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Investigation of optical properties of praseodymium and cerium chlorides in nonaqueous solutions
Spectrochimica Acta Part A 54 (1998) 2207 – 2213
Investigation of optical properties of praseodymium and cerium chlorides in nonaqueous solutions Barbara Keller, Janina Legendziewicz *, Jacek Glin´ski Faculty of Chemistry, Uni6ersity of Wrocl*aw, 14F. Joliot-Curie Street, 50 -383 Wrocl*aw, Poland Received 4 December 1997; received in revised form 28 January 1998; accepted 28 January 1998
Abstract Absorption, emission and excitation spectra of anhydrous and hydrated praseodymium and cerium chlorides were investigated in nonaqueous solvents (CH3OH and iso-C3H7OH). The role of steric effect and solvation of salts on the intensities and energies of f–d and f–f transitions were analyzed. The changes of structure of the bands and the emission properties were discussed on the basis of the type of solvates formed in solution and compared to conductivity and ultrasonic velocity results. Effect of concentration on the excitation and emission spectra was also considered. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Cerium chloride; Praseodymium chloride; Methanol; Iso-propanol; Absorption; Excitation; Emission spectra
1. Introduction In the recent decade the great effort was made to investigate the organically modified glasses obtained by sol – gel technique. Most of these glasses were obtained by applying the alcohol method of synthesis. Thus, the optical properties of the glasses depend on the structure of active ions, their oxidation state and the type of cluster and/ or polymeric forms created in the material. Recently we reported the investigations of the structure of active centres using Eu3 + ion as a spectroscopic probe [1,2]. Now, we have undertaken the investigations of the structure and emis* Corresponding author. Fax: + 48 71 22348; e-mail: [email protected]
sion properties of hydrated and anhydrous cerium and praseodymium chlorides in different alcoholic solutions. The aim of it was to find the best conditions for strong emission of the systems studied. Moreover, we are trying to explain how the type of alcohol affects the optical properties of the solution (absorption, excitation and emission), how the equilibria of different oxidation states of metal existing in the solution (M3 + and M4 + ) influence the emission intensity, and how concentration of active ions affect the electronic transition probabilities, emission and M3 + /M4 + equilibria. The results could help in explaining the properties of more complex systems in gels and glasses, and help in determining the best conditions for the synthesis of glasses with better optical properties.
1386-1425/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S1386-1425(98)00139-5
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
2208
Fig. 1. Absorption spectra of hydrated and anhydrous cerium chlorides in alcohol solutions.
Except the spectroscopic methods, we also used the ultrasonic and conductivity measurements, which are useful in explaining the equilibria of different complexes and in determining the solvation numbers.
2. Experimental Cerium and praseodymium hydrated and anhydrous chlorides were obtained by the modified technique described earlier in details for lanthanide perchlorates [3]. IR spectra were used to detect water in the anhydrous salts. Purification of solvents was reported in [4]. Absorption spectra were measured at room temperature with a Cary-Varian 5 spectrophotometer. Emission and excitation spectra were recorded at 77 and 293 K using a SPF 500 spectrofluorometer. The details of sound velocity measurements are given in Ernst et al. and Jez; owska-Trzebiatowska et al. [5,6].
3. Results and discussion Two types of compounds were chosen for the
optical properties of alcoholic solutions investigations: cerium and praseodymium chlorides (anhydrous and hydrated). Cerium chlorides were used because of the possibility of existence of Ce3 + / Ce4 + equilibrium in solutions, while praseodymium chlorides-because of the application of f–f transition intensities in studies of the structure of metal ions in solutions, as well as the f–d transitions available in measured spectral region, what allows correlations with the properties of cerium compounds.
