 11 119 vw 11 113 vw
6 7 8
11 118 sh
3
11 104 w
9
11 103 mw
4
11 082 w 11 072 vw
10 11
11 080 mw
5
11 057 vw
12
11 060 w
6
11 142 m
11 105 w 11 080 m 11 069 w 11 046 shw 11 037 m (1 1033)
13
11 017 w 11 009 mw 10998 w
14
10997 mw
7
10958 mw 10952 w (10941) 10930 m
15 16 17 18
10959 m
8
10930 ms
9
10921 w 10914 m
19 20
10913 ms
10
10890 w 10884 m
21 22
10888 m
11
10875 ms
23
10875 vs
12
10855 sh 10846 s 10824 ms 10801 w
24 25 26 27
10859 sh 10846 vs 10825 s 10805 mw
13 14 15 16
10777 vw
28
10 752 mw 10738 m 10714 mw
29 30 31
10991 mw 10967 m
10923 ms
10911 sh 10881 s
10858 sh
1
10849 s 10823 m 10801 vw 10776 w 10751 bmw 10738 bmw 10718 w 10708 mw
I I
10688 m 10674 ms 10648 mw 'Electronic origins other a r s (11 0181, r3 br,, b r s (11008).
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17
10755 m 10743 m 10717 m
18 19 20
10700 mw 32 10 704 ms 21 10685 m 33 10688 ms 22 34 23 10670 ms 10674 s 35 24 10644 mw 10647 m than those directly observed are inferred from vibronic structure at the following wavenumbers: r3 a r , (1 1246), r3 b r s and rs ars(10989), r5 a r 3 (11 227), rs r, (11 188), rs b r 4 (11 028), b r 4 b r 4 (11 074), b r 4 br,,
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J . Phys. Chem. 1986, 90, 5608-561 1
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highest energy weak bands (lines 1 and 2) are readily associated with the (515)aF4 a r 3 , aI'JIs) transitions. All other M D electronic origins in the transitions originating from a r 4 are also a r 5 and a r , b r 3 , b r 5 are observed. In particular, a r 4 identified and distinguished from a r 4 a r 3 , a r 4 + s6 since the transverse optic (TO) component of s6 near 260 cm-' is expected to be stronger than the zone boundary (ZB) mode near 245 cm-I, the former not being observed. From Figure 2a the measured intensity ratios of the a r 4 a r 3 , a r 4 , rl,b r 4 , a r 5 , and b r 5 , b r 3 transitions are 0.2:0.2:0.03:1.00:2.5 f 1:9 f 2, whereas those calculated are 1.3:0.3:0.06:1.OO:1.6:37.7, respectively. The location of the electronic origins is confirmed from the analysis of the extensive vibronic structure (Table IV). The derived wavenumbers of the ungerade moiety modes of HoCls3- are (in cm-I) at 77, 84 (Slo),104, 128 (S,), and 245, 260, 285 ( S 6 ) . Lattice mode structure is observed at 45 (S5), 56 (S9), and 181 cm-l (S8). The luminescence from CS2NaGdC16-HOC163- is rather weaker under the same excitation conditions and fewer bands are observed (Figure 4b). Many new features are observed in the 85 K spectrum, and most of these are associated with luminescence originating from the excited crystal field components of 515. All bands may be assigned (Table IV) with the energy level scheme from the previous sections, and the expected M D electronic origins are observed.
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Conclusion The analyses of the 5F3,5F5 'I5 and 515 luminescence transitions of has enabled the location of all the crys-
tal-field components of the 515Russell-Saunders term of Ho3+ in cubic symmetry. The energy levels are given in the abstract to this paper. The observed crystal-field splittings, ar4-r5, ar4-r3, and ar4-br4 are 3 1,60, and 75 cm-I, respectively, whereas those calcu1atedf3J6are 48, 78, and 103 cm-'. In the 5F5 515transition of Cs2NaHoC1, most of the observed intensity is due to MD-allowed electronic origins. While origins are directly observed in the 5F3 515and 515 518transitions, their locations may also be inferred from extensive vibronic structure. The 515level in Cs2NaHoC1, is long-lived, the measured values of the (e-') decay lifetimes at 300 and 85 K being 48 and 108 ms, respectively. Between the wavenumber of the argon ion excitation source (20492 cm-I) and that of the 515 levels there are five other energy levels of Ho3+. Only two of these levels are significantly populated in C S ~ N ~ H at O or C above ~ ~ 85 K. One of these, 5F3,exhibits fast nonradiative decay, predominantly via the cross-relaxation to 5F5,and other level, 5F5,has a lifetime near 10 ms at 85 K. The energy gap 5F5-515is -4080 cm-' so that no cross-relaxation mechanisms are available for populating 'I5 from 5F5. The radiative population mechanism has however been shown to occur. The separation 5F3-515is -9150 cm-l so that a two-phonon-assisted cross-relaxation from 5F5to 515via 516is possible in neat Cs2NaHoC1,. The measured rise time of the '1, emission at liquid nitrogen temperature is more rapid than that expected for the population of 'I5 from 5F5alone, showing that the cross-relaxation from 5F3may be important. Registry No. HoCI,", 336 13-65-9; CsNaHoCl,, 52542-88-8; Cs,NaGdCI,, 27880-1 9-9.