3.1. Cerium chloride solutions Absorption spectra of anhydrous and hydrated CeCl3 in CH3OH (I) and in iso-C3H7OH (II) are presented in Fig. 1. Note five components in the region of metal f–d transitions. The transitions in UV region are composed of five bands with different both shapes and relative intensities of separated components. Moreover, the half-width of the band located at 312 nm is relatively higher than those of other components. Resolving the UV spectra using Gauss–Lorentz functions leads to the evaluation of separated bands. Their intensities are collected in Table 1(a). The Ce3 + ion has one optically active electron; in 4f configuration for free ion spectra two levels
211.8
205.1
207.4
207.4
350.0 317.8 306.8 284.7 387.0 300.2 338.9 255.3 227.2
226.0
218.8
218.2
303.5 326.6 218.3 289.9 330.2 274.0 330.5 288.4
P · 105 o
244.9
243.4
334.9
234.0
l (nm)
3
343.8 303.8 292.5 271.3 283.1 311.0 408.8 403.4
P · 105 o
259.1
259.1
254.9
254.8
l (nm)
4
553.8 438.2 541.7 404.2 415.5 333.7 579.8 375.3
P · 105 o
313.5
312.7
313.7
313.5
l (nm)
5
733.1 821.8 678.5 767.7 848.9 772.2 1280 946.9
P · 105 o
2937
2304
2112
2284
PS · 105
(b) PrCl3 · 6H2O+CH3OH c=5.0 · 10−2 M PrCl3+CH3OH c =5.0 · 10−2 M PrCl3 · 6H2O+i-C3H7OH c=2.8 · 10−2 M PrCl3+i-C3H7OH c= 5.3 · 10−3 M 919.6 1137.0 1505.8 2128.9
226.5 224.2 226.3
P · 105
226.4
l (nm)
4f–5d
447.4
446.3
446.8
446.6
l (nm)
P2
3
4f–4f
1330.4
1352.3
1297.6
1262.1
P · 108
471.7
471.0
471.4
471.1
l (nm)
1 3 I6P1
667.6
624.5
661.0
625.0
P · 108
P0
484.5
483.3
483.4
483.2
l (nm)
3
449.1
372.9
384.8
359.3
P · 108
594.0
594.0
595.8
595.3
l (nm)
D2
1
294.0
278.6
294.8
297.9
P · 108
1012.4
1028.9
1029.5
1029.5
l (nm)
G4
1
51.77
51.65
33.38
34.45
P · 108
Oscillator strengths for separated components were obtained after resolving the bands using the Gauss function. PS, ‘total’ oscillator strength calculated from the area under full experimental band
(a) CeCl3 · 7H2O+CH3OH c= 1.2 · 10−2 M CeCl3+CH3OH c=1.3 · 10−2 M CeCl3 · 7H2O+i-C3H7OH c=1.0 · 10−2 M CeCl3+I-C3HOH c=1.7 · 10−3 M
l (nm)
l (nm) P · 105 o
2
1
4f–5d
Table 1 Intensity analysis of f–d and f–f transitions in cerium and praseodymium systems investigated
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213 2209
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B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
F5/2 ground state and 2F7/2 excited one are separated by 2200 cm − 1. In the 5d configuration it has two levels, 2D3/2 and 2D5/2. Oh symmetry splits them into three components and at lower symmetries degeneration of these levels is removed. Crystal field splits these levels into two and three components. In this way, there are five transitions from the 2F5/2 ground state of Ce3 + to the excited ones. In fact, we observed five components in the UV region for (I) and (II) systems. However, if we relate this result to those reported by Szczurek and Schlesinger in solids [7] (CaF2:Ce3 + ): the component at 260 cm − 1 can not belong to one Ce3 + centre in solution, with C46 symmetry. Thus, we miss one band. However, when considering C36 symmetry assuming CN= 9 and three inner coordinated chloride ions, the band at 260 nm may correspond to our system, at least that in metanolic solution. In solution one could expect one dominating form (in limited concentration range), but variation of concentration should modulate the equilibrium forms. The problem is, whether those fluctuations caused by dissociation of Cl − ions lead to strong differences in splitting of d levels. Coordination number seems to be 9, as it was found by the C – T band analysis in H2O –CH3OH solutions [8,9] and by our conductivity measurements. Inspection of the absorption spectra of CeCl3 and CeCl3 · 6H2O for two kinds of solvents (CH3OH and i-C3H7OH) indicates a relatively large differences in the distribution of separated components, as well as the presence of oxidation processes. In result an additional band given by C–T ligand to Ce4 + ion, occurs in the spectra. Intensity of this additional line depends on time (Fig. 1(d)). Moreover, the differences found between Ce + 3 spectra in CH3OH and iso-C3H7OH solvents point on coordination number changes. In consequence it can lead to the changes of the crystal field splitting. The above is better manifested in anhydrous systems (Fig. 1 (b, d)). Note also that splitting patterns of CeCl3 · 7H2O in iso-propanol are close to those in methanolic solutions. Comparison of the absorption spectra in alcohols with the spectrum of CeCl3 · 7H2O in water (where the aquoion is the domination form) point that both the splitting and shift of the
absorption bands are done by inner sphere coordination of chloride ions. Intensity analysis of the separated components of f–d bands in absorption (Fig. 1) clearly shows drastic differences of cerium ion surrounding in the anhydrous system in i-C3H7OH. Table 1a presents the oscillator strenght values of the separated components of f–d transitions. Instead of a decrease of intensities, like in the system CeCl3(anhydrous) in methanol, a strong increase of intensities of all the components was found in CeCl3(anhydrous) in iso-propanolic spectra. That confirms well our conclusion about the decreasing CN in anhydrous i-C3H7OH system and the symmetry change. Available X-ray data for Ce and Nd solvates [10,11] with iso-propanol of composition [NdCl2(i-C3H7OH)3]2Cl2 show creation of dimeric forms in solids with CN=7 and two inner sphere Cl − ions, and one bridging Cl − anion. Three iso-propanol molecules complete the CN to 7. It is worthy noting that M–Cl bonding lengths with bridging ions are significantly longer in comparison to other M–Cl bonds. This result corresponds to our spectroscopic ones. Analysis of the emission spectra versus lexc shows again the differences in the emission intensities and changes in intensity distribution of separated components for the two anhydrous and hydrated systems tested (Fig. 2). Decreasing temperature down to 77 K changes the Stokes shifts of the bands from 3600 cm − 1 at 293 K to 3150 cm − 1 at 77 K in CH3OH. Note strong decrease of the Stokes shift in isopropanol+ anhydrous CeCl3 system (from 4620 to 2800 cm − 1). The effect of salt concentration on the emission intensities is worthy analysis, too. Decreasing concentration is accompanied by an increase of the intensity of emission lines, reaching maximum at 0.002 M (Fig. 3). In low concentrations conductivity shows creation of solvates involving two or one Cl − ions instead of three in high concentrations (Section 3.2). Since oxidation of Ce3 + occurs, the role of C–T transition from Cl − to Ce4 + can be considered in the quenching processes in solution. The above is in straight relation
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
2211
to the shift of energy of the C – T band versus the number of inner-sphere Cl − ions [8,12,13], as well as to the changes of splitting of the f – d bands, depending on the composition of solvates formed (Fig. 1(d)). Low temperature luminescence spectra of hydrated CeCl3 show two strong components with splitting equal to 1700 cm − 1, and two low intensity ones separated by 2400 cm − 1. Judging from the above, two optically active metal centres with different symmetries exhibit emission at different energies. Moreover, their concentra-
Fig. 3. Emission spectra for different concentrations of CeCl3 · 7H2O in solutions.
tions in the solution differ meaningly, too. In fact, for anhydrous CeCl3 in iso-propanol, both low and high temperature emission also seem to show equilibria of more than one species with relatively different concentrations Fig. 4.