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Appkation of Carbon-I 3 Solid-state High-Resolution NMR to the Study of Proton Mobility. Separation of Rigid and Mobile Components in Coal Structure P. Tekely, D. Nicole, J. Brondeau, and J.-J. Delpuech* Laboratoire d'Etude des Solutions Organiques et Colloidales, UA CNRS 406. Universitd de Nancy I , B.P. 239, 54506 Vandoeuvre-les- Nancy Cedex, France (Received: March 13, 1986)
A pulse sequence, in which I3C CP/MAS NMR signals are monitored after the partial transverse relaxation of attached hydrogens, is described for indirect measurement of T2* relaxation times of protons in solid materials. The method was independently applied to the discrimination between rigid/macromolecular and mobile/molecular components in two coal samples for aromatic and aliphatic protons. Closely similar T2* values are obtained for aromatic and aliphatic protons in the rigid component, while they differ by a factor of ca. 2 in the mobile component. A predominance of aromatic hydrogens was found in the mobile component with even an absence of aliphatic hydrogens for the higher rank coal sample.
Introduction The organic matter in coal is generally considered as being distributed in two "components". The first component, which is given the name of "macromolecular" or "rigidn component,'" is assumed to be an insoluble three-dimensional network composed of condensed aromatic and hydroaromatic units cross-linked by ether, methylene, or alkyl bridges. The second component, which is the "molecular" or "mobile" component, consists of organic molecules with moderately high molecular weights (< 1000 Considerable proportions of the latter material may (1) Larsen, J. W.; Green, T. K.;Chiri, I. In Proceedings of the Coal Science Conference 1983, Pittsburgh, PA, 1983; p 277. (2) Lucht, L. M.; Peppas, N. A. In Chemistry and Physics of Coal Utilization; Cooper,B. R., Petrakis, L., Eds.; American Institute of Physics: New York, 1980 p 28. (3) Lucht, L. M.; Peppas, N. A. ACS Symp. Ser. 1981, No. 169, 43. ( 4 ) Nelson, J. R. Fuel 1983, 62, 1 12. ( 5 ) Grimes, W. R. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic: New York, 1982; Vol. 1, p 21. (6) Weller, M.; Wert, C . H. Fuel 1984, 63, 891. (7) Davidson, R. M. In Coal Science; Gorbaty, M. L., Larsen, J. W., Wender, I., Eds.; Academic: New York, 1982; Vol. 1, p 83.
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be present in coal s t r ~ c t u r e .The ~ presence of the mobile component seems to be responsible for ihe thermoplastic properties of coal which are lost by removing large proportions of the organic molecules by solvent extraction.IO According to Hayatsu et al.,11-'3 lignites and anthracites contain smaller proportions of the mobile organic component than bituminous coals; this suggests that the organic molecules trapped within the macromolecular matrix are produced after the lignite stage and that they are subsequently either rejected from or incorporated into the coal matrix at the anthracite stage. In the investigations mentioned above, the proportions of mobile component are estimated as the fraction of organic matter removed ~~
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(8) Bodzek, D.; Marzec, A. Fuel 1981, 60, 47. (9) Vahrman, M. Chem. Br. 1972, 8 , 16. (10) Grint, A.; Mehani, S.; Trewhella, M.; Crook, M. J. Fuel 1985, 64,
1355. (11) Hayatsu, R.; Winans, R. E.; Scott, R. G.; Moore, L. P.; Studier, M. H. Fuel 1978, 57, 541. (12) Hayatsu, R.; Winans, R. E.; Scott, R. G . ;Moore, L. P.; Studier, M. H. Nature (London) 1978, 275, 116. (1 3) Studier, M. H.; Hayatsu, R.; Winans, R. E. In Analytical Methods for Coal and Coal Products; Karr, C . , Jr., Ed.; Academic: New York, 1978; VOl. 2, p 43
0 1986 American Chemical Society