3.2. Praseodymium chloride solutions
Fig. 2. Excitation (left side) and emission (right side) spectra: the dependence of emission on the excitation energy.
Unexpectedly, the intensity analysis of 3P2, 3P0 transition for PrCl3 and PrCl3 · 6H2O in two alcohols studied shows only minor changes and does not allow to detect any structural modifications. Better results were obtained from analysis of the splitting and intensity of f–d bands. They show
2212
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
Fig. 4. Emission and excitation spectra of anhydrous and hydrated cerium chlorides at room and liquid nitrogen temperatures.
sufficiently strong differences in intensities of the f–d components (at 226.3 nm), confirming the conclusions drawn from the optical properties of cerium systems. Table 1(b) collects the results of oscillator strength calculations of f – d and f – f transitions in the absorption spectra of praseodymium chlorides in the solvents under investigation. Intensities of the f–d bands are by three orders of magnitude higher than those of the f – f ones. Besides, going to absolutely anhydrous systems, an increase of intensities of f – d bands was observed. Moreover, in the iso-propanolic system, this increase is significantly higher than in methanol+ PrCl3(anhydrous) solution spectra. The increase of intensities in the former system is accompagnied by a shift of energy of the f – d band. It is worthy noting that energy of the f – d band in Pr3 + ion spectra is shifted from the first Ce3 + band by 89 nm (towards higher energies). Note
also that intensities of f–d bands in Pr3 + system are higher than that of the first f–d band in cerium systems. Finally, detectable differences in intensities for different systems in the range of f–f bands were detected for 3H4 3P0;1G4 transitions only. Our acoustical investigation (to be published separately) seems to confirm the spectroscopic results.
4. Conclusions Comparison of the f–d transitions of anhydrous CeCl3 in CH3OH and in iso-propanol shows drastic differences both in splitting and intensities of respective components. Analysis of the f–d-transitions of respective PrCl3 systems seems to confirm these relations. The above suggests changes of CN in iso-propanol as a result of steric factors. X-ray data in solids seem to confirm
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
our conclusions. The hydrated systems differ significantly from the CeCl3(anhydrous)+ isopropanol one. Note that the results of both splitting and intensity of the f – d components better reflect the equilibria in the systems under investigation then the f-f ones. The essential differences of Stokes shifts were detected in the spectra of anhydrous cerium solutions. UV region (200–350 nm) in absorption and excitation spectra is composed of five components (2D3/2, 2D5/2 splitting levels) what indicates low symmetry (C26, C36, C46 ) of metal centres. Thus, the splitting given by the crystal field changes with concentration of salts shows equilibria of the centres with different symmetries. Sound velocity and conductivity investigations indicate the presence of equilibria of different chlorocomplexes created in the concentration range tested, confirming the spectroscopic results. They will be the subject of future publication.
Acknowledgements The authors are grateful to J. Sokolnicki and B. Urban´ski for their help in preparation of this paper.
.
2213
References [1] J. Legendziewicz, W. Stre¸k, J. Sokolnicki, B. Keller, M. Borzechowska, Acta Phys. Polon. A90 (1996) 461. [2] J. Legendziewicz, W. Stre¸k, J. Sokolnicki, B. Keller, M. Borzechowska, Proc. SPIE 3176 (1997) 255. [3] G. Oczko, J. Legendziewicz, B. Keller, B. Jez; owskaTrzebiatowska, Spectrochim. Acta Part A 45A (1989) 945. [4] K. Bukietyn´ska, B. Jez; owska-Trzebiatowska, B. Keller, J. Inorg. Nucl. Chem. 43 (1981) 1065. [5] S. Ernst, J. Glin´ski, B. Jez; owska-Trzebiatowska, Acta Phys. Polon. A55 (1979) 501. [6] B. Jez; owska-Trzebiatowska, S. Ernst, J. Legendziewicz, G. Oczko, Bull. Acad. Polon. Sci.: Chim. 26 (1978) 805. [7] T. Szczurek, M. Schlesinger, Proceedings of the International Symposium on Rare Earth Spectroscopy, World Scientific Publishers Singapore,Wrocl*aw, Poland, 1984, pp. 309. [8] B. Keller, K. Bukietyn´ska, B. Jez; owska-Trzebiatowska, Bull. Acad. Polon. Sci.; Ser. Chim. 24 (1976) 763. [9] B. Keller, PhD Thesis, University of Wrocl*aw, Poland, 1975. [10] J. Zhongsheng, et al., Chem. J. Chin. Univ. 6 (1989) 735. [11] M. Schofer, R. Herbst-Irmer, U. Groth, T. Kohler, Acta Crystallogr. C 50 (1991) 1256. [12] J. D. Ryan, C. K. Jorgensen, J. Phys. Chem. 70 (1966) 2845. [13] J. Legendziewicz, G. Oczko, B. Keller, W. Stre¸k, B. Jez; owska-Trzebiatowska, Bull. Acad. Polon. Sci. Ser. Chem. 32 (1984) 301.
Investigation of optical properties of praseodymium and cerium chlorides in nonaqueous solutions Barbara Keller, Janina Legendziewicz *, Jacek Glin´ski Faculty of Chemistry, Uni6ersity of Wrocl*aw, 14F. Joliot-Curie Street, 50 -383 Wrocl*aw, Poland Received 4 December 1997; received in revised form 28 January 1998; accepted 28 January 1998
Abstract Absorption, emission and excitation spectra of anhydrous and hydrated praseodymium and cerium chlorides were investigated in nonaqueous solvents (CH3OH and iso-C3H7OH). The role of steric effect and solvation of salts on the intensities and energies of f–d and f–f transitions were analyzed. The changes of structure of the bands and the emission properties were discussed on the basis of the type of solvates formed in solution and compared to conductivity and ultrasonic velocity results. Effect of concentration on the excitation and emission spectra was also considered. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Cerium chloride; Praseodymium chloride; Methanol; Iso-propanol; Absorption; Excitation; Emission spectra
1. Introduction In the recent decade the great effort was made to investigate the organically modified glasses obtained by sol – gel technique. Most of these glasses were obtained by applying the alcohol method of synthesis. Thus, the optical properties of the glasses depend on the structure of active ions, their oxidation state and the type of cluster and/ or polymeric forms created in the material. Recently we reported the investigations of the structure of active centres using Eu3 + ion as a spectroscopic probe [1,2]. Now, we have undertaken the investigations of the structure and emis* Corresponding author. Fax: + 48 71 22348; e-mail: [email protected]
sion properties of hydrated and anhydrous cerium and praseodymium chlorides in different alcoholic solutions. The aim of it was to find the best conditions for strong emission of the systems studied. Moreover, we are trying to explain how the type of alcohol affects the optical properties of the solution (absorption, excitation and emission), how the equilibria of different oxidation states of metal existing in the solution (M3 + and M4 + ) influence the emission intensity, and how concentration of active ions affect the electronic transition probabilities, emission and M3 + /M4 + equilibria. The results could help in explaining the properties of more complex systems in gels and glasses, and help in determining the best conditions for the synthesis of glasses with better optical properties.
1386-1425/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S1386-1425(98)00139-5
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
2208
Fig. 1. Absorption spectra of hydrated and anhydrous cerium chlorides in alcohol solutions.
Except the spectroscopic methods, we also used the ultrasonic and conductivity measurements, which are useful in explaining the equilibria of different complexes and in determining the solvation numbers.
2. Experimental Cerium and praseodymium hydrated and anhydrous chlorides were obtained by the modified technique described earlier in details for lanthanide perchlorates [3]. IR spectra were used to detect water in the anhydrous salts. Purification of solvents was reported in [4]. Absorption spectra were measured at room temperature with a Cary-Varian 5 spectrophotometer. Emission and excitation spectra were recorded at 77 and 293 K using a SPF 500 spectrofluorometer. The details of sound velocity measurements are given in Ernst et al. and Jez; owska-Trzebiatowska et al. [5,6].
3. Results and discussion Two types of compounds were chosen for the
optical properties of alcoholic solutions investigations: cerium and praseodymium chlorides (anhydrous and hydrated). Cerium chlorides were used because of the possibility of existence of Ce3 + / Ce4 + equilibrium in solutions, while praseodymium chlorides-because of the application of f–f transition intensities in studies of the structure of metal ions in solutions, as well as the f–d transitions available in measured spectral region, what allows correlations with the properties of cerium compounds.
3.1. Cerium chloride solutions Absorption spectra of anhydrous and hydrated CeCl3 in CH3OH (I) and in iso-C3H7OH (II) are presented in Fig. 1. Note five components in the region of metal f–d transitions. The transitions in UV region are composed of five bands with different both shapes and relative intensities of separated components. Moreover, the half-width of the band located at 312 nm is relatively higher than those of other components. Resolving the UV spectra using Gauss–Lorentz functions leads to the evaluation of separated bands. Their intensities are collected in Table 1(a). The Ce3 + ion has one optically active electron; in 4f configuration for free ion spectra two levels
211.8
205.1
207.4
207.4
350.0 317.8 306.8 284.7 387.0 300.2 338.9 255.3 227.2
226.0
218.8
218.2
303.5 326.6 218.3 289.9 330.2 274.0 330.5 288.4
P · 105 o
244.9
243.4
334.9
234.0
l (nm)
3
343.8 303.8 292.5 271.3 283.1 311.0 408.8 403.4
P · 105 o
259.1
259.1
254.9
254.8
l (nm)
4
553.8 438.2 541.7 404.2 415.5 333.7 579.8 375.3
P · 105 o
313.5
312.7
313.7
313.5
l (nm)
5
733.1 821.8 678.5 767.7 848.9 772.2 1280 946.9
P · 105 o
2937
2304
2112
2284
PS · 105
(b) PrCl3 · 6H2O+CH3OH c=5.0 · 10−2 M PrCl3+CH3OH c =5.0 · 10−2 M PrCl3 · 6H2O+i-C3H7OH c=2.8 · 10−2 M PrCl3+i-C3H7OH c= 5.3 · 10−3 M 919.6 1137.0 1505.8 2128.9
226.5 224.2 226.3
P · 105
226.4
l (nm)
4f–5d
447.4
446.3
446.8
446.6
l (nm)
P2
3
4f–4f
1330.4
1352.3
1297.6
1262.1
P · 108
471.7
471.0
471.4
471.1
l (nm)
1 3 I6P1
667.6
624.5
661.0
625.0
P · 108
P0
484.5
483.3
483.4
483.2
l (nm)
3
449.1
372.9
384.8
359.3
P · 108
594.0
594.0
595.8
595.3
l (nm)
D2
1
294.0
278.6
294.8
297.9
P · 108
1012.4
1028.9
1029.5
1029.5
l (nm)
G4
1
51.77
51.65
33.38
34.45
P · 108
Oscillator strengths for separated components were obtained after resolving the bands using the Gauss function. PS, ‘total’ oscillator strength calculated from the area under full experimental band
(a) CeCl3 · 7H2O+CH3OH c= 1.2 · 10−2 M CeCl3+CH3OH c=1.3 · 10−2 M CeCl3 · 7H2O+i-C3H7OH c=1.0 · 10−2 M CeCl3+I-C3HOH c=1.7 · 10−3 M
l (nm)
l (nm) P · 105 o
2
1
4f–5d
Table 1 Intensity analysis of f–d and f–f transitions in cerium and praseodymium systems investigated
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213 2209
2210 2
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
F5/2 ground state and 2F7/2 excited one are separated by 2200 cm − 1. In the 5d configuration it has two levels, 2D3/2 and 2D5/2. Oh symmetry splits them into three components and at lower symmetries degeneration of these levels is removed. Crystal field splits these levels into two and three components. In this way, there are five transitions from the 2F5/2 ground state of Ce3 + to the excited ones. In fact, we observed five components in the UV region for (I) and (II) systems. However, if we relate this result to those reported by Szczurek and Schlesinger in solids [7] (CaF2:Ce3 + ): the component at 260 cm − 1 can not belong to one Ce3 + centre in solution, with C46 symmetry. Thus, we miss one band. However, when considering C36 symmetry assuming CN= 9 and three inner coordinated chloride ions, the band at 260 nm may correspond to our system, at least that in metanolic solution. In solution one could expect one dominating form (in limited concentration range), but variation of concentration should modulate the equilibrium forms. The problem is, whether those fluctuations caused by dissociation of Cl − ions lead to strong differences in splitting of d levels. Coordination number seems to be 9, as it was found by the C – T band analysis in H2O –CH3OH solutions [8,9] and by our conductivity measurements. Inspection of the absorption spectra of CeCl3 and CeCl3 · 6H2O for two kinds of solvents (CH3OH and i-C3H7OH) indicates a relatively large differences in the distribution of separated components, as well as the presence of oxidation processes. In result an additional band given by C–T ligand to Ce4 + ion, occurs in the spectra. Intensity of this additional line depends on time (Fig. 1(d)). Moreover, the differences found between Ce + 3 spectra in CH3OH and iso-C3H7OH solvents point on coordination number changes. In consequence it can lead to the changes of the crystal field splitting. The above is better manifested in anhydrous systems (Fig. 1 (b, d)). Note also that splitting patterns of CeCl3 · 7H2O in iso-propanol are close to those in methanolic solutions. Comparison of the absorption spectra in alcohols with the spectrum of CeCl3 · 7H2O in water (where the aquoion is the domination form) point that both the splitting and shift of the
absorption bands are done by inner sphere coordination of chloride ions. Intensity analysis of the separated components of f–d bands in absorption (Fig. 1) clearly shows drastic differences of cerium ion surrounding in the anhydrous system in i-C3H7OH. Table 1a presents the oscillator strenght values of the separated components of f–d transitions. Instead of a decrease of intensities, like in the system CeCl3(anhydrous) in methanol, a strong increase of intensities of all the components was found in CeCl3(anhydrous) in iso-propanolic spectra. That confirms well our conclusion about the decreasing CN in anhydrous i-C3H7OH system and the symmetry change. Available X-ray data for Ce and Nd solvates [10,11] with iso-propanol of composition [NdCl2(i-C3H7OH)3]2Cl2 show creation of dimeric forms in solids with CN=7 and two inner sphere Cl − ions, and one bridging Cl − anion. Three iso-propanol molecules complete the CN to 7. It is worthy noting that M–Cl bonding lengths with bridging ions are significantly longer in comparison to other M–Cl bonds. This result corresponds to our spectroscopic ones. Analysis of the emission spectra versus lexc shows again the differences in the emission intensities and changes in intensity distribution of separated components for the two anhydrous and hydrated systems tested (Fig. 2). Decreasing temperature down to 77 K changes the Stokes shifts of the bands from 3600 cm − 1 at 293 K to 3150 cm − 1 at 77 K in CH3OH. Note strong decrease of the Stokes shift in isopropanol+ anhydrous CeCl3 system (from 4620 to 2800 cm − 1). The effect of salt concentration on the emission intensities is worthy analysis, too. Decreasing concentration is accompanied by an increase of the intensity of emission lines, reaching maximum at 0.002 M (Fig. 3). In low concentrations conductivity shows creation of solvates involving two or one Cl − ions instead of three in high concentrations (Section 3.2). Since oxidation of Ce3 + occurs, the role of C–T transition from Cl − to Ce4 + can be considered in the quenching processes in solution. The above is in straight relation
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
2211
to the shift of energy of the C – T band versus the number of inner-sphere Cl − ions [8,12,13], as well as to the changes of splitting of the f – d bands, depending on the composition of solvates formed (Fig. 1(d)). Low temperature luminescence spectra of hydrated CeCl3 show two strong components with splitting equal to 1700 cm − 1, and two low intensity ones separated by 2400 cm − 1. Judging from the above, two optically active metal centres with different symmetries exhibit emission at different energies. Moreover, their concentra-
Fig. 3. Emission spectra for different concentrations of CeCl3 · 7H2O in solutions.
tions in the solution differ meaningly, too. In fact, for anhydrous CeCl3 in iso-propanol, both low and high temperature emission also seem to show equilibria of more than one species with relatively different concentrations Fig. 4.
3.2. Praseodymium chloride solutions
Fig. 2. Excitation (left side) and emission (right side) spectra: the dependence of emission on the excitation energy.
Unexpectedly, the intensity analysis of 3P2, 3P0 transition for PrCl3 and PrCl3 · 6H2O in two alcohols studied shows only minor changes and does not allow to detect any structural modifications. Better results were obtained from analysis of the splitting and intensity of f–d bands. They show
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Fig. 4. Emission and excitation spectra of anhydrous and hydrated cerium chlorides at room and liquid nitrogen temperatures.
sufficiently strong differences in intensities of the f–d components (at 226.3 nm), confirming the conclusions drawn from the optical properties of cerium systems. Table 1(b) collects the results of oscillator strength calculations of f – d and f – f transitions in the absorption spectra of praseodymium chlorides in the solvents under investigation. Intensities of the f–d bands are by three orders of magnitude higher than those of the f – f ones. Besides, going to absolutely anhydrous systems, an increase of intensities of f – d bands was observed. Moreover, in the iso-propanolic system, this increase is significantly higher than in methanol+ PrCl3(anhydrous) solution spectra. The increase of intensities in the former system is accompagnied by a shift of energy of the f – d band. It is worthy noting that energy of the f – d band in Pr3 + ion spectra is shifted from the first Ce3 + band by 89 nm (towards higher energies). Note
also that intensities of f–d bands in Pr3 + system are higher than that of the first f–d band in cerium systems. Finally, detectable differences in intensities for different systems in the range of f–f bands were detected for 3H4 3P0;1G4 transitions only. Our acoustical investigation (to be published separately) seems to confirm the spectroscopic results.
4. Conclusions Comparison of the f–d transitions of anhydrous CeCl3 in CH3OH and in iso-propanol shows drastic differences both in splitting and intensities of respective components. Analysis of the f–d-transitions of respective PrCl3 systems seems to confirm these relations. The above suggests changes of CN in iso-propanol as a result of steric factors. X-ray data in solids seem to confirm
B. Keller et al. / Spectrochimica Acta Part A 54 (1998) 2207–2213
our conclusions. The hydrated systems differ significantly from the CeCl3(anhydrous)+ isopropanol one. Note that the results of both splitting and intensity of the f – d components better reflect the equilibria in the systems under investigation then the f-f ones. The essential differences of Stokes shifts were detected in the spectra of anhydrous cerium solutions. UV region (200–350 nm) in absorption and excitation spectra is composed of five components (2D3/2, 2D5/2 splitting levels) what indicates low symmetry (C26, C36, C46 ) of metal centres. Thus, the splitting given by the crystal field changes with concentration of salts shows equilibria of the centres with different symmetries. Sound velocity and conductivity investigations indicate the presence of equilibria of different chlorocomplexes created in the concentration range tested, confirming the spectroscopic results. They will be the subject of future publication.
Acknowledgements The authors are grateful to J. Sokolnicki and B. Urban´ski for their help in preparation of this paper.
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